論文 - 大橋　洋士

Shear viscosity and hierarchical structure of matter

Kagamihara D., Horikoshi M., Nonaka C., Ohashi Y., Shigaki K.

European Physical Journal A （Springer）  61 （ 7 ） 180(1-12) 2025年07月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  14346001

　概要を見る

This article deals with the shear viscosity and effects of inter-particle correlations from the viewpoint of the hierarchical structure of matter. Although the change in hierarchical levels, which means that one kind of dominant degrees of freedom transitions into another, does not necessarily accompany the phase transition, and then we cannot expect a clear boundary between them in general, we propose that the shear viscosity is a useful probe to detect it: the shear viscosity takes a minimum around the boundary of the change in hierarchies. The minimum of the shear viscosity itself has attracted much attention and has widely been investigated from various research fields, ranging from low-energy condensed matter physics to high-energy nuclear physics, due to the so-called Kovtun–Son–Starinets (KSS) conjecture (which predicts the existence of a universal lower bound (KSS bound) of this quantity). We discuss a viscosity minimum and the KSS bound in two examples: quark matter in high-energy physics and an ultracold Fermi gas in low-energy physics.

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Engineering Nonequilibrium Superconducting Phases in a Voltage-Driven Superconductor Under an External Magnetic Field

Kawamura T., Ohashi Y.

Annalen Der Physik （Wiley）  537 （ 12 ） e2500102(1-14) 2025年06月

査読有り,  ISSN  00033804

　概要を見る

Nonequilibrium properties of a normal metal-superconductor-normal metal (NSN) junction are theoretically investigated under an external magnetic field. When a bias voltage is applied between the normal-metal leads, the confined superconductor is driven out of equilibrium, resulting in a nonequilibrium quasiparticle distribution function having a two-step structure. Using the nonequilibrium Green's function technique, a comprehensive phase diagram of the nonequilibrium superconductor is determined. This analysis reveals that the interplay between Zeeman-split energy bands and the nonequilibrium distribution function gives rise to a rich phase structure. Notably, it is found that superconductivity destroyed by a strong external magnetic field revives by applying the bias voltage. This reentrant phenomenon is shown to originate from four effective “Fermi surfaces” that result from the combination of Zeeman-split energy bands and the two-step structure in the nonequilibrium distribution function. These results demonstrate the possibility of controlling quantum states of matter through the combined engineering of energy band structures and distribution functions.

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Equation of state of quantum cluster matter

Horikoshi M., Ohashi Y., Kagamihara D.

European Physical Journal A （Springer）  61 （ 6 ） 141(1-14) 2025年05月

査読有り,  ISSN  14346001

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This review article deals with the role of quantum clusters in the hierarchical structure of quantum many-body systems, especially focusing on a two-component spin-1/2 ultracold Fermi gas. Quantum clusters, which are loosely bound states with both atomic and molecular properties, play a key role in hierarchical transitions. We explain how an atom-molecule transition continuously occurs by using a Feshbach resonance. We also explain how this hierarchy is reflected in the so-called BCS-BEC crossover phase diagram, where a normal Fermi gas phase and a normal Bose gas phase form a hierarchy structure, and the pseudogap phase appears between them as an intermediate “semi-hierarchy.” The crossover region is shown to be conveniently determined by using the specific heat at constant volume, as well as the spin susceptibility. We also point out that the isothermal compressibility is a useful quantity to see how an atomic interaction gives rise to an intermolecular one. Future experiments on thermodynamic quantities would be crucial for the assessment of these theoretical results, as well as the further understanding of hierarchical many-body systems.

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Nuclear Spin-Lattice Relaxation Rate in Odd-Frequency Superconductivity

Iwasaki S.Ohashi Y.

Journal of the Physical Society of Japan （JPSJ）  94 （ 3 ） 034707(1-11) 2025年03月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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We theoretically investigate the temperature dependence of nuclear spin–lattice relaxation rate T1-1 in bulk odd-frequency superconductivity. For a model odd-frequency pairing interaction, we first evaluate the superconducting order parameter, within the framework of the combined path-integral formalism with the saddle-point approximation. We then calculate T1-1 below the superconducting phase transition temperature Tc, to see how the odd-frequency pairing affects this physical quantity. In the odd-frequency p-wave state, while the so-called coherence peak is suppressed as in the even-frequency p-wave case, T1-1 is found to exhibit the Korringa-law-like behavior (T1-1 / T) except just below Tc, even without impurity scatterings. In the odd-frequency s-wave case, the behavior of T1-1 is found to be sensitive to the detailed spin structure of the superconducting order parameter: In a case, T1-1 is enhanced far below Tc, being in contrast to the conventional (even-frequency) s-wave BCS case, where the coherence peak appears just below Tc. We also show that the calculated T1-1 in the odd-frequency p-wave case well explains the recent experiment on CeRh0.5Ir0.5In5, where the possibility of odd-frequency p-wave superconductivity was recently suggested experimentally.

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Nonequilibrium BCS-BEC crossover and unconventional FFLO superfluid in a strongly interacting driven-dissipative Fermi gas

Kawamura T., Ohashi Y

AAPPS Bulletin （AAPPS Bulletin）  34 （ 1 ） 31(1-3) 2024年11月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  02182203

　概要を見る

We present a theoretical review of the recent progress in non-equilibrium BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover physics. As a paradigmatic example, we consider a strongly interacting driven-dissipative two-component Fermi gas where the non-equilibrium steady state is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. As a powerful theoretical tool to deal with this system, we employ the Schwinger-Keldysh Green’s function technique. We systematically evaluate the superfluid transition, as well as the single-particle properties, in the non-equilibrium BCS-BEC crossover region, by adjusting the chemical potential difference between the reservoirs and the strength of an s-wave pairing interaction associated with a Feshbach resonance. In the weak-coupling BCS side, the chemical potential difference is shown to imprint a two-step structure on the particle momentum distribution, leading to an anomalous enhancement of pseudogap, as well as the emergence of exotic Fulde-Ferrell-Larkin-Ovchinnikov-type superfluid instability. Since various non-equilibrium situations have recently been realized in ultracold Fermi gases, the theoretical understanding of non-equilibrium BCS-BEC crossover physics would become increasingly important in this research field.

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Pairing properties of an odd-frequency superfluid Fermi gas

Iwasaki S., Kawamura T., Manabe K., Ohashi Y.

Physical Review A （Physical Review A）  109 （ 6 ） 063309(1-15) 2024年06月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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We theoretically investigate strong-coupling properties of an odd-frequency Fermi superfluid. This pairing state has the unique property that Cooper pairs are formed between fermions, not at the same time, but at different times. To see whether or not such unequal-time pairs still exhibit bosonic behavior, we examine the space-time structure of the odd-frequency Cooper-pair wave function at T=0, by employing the combined path-integral formalism with the BCS-Eagles-Leggett-type superfluid theory. In the strong-coupling regime, the odd-frequency pair wave function still has different space-time structure from that in the ordinary even-frequency s-wave superfluid state; their magnitudes are found to become close to each other, except for the equal-time pairing component. In this regime, we also evaluate the superfluid phase transition temperature Tc within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink. The calculated Tc in the strong-coupling regime of the odd-frequency system is found to be well described by the Bose-Einstein condensation of tightly bound Bose molecules. Our results indicate that, in spite of vanishing equal-time pairing, odd-frequency Cooper pairs still behave like bosons in the strong-coupling regime, as in the even-frequency s-wave superfluid case.

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Emergence of Larkin-Ovchinnikov-type superconducting state in a voltage-driven superconductor

Kawamura T., Ohashi Y., Stoof H.T.C.

Physical Review B （Physical Review B）  109 （ 10 ） 104502(1-19) 2024年03月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  24699950

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We theoretically investigate a voltage-biased normal metal-superconductor-normal metal (NSN) junction. Using the nonequilibrium Green's function technique, we derive a quantum kinetic equation to determine the superconducting order parameter self-consistently. The derived equation is an integral-differential equation with memory effects. We solve this equation by converting it into a system of ordinary differential equations with the use of a pole expansion of the Fermi-Dirac function. When the applied voltage exceeds the critical value, the superconductor switches to the normal state. We find that when the voltage is decreased from the normal phase, the system relaxes to a Larkin-Ovchinnikov-type (LO) inhomogeneous superconducting state, even in the absence of a magnetic Zeeman field. We point out that the emergence of the LO-type state can be attributed to the nonequilibrium energy distribution of electrons due to the bias voltage. We also point out that the system exhibits bistability, which leads to hysteresis in the voltage-current characteristic of the NSN junction.

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Stable nonequilibrium Fulde-Ferrell-Larkin-Ovchinnikov state in a spin-imbalanced driven-dissipative Fermi gas loaded on a three-dimensional cubic optical lattice

Kawamura T., Ohashi Y.

Physical Review A （Physical Review A）  108 （ 1 ） 013321 2023年07月

研究論文（学術雑誌）, 最終著者, 査読有り,  ISSN  24699926

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We theoretically investigate a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type superfluid phase transition in a driven-dissipative two-component Fermi gas. The system is assumed to be in the nonequilibrium steady state, which is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. Including pairing fluctuations by extending the strong-coupling theory developed in the thermal-equilibrium state by Nozières and Schmitt-Rink to this nonequilibrium case, we show that a nonequilibrium FFLO (NFFLO) phase transition can be realized without spin imbalance, under the conditions that (1) the two reservoirs imprint a two-edge structure on the momentum distribution of Fermi atoms and (2) the system is loaded on a three-dimensional cubic optical lattice. While the two edges work like two Fermi surfaces with different sizes, the role of the optical lattice is to prevent the NFFLO long-range order from destruction by NFFLO pairing fluctuations. We also draw the nonequilibrium mean-field phase diagram in terms of the chemical potential difference between the two reservoirs, a fictitious magnetic field to tune the spin imbalance of the system, and the environmental temperature of the reservoirs to clarify the relation between the NFFLO state and the ordinary thermal-equilibrium FFLO state discussed in spin-imbalanced Fermi gases.

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Isothermal compressibility and effects of multi-body molecular interactions in a strongly interacting ultracold Fermi gas

Daichi Kagamihara, Ryohei Sato, Koki Manabe, Hiroyuki Tajima, Yoji Ohashi

Physical Review A （Physical Review A）  106 （ 3 ） 033308  2022年09月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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We theoretically investigate the isothermal compressibility κT in the normal state of an ultracold Fermi gas. Including pairing fluctuations, as well as preformed-pair formations, within the framework of the self-consistent T-matrix approximation, we evaluate the temperature dependence of this thermodynamic quantity over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. While κT in the weak-coupling BCS regime is dominated by Fermi atoms near the Fermi surface, correlations between tightly bound Cooper-pair molecules are found to play crucial roles in the strong-coupling BEC regime. In the latter region, besides a two-body molecular interaction, a three-body one is shown to sizably affect κT near the superfluid phase-transition temperature. Our results indicate that the strong-coupling BEC regime of an ultracold Fermi gas would provide a unique opportunity to study multibody correlations between Cooper-pair molecules.

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Feasibility of a Fulde-Ferrell-Larkin-Ovchinnikov superfluid Fermi atomic gas

Kawamura T., Ohashi Y.

Physical Review A （Physical Review A）  106 （ 3 ）  2022年09月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

　概要を見る

We theoretically explore a promising route to achieve the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in a spin-imbalanced ultracold Fermi gas. In the current stage of cold atom physics, search for this exotic Fermi superfluid is facing two serious difficulties: One is the desperate destruction of the FFLO long-range order by FFLO pairing fluctuations, which precludes entering the phase through a second-order transition, even in three dimension. The other is the fierce competition with the phase separation into the BCS (Bardeen-Cooper-Schrieffer) state and the spin-polarized normal state. By including strong FFLO pairing fluctuations within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that the anisotropy of Fermi surface introduced by an optical lattice makes the FFLO state stable against the paring fluctuations. This stabilized FFLO state is also found to be able to overcome the competition with the phase separation under a certain condition. Since the realization of unconventional Fermi superfluids is one of the most exciting challenges in cold atom physics, our results would contribute to the further development of this field.

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Proposed Fermi-surface reservoir engineering and application to realizing unconventional Fermi superfluids in a driven-dissipative nonequilibrium Fermi gas

Kawamura Taira, Hanai Ryo, Ohashi Yoji

PHYSICAL REVIEW A （Physical Review A）  106 （ 1 ）  2022年07月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  2469-9926

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We develop a theory to describe the dynamics of a driven-dissipative many-body Fermi system to pursue our proposal to realize exotic quantum states based on reservoir engineering. Our idea is to design the shape of a Fermi surface so as to have multiple Fermi edges by properly attaching multiple reservoirs with different chemical potentials to a fermionic system. These emerged edges give rise to additional scattering channels that can destabilize the system into unconventional states, which is exemplified in this work by considering a driven-dissipative attractively interacting Fermi gas. By formulating a quantum kinetic equation using the Nambu-Keldysh Green's function technique, we explore nonequilibrium steady states in this system and assess their stability. We find that, in addition to the Bardeen - Cooper - Schrieffer-type isotropic pairing state, a Fulde-Ferrell-type anisotropic superfluid state being accompanied by Cooper pairs with nonzero center-of-mass momentum exists as a stable solution, even in the absence of a magnetic Zeeman field. Our result implies a great potential of realizing quantum matter beyond the equilibrium paradigm by engineering the shape and topology of Fermi surfaces in both electronic and atomic systems.

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Beyond Gaussian pair fluctuation theory for strongly interacting Fermi gases II: The broken-symmetry phase

Brendan C. Mulkerin, Xing-Can Yao, Yoji Ohashi, Xia-Ji Liu, Hui Hu

arXiv:2201.04798 （arXiv (preprint server)）  2022年01月

研究論文（学術雑誌）, 共著

　概要を見る

We theoretically investigate the superfluid properties of a two-band gas of Yb173 Fermi atoms with an orbital Feshbach resonance (OFR). To describe the BCS-BEC crossover region, we include superfluid fluctuations caused by interband and intraband pairing interactions associated with OFR by extending the strong-coupling theory developed by Nozières and Schmitt-Rink to the two-band case below the superfluid phase transition temperature; however, the effects of an experimentally inaccessible deep bound state are removed to model a real Yb173 Fermi gas near OFR. We show that the condensate fraction in the upper closed channel gradually becomes smaller than that in the lower open channel as one moves from the strong- to the weak-coupling regime, because the OFR-pairing mechanism tunes the interaction strengths by adjusting the energy difference between the two bands. However, even when the closed-channel band is much higher in energy than the open-channel band in the weak-coupling regime, the magnitude of the superfluid order parameter in the closed channel is found to be still comparable to that in the open channel. As the reason for this, we point out a pair-tunneling effect by the OFR-induced interband interaction. In addition to these superfluid quantities, we also examine collective modes, such as the Goldstone mode, the Schmid (Higgs) mode, and Leggett mode, to clarify how they appear in the spectral weights of pair-correlation functions in each band. Since the realization of a multiband superfluid Fermi gas is a crucial issue in cold Fermi gas physics, our results would contribute to the basic understanding of this type of Fermi superfluid in the BCS-BEC crossover region.

Thermodynamic stability, compressibility matrices, and effects of mediated interactions in a strongly interacting Bose-Fermi mixture

Koki Manabe, Ohashi Yoji

PHYSICAL REVIEW A （Physical Review A）  103 （ 6 ） 063317(1-12) 2021年06月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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We theoretically investigate the thermodynamic stability of a normal-state Bose-Fermi mixture, with a tunable Bose-Fermi pairing interaction -UBF<0 associated with a heteronuclear Feshbach resonance, as well as a weak repulsive Bose-Bose interaction UBB≥0. Including strong heteropairing fluctuations associated with the former interaction within the self-consistent T-matrix approximation, as well as the latter within the mean-field level, we calculate the compressibility matrix, to assess the stability of this system against density fluctuations. In the weak- and the intermediate-coupling regimes with respect to -UBF, we show that an effective attractive interaction between bosons mediated by density fluctuations in the Fermi component makes the system unstable below a certain temperature Tclp (leading to density collapse). When UBB=0, Tclp is always higher than the Bose-Einstein condensation (BEC) temperature Tc. When UBB>0, the density collapse is suppressed, and the BEC transition becomes possible. It is also suppressed by the formation of tightly bound Bose-Fermi molecules when the heteropairing interaction -UBF is strong; however, since the system may be viewed as a molecular Fermi gas in this case, the BEC transition does not also occur. Since quantum gases involving Bose atoms are known to be sensitive to interparticle correlations, our results would be useful for the study of many-body properties of a Bose-Fermi mixture in a stable manner, without facing the unwanted density collapse.

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Superfluid properties of an ultracold Fermi gas with an orbital Feshbach resonance in the BCS-BEC crossover region

Kamihori Taro, Kagamihara Daichi, Ohashi Yoji

PHYSICAL REVIEW A （Physical Review A）  103 （ 5 ） 053319(1-14) 2021年05月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  2469-9926

　概要を見る

We theoretically investigate the superfluid properties of a two-band gas of Yb173 Fermi atoms with an orbital Feshbach resonance (OFR). To describe the BCS-BEC crossover region, we include superfluid fluctuations caused by interband and intraband pairing interactions associated with OFR by extending the strong-coupling theory developed by Nozières and Schmitt-Rink to the two-band case below the superfluid phase transition temperature; however, the effects of an experimentally inaccessible deep bound state are removed to model a real Yb173 Fermi gas near OFR. We show that the condensate fraction in the upper closed channel gradually becomes smaller than that in the lower open channel as one moves from the strong- to the weak-coupling regime, because the OFR-pairing mechanism tunes the interaction strengths by adjusting the energy difference between the two bands. However, even when the closed-channel band is much higher in energy than the open-channel band in the weak-coupling regime, the magnitude of the superfluid order parameter in the closed channel is found to be still comparable to that in the open channel. As the reason for this, we point out a pair-tunneling effect by the OFR-induced interband interaction. In addition to these superfluid quantities, we also examine collective modes, such as the Goldstone mode, the Schmid (Higgs) mode, and Leggett mode, to clarify how they appear in the spectral weights of pair-correlation functions in each band. Since the realization of a multiband superfluid Fermi gas is a crucial issue in cold Fermi gas physics, our results would contribute to the basic understanding of this type of Fermi superfluid in the BCS-BEC crossover region.

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Strong-Coupling Effects on Quantum Transport in an Ultracold Fermi Gas

Furutani K., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  201 （ 1-2 ） 49 - 57 2020年06月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We theoretically investigate quantum conductance across a point contact between two strongly-interacting ultracold Fermi gases. We evaluate the tunneling current, taking into account pairing fluctuations in the Fermi gases within the framework of a combined strong-coupling T-matrix approximation with the Schwinger-Keldysh non-equilibrium formalism. We show that quasi-particle conductance is gradually suppressed, as one passes through the BCS–BEC crossover region from the weak-coupling side. On the other hand, with increasing the strength of a pairing interaction, the fluctuation-pair conductance is found to remarkably increase in the crossover region, which is followed by the suppression in the strong-coupling regime.

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Kovtun-Son-Starinets Conjecture and Effects of Mass Imbalance in the Normal State of an Ultracold Fermi Gas in the BCS-BEC Crossover Region

Kagamihara Daichi, Ohashi Yoji

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN （Journal of the Physical Society of Japan）  89 （ 4 ） 044005(1-10) 2020年03月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  0031-9015

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We theoretically assess the conjecture proposed by Kovtun, Son, and Starinets, stating that the ratio η/s of the shear viscosity η to the entropy density s has the lower bound as η/s ≥ ħ/(4πkB). In the normal state of a mass-imbalanced ultracold Fermi gas, consistently including strong-coupling corrections to both η and s within the self-consistent T-matrix approximation, we evaluate η/s over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region, in the presence of mass imbalance. We find that η/s achieves the minimum value 4.5 × ħ/(4πkB), not at the unitarity, but slightly in the BEC regime, (kFas)-1 ≃ 0:4 > 0 (where as is the s-wave scattering length, and kF is the Fermi momentum). In contract to the previous expectation, we find that this lower bound is almost independent of mass imbalance: Our results predict that all the mass-balanced 6Li-6Li and 40K-40K mixtures and the mass-imbalanced 40K-161Dy mixture give almost the same lower bound of η/s. We also point out that the two quantum phenomena, Pauli blocking and bound-state formation, are crucial keys for the lower bound of η/s.

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BCS–BEC crossover in cold atomic and in nuclear systems

Ohashi Y., Tajima H., van Wyk P.

Progress in Particle and Nuclear Physics （Progress in Particle and Nuclear Physics）  111   103739(1-54) 2020年03月

研究論文（学術雑誌）, 共著, 筆頭著者, 責任著者, 査読有り,  ISSN  01466410

　概要を見る

© 2019 Elsevier B.V. We review the BCS (Bardeen–Cooper–Schrieffer)–BEC (Bose–Einstein condensation) crossover phenomenon discussed in an ultracold Fermi atomic gas and a neutron superfluid in the low-density crust regime of a neutron star. A purpose of this paper is to show that these two very different atomic and nuclear systems can be closely related to each other from the viewpoint of this quantum many-body phenomenon. We explain how the BCS–BEC crossover is realized in the former atomic system by using the novel pairing mechanism called Feshbach resonance. We present a simple explanation for this crossover phenomenon to grasp the essence, as well as detailed microscopic theories that can cover the entire BCS–BEC crossover region. In the latter, we point out that the ordinary BCS theory already has the ability to describe the BCS–BEC crossover at T=0. At finite temperatures T>0, however, we need to go beyond this mean-field theory. Besides general aspects of the BCS–BEC crossover phenomenon, we also pick up special topics peculiar to each atomic gas and neutron fluid. The first one is the pseudogap phenomenon in the normal state of a Fermi atomic gas. The second one is the problem of non-zero effective range in an s-wave neutron superfluid.

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Thermodynamic Stability and Effects of Bose–Bose Repulsion in an Ultracold Bose–Fermi Mixture with Strong Hetero-Pairing Fluctuations

Manabe K., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  201 （ 1-2 ） 65 - 72 2020年01月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We theoretically investigate the thermodynamic stability of an ultracold Bose–Fermi mixture with a tunable inter-species interaction associated with a hetero-nuclear Feshbach resonance. To include hetero-pairing fluctuations, we employ an improved T-matrix approximation (iTMA), where all Bose Green’s functions involved in the self-energy are modified so as to satisfy the required gapless Bose excitations at the BEC phase transition. Using iTMA, we evaluate the compressibility matrix in the normal state, to examine the stability of the system against an effective Bose–Bose attraction, mediated by Fermi atoms. We clarify how the mechanical collapse of a Bose–Fermi mixture can be avoided by a direct Bose–Bose repulsive interaction. We also show that this thermodynamic instability cannot be captured by the ordinary non-self-consistent TMA, where the bare Bose Green’s function is used in the self-energy.Since the thermodynamic stability is crucial for the experimental realization of a resonant Bose–Fermi mixture, our results would contribute to the research for many-body properties of this system.

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Spin-dipole mode in a trapped Fermi gas near unitarity

Tajima H., Recati A., Ohashi Y.

Physical Review A （Physical Review A）  101 （ 1 ） 013610(1-7) 2020年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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We theoretically investigate the spin-dipole oscillation of a strongly interacting Fermi gas confined by a harmonic trapping potential. By using a diagrammatic strong-coupling theory combined with a local density approximation and a sum rule approach, we study the temperature dependence of the spin-dipole frequency near unitarity. The connection of the spin-dipole frequency with the spin susceptibility and the pairing correlations is exploited. While the spin-dipole frequency exactly coincides with the trap frequency in a noninteracting Fermi gas, it is shown to be strongly enhanced in the superfluid state, because of the suppression of the spin degree of freedom due to the spin-singlet Cooper-pair formation. In strongly interacting Fermi gases, such enhancement occurs even above the superfluid phase transition temperature, due to the strong pairing correlations.

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Nonequilibrium strong-coupling theory for a driven-dissipative ultracold Fermi gas in the BCS-BEC crossover region

Kawamura T., Hanai R., Kagamihara D., Inotani D., Ohashi Y.

Physical Review A （Physical Review A）  101 （ 1 ） 013602(1-20) 2020年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

　概要を見る

We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC crossover regime in the nonequilibrium steady state, being coupled with two fermion baths. By developing a nonequilibrium strong-coupling theory based on the combined T-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the nonequilibrium superfluid phase transition in the Bardeen-Cooper-Schrieffer unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various nonequilibrium phenomena have recently been measured in ultracold Fermi gases, our nonequilibrium strong-coupling theory would be useful to catch up with this experimental development in this research field.

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Strong-Coupling Theory for a Non-equilibrium Unitary Fermi Gas

Kawamura T., Kagamihara D., Hanai R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  201 （ 1-2 ） 41 - 48 2019年12月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We study a non-equilibrium unitary Fermi gas, being coupled with two baths with different values of chemical potential. Extending a T-matrix approximation developed in equilibrium Fermi gases to the non-equilibrium steady state by using the Keldysh Green’s function technique, we examine the superfluid phase transition out of equilibrium. We also clarify non-equilibrium effects on the pseudogap phenomenon by strong pairing fluctuations.

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Superfluid Phase Transitions and Effects of Thermal Pairing Fluctuations in Asymmetric Nuclear Matter

Tajima H., Hatsuda T., van Wyk P., Ohashi Y.

Scientific Reports （Scientific Reports）  9 （ 1 ） 1 - 13 2019年12月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  2045-2322

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© 2019, The Author(s). We investigate superfluid phase transitions of asymmetric nuclear matter at finite temperature (T) and density (ρ) with a low proton fraction (Yp ≤ 0.2), which is relevant to the inner crust and outer core of neutron stars. A strong-coupling theory developed for two-component atomic Fermi gases is generalized to the four-component case, and is applied to the system of spin-1/2 neutrons and protons. The phase shifts of neutron-neutron (nn), proton-proton (pp) and neutron-proton (np) interactions up to k = 2 fm−1 are described by multi-rank separable potentials. We show that the critical temperature Tcnn of the neutron superfluidity at Yp = 0 agrees well with Monte Carlo data at low densities and takes a maximum value Tcnn= 1.68 MeV at ρ/ρ=0.14 with ρ0 = 0.17 fm−3. Also, the critical temperature Tcnn of the proton superconductivity for Yp ≤ 0.2 is substantially suppressed at low densities due to np-pairing fluctuations, and starts to dominate over Tcnn only above ρ/ρ=0.70(0.77) for Yp = 0.1(0.2), and (iii) the deuteron condensation temperature Tcd is suppressed at Yp ≤ 0.2 due to a large mismatch of the two Fermi surfaces.

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Single-particle properties of a strongly interacting Bose-Fermi mixture with mass and population imbalance

Manabe K., Inotani D., Ohashi Y.

Physical Review A （Physical Review A）  100 （ 6 ） 063609(1-13) 2019年12月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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We theoretically investigate strong-coupling properties of a Bose-Fermi mixture. In the mass- A nd population-balanced case, two of the authors have shown that a strong heteropairing interaction in this mixture brings about coupling phenomena between Fermi atomic excitations and Bose atomic and composite molecular excitations, that appear as an anomalous multiple peak structure in the single-particle spectral weight (SW) [Kharga, Inotani, Hanai, and Ohashi, J. Phys. Soc. Jpn. 86, 084301 (2017)JUPSAU0031-901510.7566/JPSJ.86.084301]. In this paper, we show that, although these many-body phenomena are sensitive to mass and population imbalances between the Bose and Fermi components, SW still exhibits the multiple peak structure in moderately mass-imbalanced Rb87-K40 and Na23-K40 mixtures. We also point out that the photoemission spectrum is a useful quantity to observe this spectral anomaly. Since a real trapped Bose-Fermi mixture is usually accompanied by mass and (local) population imbalances, our results would contribute to the study of a strongly interacting Bose-Fermi mixture, under realistic imbalanced conditions.

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Shear viscosity and strong-coupling corrections in the BCS–BEC crossover regime of an ultracold Fermi gas

Kagamihara D., Inotani D., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  88 （ 11 ） 114001(1-10) 2019年10月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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We theoretically investigate the shear viscosity η in the BCS–BEC crossover regime of an ultracold Fermi gas with a Feshbach resonance. Within the framework of the strong-coupling self-consistent T-matrix approximation, we examine how a strong pairing interaction associated with a Feshbach resonance affects this transport coefficient, in the normal state above the superfluid phase transition temperature T . We show that, while η diverges in both the weak-coupling BCS and strong-coupling BEC limits, it becomes small in the unitary regime. The minimum of η is obtained, not at the unitarity, but slightly in the strong-coupling BEC side. This deviation is consistent with the recent experiment on a Li Fermi gas. In the weak-coupling BCS regime, we also find that η exhibits anomalous temperature dependence near T , which is deeply related to the pseudogap phenomenon originating from strong pairing fluctuations. c c 6

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Isothermal Compressibility of an Ultracold Fermi Gas in the BCS–BEC Crossover

Sato R., Kagamihara D., Manabe K., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  196 （ 1-2 ） 119 - 125 2019年07月

研究論文（国際会議プロシーディングス）, 共著, 最終著者,  ISSN  00222291

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© 2018, Springer Science+Business Media, LLC, part of Springer Nature. We theoretically investigate the isothermal compressibility κT in the normal state of an ultracold Fermi gas with a tunable attractive interaction. We calculate this thermodynamic quantity by considering fluctuations in the Cooper channel, within the framework of the self-consistent T-matrix approximation (SCTMA). For comparison, we also evaluate this quantity in a “non”-self-consistent T-matrix approximation (TMA). We show that the calculated κT diverges at Tc in the BCS–BEC crossover region. On the other hand, such a singular behavior is absent when we deal with this quantity in SCTMA. We point out that the origin of this difference is the neglect of an effective inter-pair interaction in the former approximation. We also explicitly show how such an interaction is involved in the theory when one deals with pairing fluctuations in SCTMA. Our results indicate that the isothermal compressibility is a useful quantity in considering how preformed Cooper pairs interact with one another in the BCS–BEC crossover regime of an ultracold Fermi gas.

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Non-Hermitian phase transition from a polariton Bose-Einstein condensate to a photon laser

R. Hanai, A. Edelman, Y. Ohashi, and P. B. Littlewood, Yoji Ohashi

Physical Review Letters （Physical Review Letters）  122 （ 18 ） 185301(1-6) 2019年05月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319007

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We propose a novel mechanism for a nonequilibrium phase transition in a U(1)-broken phase of an electron-hole-photon system, from a Bose-Einstein condensate of polaritons to a photon laser, induced by the non-Hermitian nature of the condensate. We show that a (uniform) steady state of the condensate can always be classified into two types, namely, arising either from lower or upper-branch polaritons. We prove (for a general model) and demonstrate (for a particular model of polaritons) that an exceptional point where the two types coalesce marks the end point of a first-order-like phase boundary between the two types, similar to a critical point in a liquid-gas phase transition. Since the phase transition found in this paper is not in general triggered by population inversion, our result implies that the second threshold observed in experiments is not necessarily a strong-to-weak-coupling transition, contrary to the widely believed understanding. Although our calculation mainly aims to clarify polariton physics, our discussion is applicable to general driven-dissipative condensates composed of two complex fields.

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Strong-Coupling Effects on Specific Heat in the BCS–BEC Crossover

Inotani D., van Wyk P., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  196 （ 1-2 ） 111 - 118 2019年05月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2019, Springer Science+Business Media, LLC, part of Springer Nature. We theoretically investigate strong-coupling effects on specific heat at constant volume CV in a superfluid Fermi gas with a tunable interaction associated with Feshbach resonance. Including fluctuations of the superfluid order parameter within the strong-coupling theory developed by Nozières and Schmitt-Rink, we calculate the temperature dependence of CV at the unitarity limit in the superfluid phase. We show that, in the low-temperature region, T3-behavior is shown in the temperature dependence of CV. This result indicates that the low-lying excitations are dominated by the gapless Goldstone mode, associated with the phase fluctuations of the superfluid order parameter. Since the Goldstone mode is one of the most fundamental phenomena in the Fermionic superfluidity, our results are useful for further understanding how the pairing fluctuations affect physical properties in the BCS–BEC crossover physics below the superfluid transition temperature.

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Single-Particle Structures in the Normal State of a Strongly Interacting Bose–Fermi Mixture with Mass Imbalance

Manabe K., Kagamihara D., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Springer）  196 （ 1-2 ） 126 - 132 2018年12月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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We extend some of the authors’ previous work for a mass-balanced Bose–Fermi mixture to include effects of realistic mass imbalance in a Na– K and a Rb– K mixtures. Including effects of pairing fluctuations within the framework of an improved T-matrix theory, we calculate the single-particle spectral weight (SW), as well as the photoemission spectrum (PES) in the unitary regime at the Bose–Einstein condensation temperature T . We show that the characteristic three-peak structure induced by paring fluctuations, being predicted in the mass-balanced case, remains to exist in SW even in the presence of mass difference. We also show that PES is a useful experimental method to observe this phenomenon in a Na– K, as well as Rb– K, Bose–Fermi mixtures. Since this strong-coupling phenomenon is qualitatively different from that in a two-component Fermi gas, our results would be useful in clarifying how the quantum statistics affects Bose and Fermi pairing phenomena in a two-component gas mixture. 23 40 87 40 23 40 87 40 BEC

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Photoemission spectrum in the BCS-BEC Crossover Regime of a Rare-Earth Fermi Gas with an Orbital Feshbach Resonance

S. Mondai, D. Inotani, Yoji Ohashi

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  87 （ 9 ） 094301(1-9) 2018年08月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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© 2018 The Physical Society of Japan. We theoretically investigate photoemission spectrum (PES) in a 173Yb Fermi gas with a tunable pairing interaction associated with an orbital Feshbach resonance. Including the two-channel character of this rare-earth atomic system, we examine strong-coupling effects on single-particle properties of the open and closed channels, within the framework of an T-matrix approximation in terms of fluctuations in the Cooper channel. In the BCS-BEC crossover region, PES in the closed channel is shown to exhibit non-vanishing intensity in the negative energy region, as an effect of pairing fluctuations. On the other hand, PES in the open channel exhibits a pseudogap phenomenon, as in the case of a broad magnetic Feshbach resonance discussed in 40K and 6Li Fermi gases. We also examine effects of a harmonic trap in the local density approximation, to see how the spatial inhomogeneity affects strong-coupling corrections to PES.

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Single-particle Excitations and Strong Coupling Effects in the BCS-BEC Crossover Regime of a Rare-Earth Fermi Gas with an Orbital Feshbach Resonance

S. Mondal, D. Inotani, Yoji Ohashi

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  87 （ 8 ） 084302(1-10) 2018年08月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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We theoretically investigate normal-state properties of an ultracold Fermi gas with an orbital Feshbach resonance (OFR). Recently, OFR has attracted much attention as a promising pairing mechanism to realize a superfluid Yb Fermi gas. Including pairing fluctuations within a T-matrix approximation, and removing effects of an experimentally inaccessible deep bound state, we evaluate strong-coupling corrections to single-particle excitations. With increasing the strength of an OFR-induced tunable pairing interaction, the open channel is shown to exhibit the pseudogap phenomenon in the BCS–BEC crossover region, as in the case of a broad magnetic Feshbach resonance (MFR) in Li and K Fermi gases. We also show that the strong pairing interaction affects the closed channel, leading to the coexistence of particle and hole branches in the single-particle spectral weight. Since the latter phenomenon cannot be observed in the conventional MFR case, it may be viewed as a characteristic strong-coupling phenomenon peculiar to the OFR case. 173 6 40

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Strong-Coupling and Finite Temperature Effects on p-wave Contacts

D. Inotani, Yoji Ohashi

Physical Review A （Physical Review A）  98 （ 2 ） 023603(1-12) 2018年08月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2018 American Physical Society. We investigate theoretically strong-coupling and finite-temperature effects on the p-wave contacts, as well as the asymptotic behavior of the momentum distribution in the large-momentum region in a one-component Fermi gas with a tunable p-wave interaction. Including p-wave pairing fluctuations within a strong-coupling theory, we calculate the p-wave contacts above the superfluid transition temperature Tc from the adiabatic energy relations. We show that, while the p-wave contact related to the scattering volume monotonically increases with increasing interaction strength, the one related to the effective range depends nonmonotonically on interaction strength and its sign changes in the intermediate-coupling regime. The nonmonotonic interaction dependence of these quantities is shown to originate from the competition between the increase of the cutoff momentum and the decrease of the coupling constant of the p-wave interaction with increasing effective range. We also analyze the asymptotic form of the momentum distribution in the large-momentum region. In contrast to the conventional s-wave case, we show that the asymptotic behavior cannot be completely described by only the p-wave contacts, and the extra terms, which are not related to the thermodynamic properties, appear. Furthermore, in the high-temperature region, we find that the extra terms dominate the subleading term of the large-momentum distribution. We also directly compare our results with the recent experimental measurement by including the effects of a harmonic trap potential within the local density approximation. We show that, while our model qualitatively explains the dependence on the interaction strength of the p-wave contacts, there are some quantitative disagreements between our theory and recent experiment; for example, the peak position of the p-wave contact related to the effective range. Since the p-wave contacts connect the microscopic properties to the thermodynamic properties of the system, our results would be helpful for further understanding many-body properties of this anisotropic interacting Fermi gas in the normal phase.

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Photoluminescence and gain/absorption spectra of a driven-dissipative electron-hole-photon condensate

Hanai R., Littlewood P., Ohashi Y.

Physical Review B （Physical Review B）  97 （ 24 ） 245302(1-22) 2018年06月

研究論文（学術雑誌）, 共著, 最終著者,  ISSN  24699950

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© 2018 American Physical Society. We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a driven-dissipative exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are obtained. We show that the nonequilibrium nature of the exciton-polariton condensate strongly suppresses the visibility of the Bogoliubov dispersion in the negative energy branch (ghost branch) in photoluminescence spectra. We also show that the trace of this branch can be captured as a hole burning effect in gain/absorption spectra. Our results indicate that the nonequilibrium nature of the exciton-polariton condensate strongly reduces quantum depletion, while a scattering channel to the ghost branch is still present.

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Specific heat and effects of strong pairing fluctuations in a superfluid Fermi atom gas in the BCS-BEC crossover region

Van Wyk P., Inotani D., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  969 （ 1 ） 012013(1-6) 2018年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

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© Published under licence by IOP Publishing Ltd. We theoretically investigate the specific heat at constant volume C V in the BCS(Bardeen-Cooper-Schrieffer)-BEC(Bose-Einstein-condensation)-crossover regime of an ultracold Fermi gas, below the superfluid phase transition temperature T c. Within the strong-coupling framework developed by Nozières and Schmitt-Rink, we show that the temperature dependence of C V drastically changes as one passes through the crossover region, and is sensitive to strong fluctuations in the Cooper channel near the unitarity limit. We also compare our results to a recent experiment on a 6Li unitary Fermi gas. Since fluctuation effects are a crucial key in the BCS-BEC-crossover phenomenon, our results would be helpful in considering how the fermionic BCS superfluid changes into BEC with increasing the interaction strength, from the viewpoint of specific heat.

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Pairing fluctuations in a strongly interacting two-dimensional Fermi gas

Matsumoto M., Hanai R., Inotani D., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  969 （ 1 ） 012012(1-6) 2018年04月

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  17426588

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© Published under licence by IOP Publishing Ltd. We theoretically investigate strong-coupling properties of a two-dimensional Fermi gas in the normal phase. Including pairing fluctuations within the framework of a self-consistent T-matrix approximation, we calculate the single-particle density of states (DOS) near the observed Berezinskii-Kosterlitz-Thouless phase transition temperature T BKT. We show that the pseudogap phenomenon associated with strong pairing fluctuations appears as an almost fully gapped single-particle density of states in the strong-coupling regime, indicating that amplitude fluctuations of the order parameter are almost absent and the system is dominated by phase fluctuations of the order parameter, as expected in a two-dimensional system near T BKT. On the other hand, although pairing fluctuations are enhanced by the low-dimensionality of the system, they are found to be still not strong enough to open such a full gap in the intermediate coupling (crossover) regime. In this regime, a dip structure is only seen in DOS around ω = 0, indicating that amplitude of the superfluid order parameter is also fluctuating, in addition to phase fluctuations. In theoretically determining T BKT in a two-dimensional Fermi gas, it is frequently assumed that, while phase fluctuations exist, the amplitude of the superfluid order parameter is fixed. However, our results indicate the necessity of including both amplitude and phase fluctuations of the order parameter in evaluating T BKT in the crossover region. Since the BKT phase transition has recently been reported in a two-dimensional 6Li Fermi gas, our results would be useful for the theoretical study of this observed novel Fermi superfluid and effects of strong low-dimensional pairing fluctuations.

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Strongly Interacting Multi-component Fermions: From Ultracold Atomic Fermi Gas to Asymmetric Nuclear Matter in Neutron Stars

Tajima H., Hatsuda T., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  969 （ 1 ） 012003(1-6) 2018年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

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© Published under licence by IOP Publishing Ltd. We investigate an asymmetric nuclear matter consisting of protons and neutrons with spin degrees of freedom (σ = ↑, ↓). By generalizing the Nozières and Schmitt-Rink theory for two-component Fermi gases to the four-component case, we analyze the critical temperature T c of the superfluid phase transition. Although the pure neutron matter exhibits the dineutron condensation in the low-density region, the superfluid instability toward the deuteron condensation is found to take place as the proton fraction increases. We clarify the mechanism of the competition between the deuteron condensation and dineutron condensation. Our results would serve for understanding the properties of asymmetric nuclear matter realized in the interior of neutron stars.

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Closed-channel contribution in the BCS-BEC crossover regime of an ultracold Fermi gas with an orbital Feshbach resonance

Mondal S., Inotani D., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  969 （ 1 ） 012017(1-6) 2018年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

© Published under licence by IOP Publishing Ltd. We theoretically investigate strong-coupling properties of an ultracold Fermi gas with an orbital Feshbach resonance (OFR). Including tunable pairing interaction associated with an OFR within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR), we examine the occupation of the closed channel. We show that, although the importance of the closed channel is characteristic of the system with an OFR, the occupation number of the closed channel is found to actually be very small at the superfluid phase transition temperature T c, in the whole BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region, when we use the scattering parameters for an ultracold 173Yb Fermi gas. The occupation of the closed channel increases with increasing the temperature above T c, which is more remarkable for a stronger pairing interaction. We also present a prescription to remove effects of an experimentally inaccessible deep bound state from the NSR formalism, which we meet when we theoretically deal with a 173Yb Fermi gas with an OFR.

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Superfluid Fermi atomic gas as a quantum simulator for the study of the neutron-star equation of state in the low-density region

Van Wyk P., Tajima H., Inotani D., Ohnishi A., Ohashi Y.

Physical Review A （Physical Review A）  97 （ 1 ） 013601(1-13)  2018年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2018 American Physical Society. We propose a theoretical idea to use an ultracold Fermi gas as a quantum simulator for the study of the low-density region of a neutron-star interior. Our idea is different from the standard quantum simulator that heads for perfect replication of another system, such as the Hubbard model discussed in high-Tc cuprates. Instead, we use the similarity between two systems and theoretically make up for the difference between them. That is, (1) we first show that the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR) can quantitatively explain the recent experiment on the equation of state (EoS) in a Li6 superfluid Fermi gas in the BCS (Bardeen-Cooper-Schrieffer) unitary limit far below the superfluid phase-transition temperature Tc. This region is considered to be very similar to the low-density region (crust regime) of a neutron star (where a nearly unitary s-wave neutron superfluid is expected). (2) We then theoretically compensate the difference that, while the effective range reff is negligibly small in a superfluid Li6 Fermi gas, it cannot be ignored (reff=2.7 fm) in a neutron star, by extending the NSR theory to include effects of reff. The calculated EoS when reff=2.7 fm is shown to agree well with the previous neutron-star EoS in the low-density region predicted in nuclear physics. Our idea indicates that an ultracold atomic gas may more flexibly be used as a quantum simulator for the study of other complicated quantum many-body systems, when we use not only the experimental high tunability, but also the recent theoretical development in this field. Since it is difficult to directly observe a neutron-star interior, our idea would provide a useful approach to the exploration for this mysterious astronomical object.

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Pseudogap regime of a two-dimensional uniform fermi gas

Matsumoto M., Hanai R., Inotani D., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  87 （ 1 ） 014301(1-9) 2018年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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© 2018 The Physical Society of Japan. We investigate pseudogap phenomena in a two-dimensional Fermi gas. Including pairing fluctuations within a selfconsistent T-matrix approximation, we determine the pseudogap temperature T∗ below which a dip appears in the density of states ρ(ω) around the Fermi level. Evaluating T∗, we identify the pseudogap region in the phase diagram of this system. We find that, while the observed Berezinskii-Kosterlitz-Thouless (BKT) transition temperature TexpBKT in a 6Li Fermi gas is in the pseudogap regime, the detailed pseudogap structure in ρ(ω) at TexpBKT still differs from a fullygapped one, indicating the importance of amplitude fluctuations in the Cooper channel there. Since the observed TexpBKT in the weak-coupling regime cannot be explained by the recent BKT theory which only includes phase fluctuations, our results may provide a hint about how to improve this BKT theory. Although ρ(ω) has not been measured in this system, we show that the assessment of our results is still possible by using the observable Tan's contact.

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Ground-state thermodynamic quantities of homogeneous spin-1/2 fermions from the BCS region to the unitarity limit

Horikoshi M., Koashi M., Tajima H., Ohashi Y., Kuwata-Gonokami M.

Physical Review X （Physical Review X）  7 （ 4 ） 041004(1-15) 2017年10月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  2160-3308

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The understanding of physical properties of fermions in the unitary regime, where the s-wave scattering length in the collisional channel of particles is longer than both the interparticle distance and the size of the interaction potential, is a crucial issue for electron systems of high-temperature superconductivity, dilute nucleons in nuclei, and neutron stars. We experimentally determine various thermodynamic quantities of interacting two-component fermions at the zero-temperature limit from the BCS region to the unitarity limit. The obtained results are very accurate in the sense that the systematic error is within 4% in the unitary regime. Using this advantage, we can compare our data with various many-body theories. We find that an extended T-matrix approximation, which is a strong-coupling theory involving fluctuations in the Cooper channel, well reproduces our experimental results. We also find that the superfluid order parameter Δ calculated by solving the ordinary BCS gap equation with the chemical potential of interacting fermions is close to the binding energy of the paired fermions directly observed in a spectroscopic experiment and that obtained using a quantum Monte Carlo method.

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Dynamical instability of a driven-dissipative electron-hole condensate in the BCS-BEC crossover region

Hanai R., Littlewood P., Ohashi Y.

Physical Review B （Physical Review B）  96 （ 12 ） 125206(1-19) 2017年09月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699950

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© 2017 American Physical Society. We present a stability analysis on a driven-dissipative electron-hole condensate in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. Extending the combined BCS-Leggett theory with the generalized random phase approximation to the nonequilibrium case by employing the Keldysh formalism, we show that the pumping and decay of carriers causes a depairing effect on excitons. This phenomenon gives rise to an attractive interaction between excitons in the BEC regime, as well as a supercurrent that anomalously flows antiparallel to θ(r) [where θ(r) is the phase of the condensate] in the BCS regime, both leading to dynamical instabilities of an exciton BEC. Our results suggest that a substantial region of the exciton-BEC phase in the phase diagram (in terms of the interaction strength and the decay rate) is unstable.

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Spin susceptibility and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas

Tajima H., Hanai R., Ohashi Y.

Physical Review A （Physical Review A）  96 （ 3 ） 033614(1-11) 2017年09月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2017 American Physical Society. We theoretically investigate magnetic properties of a trapped ultracold Fermi gas. Including pairing fluctuations within the framework of an extended T-matrix approximation, as well as effects of a harmonic trap in the local density approximation, we calculate the local spin susceptibility χt(r,T) in the Bardeen-Cooper-Schrieffer-Bose-Einstein condensation crossover region. We show that pairing fluctuations cause nonmonotonic temperature dependence of χt(r,T). Although this behavior looks similar to the spin-gap phenomenon associated with pairing fluctuations in a uniform Fermi gas, the trapped case is found to also be influenced by the temperature-dependent density profile, in addition to pairing fluctuations. We demonstrate how to remove this extrinsic effect from χt(r,T), to study the interesting spin-gap phenomenon purely originating from pairing fluctuations. Since experiments in cold-atom physics are always done in a trap, our results would be useful for the assessment of preformed pair scenario, from the viewpoint of spin-gap phenomenon.

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Single-particle excitations and effects of hetero-pairing fluctuations in a Bose–Fermi mixture with a Feshbach resonance

Kharga D., Inotani D., Hanai R., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  86 （ 8 ） 084301 (1-8) 2017年08月

ISSN  00319015

　概要を見る

©2017 The Physical Society of Japan. We theoretically investigate normal-state properties of a gas mixture of single-component bosons and fermions with a hetero-nuclear Feshbach resonance. Including strong hetero-pairing fluctuations associated with the Feshbach resonance, we calculate single-particle density of states, as well as the spectral weight at various interaction strengths. For this purpose, we employ an improved T-matrix approximation (TMA), where the bare Bose Green’s function in the non-selfconsistent TMA self-energy is modified so as to satisfy the Hugenholtz–Pines relation at the Bose–Einstein condensation (BEC) temperature T BEC . In the unitary regime at T BEC , we show that hetero-pairing fluctuations couple Fermi atomic excitations with Fermi molecular excitations, as well as with Bose atomic excitations. Although a similar coupling phenomenon by pairing fluctuations is known to give a pseudo-gapped density of states in the unitary regime of a two-component Fermi gas, such a dip structure is found to not appear even in the unitary limit of a Bose–Fermi mixture. It only appears in the strong-coupling regime. Instead, a spectral peak along the molecular dispersion appears in the spectral weight. We also clarify how this coupling phenomenon is seen in the Bose channel. Since a hetero-nuclear Feshbach resonance, as well as the formation of Bose–Fermi molecules, have been realized, our results would be useful for the study of strong-coupling properties of this unique quantum gas.

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Normal-state properties of a unitary Bose–Fermi mixture: A combined strong-coupling approach with universal thermodynamics

Kharga D., Tajima H., Van Wyk P., Inotani D., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  86 （ 7 ） 074302(1-7) 2017年07月

最終著者, 査読有り,  ISSN  00319015

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©2017 The Physical Society of Japan. We theoretically investigate normal-state properties of a unitary Bose–Fermi mixture. Including strong hetero-pairing fluctuations, we evaluate the Bose and Fermi chemical potential, internal energy, pressure, entropy, as well as specific heat at constant volume C V , within the framework of a combined strong-coupling theory with exact thermodynamic identities. We show that hetero-pairing fluctuations at the unitarity cause non-monotonic temperature dependence of C V , being qualitatively different from the monotonic behavior of this quantity in the weak- and strong-coupling limit. On the other hand, such an anomalous behavior is not seen in the other quantities. Our results indicate that the specific heat C V , which has recently become observable in cold atom physics, is a useful quantity for understanding strong-coupling aspects of this quantum system.

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Zero-Temperature Properties of a Strongly Interacting Superfluid Fermi Gas in the BCS–BEC Crossover Region

Tajima H., van Wyk P., Hanai R., Kagamihara D., Inotani D., Horikoshi M., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  187 （ 5-6 ） 677 - 684 2017年06月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2016, Springer Science+Business Media New York. We investigate thermodynamic properties and effects of quantum fluctuations in the Bardeen–Cooper–Schrieffer (BCS)–Bose–Einstein condensation (BEC) crossover region of a superfluid Fermi gas in the low-temperature limit. Including strong-coupling corrections within the framework of an extended T-matrix approximation, we numerically compute the isothermal compressibility χn. While quantum fluctuation effects on χn in the strong-coupling BEC regime are explained by the quantum depletion due to a repulsive interaction between tightly bound molecules, effects of self-energy shift on the Fermi chemical potential are found to enhance χn in the weak-coupling BCS region. We also show that the calculated χn agrees well with the recent experiment on a 6Li Fermi gas done from the weak-coupling region to the unitarity limit. Our result would be useful for the study of many-body quantum corrections in the BCS–BEC crossover region of a strongly interacting Fermi superfluid.

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Single-Particle Properties of a Strongly Interacting Bose–Fermi Mixture Above the BEC Phase Transition Temperature

Kharga D., Inotani D., Hanai R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  187 （ 5-6 ） 661 - 667 2017年06月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2017, Springer Science+Business Media New York. We theoretically investigate the normal state properties of a Bose–Fermi mixture with a strong attractive interaction between Fermi and Bose atoms. We extend the ordinary T-matrix approximation (TMA) with respect to Bose–Fermi pairing fluctuations, to include the Hugenholtz–Pines’ relation for all Bose Green’s functions appearing in TMA self-energy diagrams. This extension is shown to be essentially important to correctly describe the physical properties of the Bose–Fermi mixture, especially near the Bose–Einstein condensation instability. Using this improved TMA, we clarify how the formation of composite fermions affects Bose and Fermi single-particle excitation spectra, over the entire interaction strength.

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Specific Heat of Ultracold Fermi Gas with a Uniaxially Anisotropic p-Wave Interaction at the Superfluid Transition Temperature

Inotani D., van Wyk P., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  187 （ 5-6 ） 685 - 691 2017年06月

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  00222291

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© 2016, Springer Science+Business Media New York. We theoretically investigate the specific heat at constant volume CV and strong-coupling effects in a Fermi gas with p-wave interaction. In a 40K Fermi gas, a uniaxial anisotropy of a p-wave interaction associated with a p-wave Feshbach resonance is expected as a result of the split of the p-wave Feshbach resonance by a dipole interaction. Including this, as well as pairing fluctuations, we show that CV is significantly affected by this anisotropy in the strong-coupling regime. We also discuss the physical origin of this effect. Our results would contribute to the further understanding of an ultracold p-wave Fermi gas.

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Zero-Temperature Properties of a Strongly Interacting Superfluid Fermi Gas in the BCS-BEC Crossover Region

H. Tajima, P. van Wyk, R. Hanai, D. Kagamihara, D. Inotani, M. Horikoshi, Y. Ohashi

J. Low Temp. Phys. 187   677 - 684 2017年06月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

Strong-Coupling Effects and Shear Viscosity in an Ultracold Fermi Gas

Kagamihara D., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  187 （ 5-6 ） 692 - 698 2017年06月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2017, Springer Science+Business Media New York. We theoretically investigate the shear viscosity η, as well as the entropy density s, in the normal state of an ultracold Fermi gas. Including pairing fluctuations within the framework of a T-matrix approximation, we calculate these quantities in the Bardeen–Cooper–Schrieffer (BCS)–Bose–Einstein condensation (BEC) crossover region. We also evaluate η/ s, to compare it with the lower bound of this ratio, conjectured by Kovtun, Son, and Starinets (KSS bound). In the weak-coupling BCS side, we show that the shear viscosity η is remarkably suppressed near the superfluid phase transition temperature T c , due to the so-called pseudogap phenomenon. In the strong-coupling BEC side, we find that, within the neglect of the vertex corrections, one cannot correctly describe η. We also show that η/ s decreases with increasing the interaction strength, to become very close to the KSS bound, ħ/ 4 πk B , on the BEC side.

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Pseudogap Phenomena Near the BKT Transition of a Two-Dimensional Ultracold Fermi Gas in the Crossover Region

Matsumoto M., Hanai R., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  187 （ 5-6 ） 668 - 676 2017年06月

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  00222291

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© 2016, Springer Science+Business Media New York. We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal phase. In the three-dimensional case, it has been shown that the so-called pseudogap phenomena can be well described by a (non-self-consistent) T-matrix approximation (TMA). In the two-dimensional case, while this strong-coupling theory can explain the pseudogap phenomenon in the strong-coupling regime, it unphysically gives large pseudogap size in the crossover region, as well as in the weak-coupling regime. We show that this difficulty can be overcome when one improves TMA to include higher-order pairing fluctuations within the framework of a self-consistent T-matrix approximation (SCTMA). The essence of this improvement is also explained. Since the observation of the BKT transition has recently been reported in a two-dimensional 6Li Fermi gas, our results would be useful for the study of strong-coupling physics associated with this quasi-long-range order.

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Local photoemission spectra and effects of spatial inhomogeneity in the BCS-BEC-crossover regime of a trapped ultracold Fermi gas

Ota M., Tajima H., Hanai R., Inotani D., Ohashi Y.

Physical Review A （Physical Review A）  95 （ 5 ） 053623(1-9) 2017年05月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2017 American Physical Society. We theoretically investigate single-particle excitations in the BCS-BEC-crossover regime of an ultracold Fermi gas. Including strong pairing fluctuations within a T-matrix approximation, as well as effects of a harmonic trap potential in the local-density approximation, we calculate the local photoemission spectrum in the normal state. Recently, the JILA group measured this quantity in a K40 Fermi gas in order to examine homogeneous single-particle properties of this system. Comparing our results with this experiment, we show that this attempt indeed succeeds under JILA's experimental condition. However, we also find that the current local photoemission spectroscopy still has room for improvement in order to examine the pseudogap phenomenon predicted in the BCS-BEC-crossover region. Since ultracold Fermi gases are always in a trap, our results would be useful in applying this system to various homogeneous Fermi systems as a quantum simulator.

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Appropriate conditions to realize a p -wave superfluid state starting from a spin-orbit-coupled s -wave superfluid Fermi gas

Yamaguchi T., Inotani D., Ohashi Y.

Physical Review A （Physical Review A）  95 （ 5 ） 053609(1-12) 2017年05月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2017 American Physical Society. We theoretically investigate a spin-orbit-coupled s-wave superfluid Fermi gas, to examine the time evolution of the system, after an s-wave pairing interaction is replaced by a p-wave one at t=0. In our recent paper [T. Yamaguchi, D. Inotani, and Y. Ohashi, J. Phys. Soc. Jpn. 86, 013001 (2017)JUPSAU0031-901510.7566/JPSJ.86.013001], we proposed that this manipulation may realize a p-wave superfluid Fermi gas because the p-wave pair amplitude that is induced in the s-wave superfluid state by a parity-broken antisymmetric spin-orbit interaction gives a nonvanishing p-wave superfluid order parameter, immediately after the p-wave interaction is turned on. In this paper, using a time-dependent Bogoliubov-de Gennes theory, we assess this idea under various conditions with respect to the s-wave and p-wave interaction strengths, as well as the spin-orbit coupling strength. From these, we clarify that the momentum distribution of Fermi atoms in the initial s-wave state (t<0) is a key to produce a large p-wave superfluid order parameter. Since the realization of a p-wave superfluid state is one of the most exciting and difficult challenges in cold Fermi gas physics, our results may provide a possible way to accomplish this.

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Specific heat and effects of uniaxial anisotropy of a p-wave pairing interaction in a strongly interacting ultracold fermi gas

Inotani D., Wyk P., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  86 （ 4 ）  2017年04月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the specific heat C at constant volume and effects of uniaxial anisotropy of a p-wave attractive interaction in the normal state of an ultracold Fermi gas. Within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we evaluate this thermodynamic quantity as a function of temperature, in the whole interaction regime. While the uniaxial anisotropy is not crucial for C in the weak-coupling regime, C is found to be sensitive to the uniaxial anisotropy in the strong-coupling regime. This originates from the population imbalance among pi-wave molecules (i = x; y; z), indicating that the specific heat is a useful observable to see which kinds of p-wave molecules dominantly exist in the strong-coupling regime when the p-wave interaction has uniaxial anisotropy. Using this strong point, we classify the strong-coupling regime into some characteristic regions. Since a p-wave pairing interaction with uniaxial anisotropy has been discovered in a K Fermi gas, our results would be useful in considering strong-coupling properties of a p-wave interacting Fermi gas, when the interaction is uniaxially anisotropic. V V V 40

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Strong-coupling corrections to ground-state properties of a superfluid Fermi gas

Tajima H., Van Wyk P., Hanai R., Kagamihara D., Inotani D., Horikoshi M., Ohashi Y.

Physical Review A （Physical Review A）  95 （ 4 ） 043625(1-5) 2017年04月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2017 American Physical Society. We theoretically present an economical and convenient way to study ground-state properties of a strongly interacting superfluid Fermi gas. Our strategy is that complicated strong-coupling calculations are used only to evaluate quantum fluctuation corrections to the chemical potential μ. Then, without any further strong-coupling calculations, we calculate the compressibility, sound velocity, internal energy, pressure, and Tan's contact, from the calculated μ without loss of accuracy, by using exact thermodynamic identities. Using a recent precise measurement of μ in a superfluid Li6 Fermi gas, we show that an extended T-matrix approximation (ETMA) is suitable for our purpose, especially in the BCS-unitary regime, where our results indicate that many-body corrections are dominated by superfluid fluctuations. Since precise determinations of physical quantities are not always easy in cold Fermi gas physics, our approach would greatly reduce experimental and theoretical efforts toward the understanding of ground-state properties of this strongly interacting Fermi system.

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Strong-coupling properties of a p-wave interacting fermi gas on the viewpoint of specific heat at constant volume

Inotani D., Van Wyk P., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  86 （ 2 ） 024302(1-6) 2017年02月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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©2017 The Physical Society of Japan. We theoretically investigate the specific heat CV at constant volume in the normal state of a p-wave interacting Fermi gas. Including fluctuations in the p-wave Cooper channel within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we clarify how CV as a function of temperature varies, as one moves from the weak-coupling regime to the strong-coupling limit. In the weak-coupling regime, CV is shown to be enhanced by p-wave pairing fluctuations, near the superfluid phase transition temperature Tc. Similar enhancement of CV(T ≃ Tc) is also obtained in the strong-coupling regime, which, however, reflects that system is close an ideal Bose gas of p-wave two-body bound molecules. Using these results, we classify the normal state into (1) the normal Fermi gas regime, (2) the p-wave molecular Bose gas regime, and (3) the region between the two, where p-wave pairing fluctuations are dominant. Since the current experiments can only access the normal phase of a p-wave interacting Fermi gas, our results would be useful for experiments to understand strong-coupling properties of this Fermi system above Tc.

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A novel route to reach a p-wave Superfluid Fermi gas

Yamaguchi T., Inotani D., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  86 （ 1 ） 013001(1-5) 2017年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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© 2017 The Physical Society of Japan. We theoretically propose an idea to realize a p-wave superfluid Fermi gas. To overcome the experimental difficulty that a p-wave pairing interaction to form p-wave Cooper pairs damages the system before the condensation growth, we first prepare a p-wave pair amplitude (Φ p ) in a spin-orbit coupled s-wave superfluid Fermi gas, without any p-wave interaction. Then, by suddenly changing the s-wave interaction with a p-wave one (U p ) by using a Feshbach resonance, we reach the p-wave superfluid phase with the p-wave superfluid order parameter being symbolically written as Δ p ∼ U p Φ p . In this letter, we assess this scenario within the framework of a time-dependent Bogoliubov-de Gennes theory. Our results would contribute to the study toward the realization of unconventional pairing states in an ultracold Fermi gas.

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Dip-hump temperature dependence of specific heat and effects of pairing fluctuations in the weak-coupling side of a P-wave interacting fermi gas

Inotani D., Van Wyk P., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  85 （ 12 ） 123301(1-5) 2016年12月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

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©2016 The Physical Society of Japan. We investigate the specific heat CV at constant volume in the normal state of a p-wave interacting Fermi gas. Including p-wave pairing fluctuations within the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that, in the weak-coupling side, CV exhibits a dip-hump behavior as a function of the temperature. While the dip is associated with the pseudogap phenomenon near Tc, the hump structure is found to come from the suppression of Fermi quasiparticle scattering into a p-wave molecular state in the Fermi degenerate regime. Since the latter phenomenon does not occur in the ordinary s-wave interacting Fermi gas, it may be viewed as a characteristic phenomenon associated with a p-wave pairing interaction.

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Triplet pair amplitude in a trapped s-wave superfluid Fermi gas with broken spin rotation symmetry. II. Three-dimensional continuum case

Inotani D., Hanai R., Ohashi Y.

Physical Review A （Physical Review A）  94 （ 4 ） 043632(1-9) 2016年10月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2016 American Physical Society. We extend our recent work [Y. Endo, Phys. Rev. A 92, 023610 (2015)]PLRAAN1050-294710.1103/PhysRevA.92.023610 for a parity-mixing effect in a model of two-dimensional lattice fermions to a realistic three-dimensional ultracold Fermi gas. Including effects of broken local spatial inversion symmetry by a trap potential within the framework of the real-space Bogoliubov-de Gennes theory at T=0, we point out that an odd-parity p-wave Cooper-pair amplitude is expected to have already been realized in previous experiments on an (even-parity) s-wave superfluid Fermi gas with spin imbalance. This indicates that when one suddenly changes the s-wave pairing interaction to an appropriate p-wave one by using a Feshbach technique in this case, a nonvanishing p-wave superfluid order parameter is immediately obtained, which is given by the product of the p-wave interaction and the p-wave pair amplitude that has already been induced in the spin-imbalanced s-wave superfluid Fermi gas. Thus, by definition, the system is in the p-wave superfluid state, at least just after this manipulation. Since the achievement of a p-wave superfluid state is one of the most exciting challenges in cold Fermi gas physics, our results may provide an alternative approach to this unconventional pairing state. In addition, since the parity-mixing effect cannot be explained as far as one deals with a trap potential in the local density approximation (LDA), it is considered as a crucial example which requires us to go beyond the LDA.

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Rashbon Bound States Associated with a Spherical Spin–Orbit Coupling in an Ultracold Fermi Gas with an s-Wave Interaction

Yamaguchi T., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  183 （ 3-4 ） 161 - 168 2016年05月

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  00222291

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© 2016, Springer Science+Business Media New York. We investigate the formation of rashbon bound states and strong-coupling effects in an ultracold Fermi gas with a spherical spin–orbit interaction, (Formula presented.) (where (Formula presented.) are Pauli matrices). Extending the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR) to include this spin–orbit coupling, we determine the superfluid phase transition temperature (Formula presented.) , as functions of the strength of a pairing interaction (Formula presented.) , as well as the spin–orbit coupling strength (Formula presented.). Evaluating poles of the NSR particle–particle scattering matrix describing fluctuations in the Cooper channel, we clarify the region where rashbon bound states dominate the superfluid phase transition in the (Formula presented.) – (Formula presented.) phase diagram. Since the antisymmetric spin–orbit interaction (Formula presented.) breaks the inversion symmetry of the system, rashbon bound states naturally have not only a spin-singlet and even-parity symmetry, but also a spin-triplet and odd-parity symmetry. Thus, our results would be also useful for the study of this parity-mixing effect in the BCS–BEC crossover regime of a spin–orbit coupled Fermi gas.

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Momentum Distribution of Cooper Pairs and Strong-Coupling Effects in a Two-Dimensional Fermi Gas Near the Berezinskii–Kosterlitz–Thouless Transition

Matsumoto M., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  183 （ 3-4 ） 199 - 207 2016年05月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2015, Springer Science+Business Media New York. We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal state. Including pairing fluctuations within the framework of a T-matrix approximation, we calculate the distribution function (Formula presented.) of Cooper pairs in terms of the center of mass momentum (Formula presented.). In the strong-coupling regime, (Formula presented.) is shown to exhibit a remarkable increase with decreasing the temperature in the low temperature region, which agrees well with the recent experiment on a two-dimensional (Formula presented.) Li Fermi gas (Ries et al. in Phys Rev Lett 114:230401, 2015). Our result indicates that the observed remarkable increase of the number of Cooper pairs with zero center of mass momentum can be explained without assuming the Berezinskii–Kosterlitz–Thouless (BKT) transition, when one properly includes pairing fluctuations that are enhanced by the low-dimensionality of the system. Since the BKT transition is a crucial topic in two-dimensional Fermi systems, our results would be useful for the study toward the realization of this quasi-long-range order in an ultracold Fermi gas.

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Non-equilibrium Properties of a Pumped-Decaying Bose-Condensed Electron–Hole Gas in the BCS–BEC Crossover Region

Hanai R., Littlewood P., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  183 （ 3-4 ） 127 - 135 2016年05月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2016, Springer Science+Business Media New York. We theoretically investigate a Bose-condensed exciton gas out of equilibrium. Within the framework of the combined BCS-Leggett strong-coupling theory with the non-equilibrium Keldysh formalism, we show how the Bose–Einstein condensation (BEC) of excitons is suppressed to eventually disappear, when the system is in the non-equilibrium steady state. The supply of electrons and holes from the bath is shown to induce quasi-particle excitations, leading to the partial occupation of the upper branch of Bogoliubov single-particle excitation spectrum. We also discuss how this quasi-particle induction is related to the suppression of exciton BEC, as well as the stability of the steady state.

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Spin Susceptibility and Effects of Inhomogeneous Strong Pairing Fluctuations in a Trapped Ultracold Fermi Gas

Tajima H., Hanai R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  183 （ 3-4 ） 175 - 182 2016年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2015, Springer Science+Business Media New York. We theoretically investigate magnetic properties of a unitary Fermi gas in a harmonic trap. Including strong pairing fluctuations within the framework of an extended T-matrix approximation, as well as effects of a trap potential within the local density approximation, we calculate the local spin susceptibility (Formula presented.) above the superfluid phase transition temperature (Formula presented.). We show that the formation of preformed singlet Cooper pairs anomalously suppresses (Formula presented.) in the trap center near (Formula presented.). We also point out that, in the unitarity limit, the spin-gap temperature in a uniform Fermi gas can be evaluated from the observation of the spatial variation of (Formula presented.). Since a real ultracold Fermi gas is always in a trap potential, our results would be useful for the study of how this spatial inhomogeneity affects thermodynamic properties of an ultracold Fermi gas in the BCS–BEC crossover region.

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Non-equilibrium properties of a pumped-decaying Bose-condensed electron-hole gas in the BCS-BEC crossover region

R. Hanai, P. Littlewood, and Y. Ohashi

J. Low Temp. Phys. 183   127 - 135 2016年04月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

Momentum distribution of Cooper-pairs and strong-coupling effects in a two-dimensional Fermi gas near the Berezinskii-Kosterlitz-Thouless transition

M. Matsumoto, D. Inotani, and Y. Ohashi

J. Low Temp. Phys. 183   199 - 207 2016年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

Strong Coupling Effects on the Specific Heat of an Ultracold Fermi Gas in the Unitarity Limit

van Wyk P., Tajima H., Hanai R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  183 （ 3-4 ） 183 - 190 2016年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

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© 2015, Springer Science+Business Media New York. We investigate strong-coupling corrections to the specific heat (Formula presented.) in the normal state of an ultracold Fermi gas in the BCS–BEC crossover region. A recent experiment on a (Formula presented.) Li unitary Fermi gas (Ku et. al. in Science 335:563 2012) shows that (Formula presented.) is remarkably amplified near the superfluid phase transition temperature (Formula presented.) , being similar to the well-known (Formula presented.) -structure observed in liquid (Formula presented.) He. Including pairing fluctuations within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that strong pairing fluctuations are sufficient to explain the anomalous behavior of (Formula presented.) observed in a (Formula presented.) Li unitary Fermi gas near (Formula presented.). We also show that there is no contribution from stable preformed Cooper pairs to (Formula presented.) at the unitarity. This indicates that the origin of the observed anomaly is fundamentally different from the case of liquid (Formula presented.) He, where stable (Formula presented.) He Bose atoms induce the (Formula presented.) -structure in (Formula presented.) near the superfluid instability. Instead, the origin is the suppression of the entropy S, near (Formula presented.) , due to the increase of metastable preformed Cooper pairs. Our results indicate that the specific heat is a useful quantity to study the effects of pairing fluctuations on the thermodynamic properties of an ultracold Fermi gas in the BCS–BEC crossover region.

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Specific heat and effects of pairing fluctuations in the BCS-BEC-crossover regime of an ultracold Fermi gas

Van Wyk P., Tajima H., Hanai R., Ohashi Y.

Physical Review A （Physical Review A）  93 （ 1 ） 013621(1-9) 2016年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2016 American Physical Society. We investigate the specific heat at constant volume CV in the Bardeen-Cooper-Schrieffer-Bose-Einstein-condensate (BCS-BEC)-crossover regime of an ultracold Fermi gas above the superfluid phase transition temperature Tc. Within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that this thermodynamic quantity is sensitive to the stability of preformed Cooper pairs. That is, while CV(TTc) in the unitary regime is remarkably enhanced by metastable preformed Cooper pairs or pairing fluctuations, it is well described by that of an ideal Bose gas of long-lived stable molecules in the strong-coupling BEC regime. Using these results, we identify the region where the system may be viewed as an almost ideal Bose gas of stable pairs, as well as the pseudogap regime where the system is dominated by metastable preformed Cooper pairs, in the phase diagram of an ultracold Fermi gas with respect to the strength of a pairing interaction and the temperature. We also show that the calculated specific heat agrees with the recent experiment on a Li6 unitary Fermi gas. Since the formation of preformed Cooper pairs is a crucial key in the BCS-BEC-crossover phenomenon, our results would be helpful in considering how fluctuating preformed Cooper pairs appear in a Fermi gas to eventually become stable as one passes through the BCS-BEC-crossover region.

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Strong-coupling corrections to spin susceptibility in the BCS-BEC-crossover regime of a superfluid Fermi gas

Tajima H., Hanai R., Ohashi Y.

Physical Review A （Physical Review A）  93 （ 1 ） 013610(1-11) 2016年01月

研究論文（学術雑誌）, 共著,  ISSN  24699926

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© 2016 American Physical Society. We theoretically investigate the uniform spin susceptibility χ in the superfluid phase of an ultracold Fermi gas in the region of the Bardeen-Cooper-Schrieffer-Bose-Einstein-condensate (BCS-BEC) crossover. In our previous paper [H. Tajima, Phys. Rev. A 89, 033617 (2014)PLRAAN1050-294710.1103/PhysRevA.89.033617], including pairing fluctuations within an extended T-matrix approximation (ETMA), we showed that strong pairing fluctuations cause the so-called spin-gap phenomenon, where χ is anomalously suppressed even in the normal state near the superfluid phase transition temperature Tc. In this paper, we extend this work to the superfluid phase below Tc, to clarify how this many-body phenomenon is affected by the superfluid order. From the comparison of the ETMA χ with the Yosida function describing the spin susceptibility in a weak-coupling BCS superfluid, we identify the region where pairing fluctuations crucially affect this magnetic quantity below Tc in the phase diagram with respect to the strength of a pairing interaction and the temperature. This spin-gap regime is found to be consistent with the previous pseudogap regime determined from the pseudogapped density of states. We also compare our results with a recent experiment on a Li6 Fermi gas. Since the spin susceptibility is sensitive to the formation of spin-singlet preformed pairs, our results would be useful for the study of pseudogap physics in an ultracold Fermi gas on the viewpoint of the spin degrees of freedom.

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Cooper pairs with zero center-of-mass momentum and their first-order correlation function in a two-dimensional ultracold Fermi gas near a Berezinskii-Kosterlitz-Thouless transition

Matsumoto M., Inotani D., Ohashi Y.

Physical Review A （Physical Review A）  93 （ 1 ） 013619(1-10) 2016年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  24699926

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© 2016 American Physical Society. We investigate the center-of-mass momentum distribution nQ of Cooper pairs and their first-order correlation function g1(r) in a strongly interacting two-dimensional Fermi gas. Recently, the Berezinskii-Kosterlitz-Thouless (BKT) transition was reported in a two-dimensional Li6 Fermi gas, based on (1) the observations of anomalous enhancement of nQ=0 [Ries, Phys. Rev. Lett. 114, 230401 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.230401], as well as (2) a power-law behavior of g1(r) [Murthy, Phys. Rev. Lett. 115, 010401 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.010401]. However, including pairing fluctuations within a T-matrix approximation (TMA), we show that these results can still be explained as strong-coupling properties of a normal-state two-dimensional Fermi gas. Our results indicate the importance of further experimental observations, to definitely confirm the realization of the BKT transition in this system. Since the BKT transition has been realized in a two-dimensional ultracold Bose gas, our results would be useful for the achievement of this quasi-long-range order in an ultracold Fermi gas.

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Specific heat and effects of pairing fluctuations in the BCS-BEC crossover regime of an ultracold Femi gas

Pieter van Wyk, Hiroyuki Tajima, Ryo Hanai, Yoji Ohashi

Physical Review A 93   013621(1-9) 2016年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

Strong-coupling corrections to spin susceptibility in the BCS-BEC crossover regime of a superfluid Fermi gas

Hiroyuki Tajima, Ryo Hanai, and Yoji Ohashi

Physical Review A 93   063610(1-11) 2016年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

Pairing fluctuations and an anisotropic pseudogap phenomenon in an ultracold superfluid Fermi gas with plural p-wave superfluid phases

Inotani D., Ohashi Y.

Physical Review A （Physical Review A）  92 （ 6 ） 063638(1-13) 2015年12月

研究論文（学術雑誌）, 共著, 最終著者,  ISSN  24699926

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© 2015 American Physical Society. We investigate the superfluid properties of a one-component Fermi gas with a uniaxially anisotropic p-wave pairing interaction, Ux>Uy=Uz [where Ui(i=x,y,z) is a pi-wave pairing interaction]. This type of interaction is considered to be realized in a K40 Fermi gas. Including pairing fluctuations within a strong-coupling T-matrix theory, we determine the px-wave superfluid phase transition temperature Tcpx, as well as the other phase transition temperature Tcpx+ipy(<Tcpx), below which the superfluid order parameter has the px+ipy-wave symmetry. In the normal state near Tcpx, px-wave pairing fluctuations are shown to induce an anisotropic pseudogap phenomenon, where a dip structure in the angle-resolved density of states around ω=0 is the most remarkable in the px direction. In the px-wave superfluid phase (Tcpx+ipy<T≤Tcpx), while the pseudogap in the px direction continuously changes to the superfluid gap, the pseudogap in the perpendicular direction to the px axis is found to continue developing because of enhanced py-wave and pz-wave pairing fluctuations around the node of the px-wave superfluid order parameter. Since pairing fluctuations are always suppressed in the isotropic s-wave superfluid state, this phenomenon is peculiar to an unconventional Fermi superfluid with a nodal superfluid order parameter. Since the p-wave Fermi superfluid is the most promising non-s-wave pairing state in an ultracold Fermi gas, our results would contribute to understanding how the anisotropic pairing fluctuations, as well as the existence of plural superfluid phases, affect many-body properties of this unconventional Fermi superfluid.

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Triplet pair amplitude in a trapped s -wave superfluid Fermi gas with broken spin rotation symmetry

Endo Y., Inotani D., Hanai R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  92 （ 2 ） 023610(1-11) 2015年10月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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© 2015 American Physical Society. We investigate the possibility that the broken spatial inversion symmetry caused by a trap potential induces a spin-triplet Cooper pair amplitude in an s-wave superfluid Fermi gas. Based on symmetry considerations, we clarify that this phenomenon may occur, when a spin rotation symmetry of the system is also broken. We also numerically confirm that a triplet pair amplitude is actually induced under this condition, using a simple model. Our results imply that this phenomenon is already present in a trapped s-wave superfluid Fermi gas with spin imbalance. As an interesting application of this phenomenon, we point out that one may produce a p-wave superfluid Fermi gas by suddenly changing the s-wave pairing interaction to a p-wave one by using the Feshbach resonance technique. Since a Cooper pair is usually classified into a spin-singlet (and even-parity) state and a spin-triplet (and odd-parity) state, our results would be useful in considering how to mix them with each other in a superfluid Fermi gas. Such an admixture has recently attracted much attention in the field of noncentrosymmetric superconductivity, so that our results would also contribute to the further development of this research field, from the viewpoint of cold Fermi gas physics.

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Proposed method to realize the p -wave superfluid state using an s -wave superfluid Fermi gas with a synthetic spin-orbit interaction

Yamaguchi T., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  92 （ 1 ） 013615(1-12)  2015年07月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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© 2015 American Physical Society. We propose a theoretical idea to reach the p-wave superfluid phase in an ultracold Fermi gas. The key of our idea is that the pairing symmetry of a Fermi superfluid is fully dominated by the symmetry of the superfluid order parameter, which is essentially given by the product of a pair amplitude and a pairing interaction. Noting this, in our proposal, we first prepare a p-wave pair amplitude not by using a p-wave interaction, but by using the phenomenon that a p-wave pair amplitude is induced in an s-wave superfluid Fermi gas with antisymmetric spin-orbit interaction. In this case, although the system is still in the s-wave superfluid state with the s-wave superfluid order parameter, when one suddenly replaces the s-wave interaction by an appropriate p-wave one (which is possible in cold Fermi gases by using a Feshbach resonance technique), the product of the p-wave interaction and the p-wave pair amplitude that has already been prepared in the spin-orbit-coupled s-wave superfluid state immediately gives a finite p-wave superfluid order parameter. Thus, at least just after this manipulation, the system is in the p-wave superfluid state, being characterized by the artificially produced p-wave superfluid parameter. In this paper, to assess our idea, we evaluate the p-wave pair amplitude in a spin-orbit-coupled s-wave superfluid Fermi gas at T=0. We determine the region where a large p-wave pair amplitude is obtained in the phase diagram with respect to the strengths of the s-wave pairing interaction and the spin-orbit coupling. We also discuss the accessibility of this optimal region on the viewpoint of the superfluid phase transition temperature. Since the achievement of a p-wave superfluid Fermi gas is one of the most crucial issues in cold-atom physics, our proposal would be useful for this exciting challenge.

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Pseudogap phenomena in a two-dimensional ultracold Fermi gas near the Berezinskii-Kosterlitz-Thouless transition

Matsumoto M., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  568   012012(1-5) 2014年12月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

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© Published under licence by IOP Publishing Ltd. We investigate single-particle excitations and strong-coupling effects in a twodimensional Fermi gas. Including pairing fluctuations within a Gaussian fluctuation theory, we calculate the density of states ρ(ω) near the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature TBKT. Near TBKT, we show that superfluid fluctuations induce a pseudogap in ρ(ω). The pseudogap structure is very similar to the BCS superfluid density of states, although the superfluid order parameter is absent in the present two-dimensional case. Since a twodimensional 40K Fermi gas has recently been realized, our results would contribute to the understanding of single-particle properties near the BKT instability.

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Spin-gap phenomenon in a strongly interacting ultracold Fermi gas

Tajima H., Hanai R., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  568   012019(1-5) 2014年12月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

© Published under licence by IOP Publishing Ltd. We investigate magnetic properties of a strongly interacting ultracold Fermi gas. Within the framework of an extended T-matrix approximation, we calculate the spin susceptibility χ in the unitarity limit. We show that effects of pairing fluctuations on this magnetic quantity are quite different in between the normal state and the superfluid phase. In the normal state, pairing fluctuations cause spin-gap phenomenon near the superfluid phase transition temperature Tc, where χ is anomalously suppressed. In the superfluid phase, on the other hand, the ordinary suppression of χ by the BCS energy gap is weakened by pairing fluctuations, because they induce finite density of states inside the gap. Our results indicate that the spin susceptibility is a useful quantity for the study of pairing fluctuations in the BCS-BEC crossover regime of an ultracold Fermi gas.

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Hetero-pairing and pseudogap phenomena in an ultracold Fermi gas with mass imbalance

Hanai R., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  568   012004(1-7) 2014年12月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

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© Published under licence by IOP Publishing Ltd. We investigate strong-coupling properties of a unitary Fermi gas consisting of two different species with different masses. Including pairing fluctuations within the self-consistent T-matrix approximation, we calculate the single-particle density of states in the normal state. We show that the pseudogap phenomenon, which is characterized by a dip structure in the density of states around ω 0, appears more remarkably in the light mass component than in the heavy mass component. As a result, the pseudogap temperature, which is determined as the temperature at which the pseudogap disappears in the density of states, is higher in the former than in the latter. We also find that this different pseudogap temperatures lead to the existence of two kinds of pseudogap regions. That is, one is the ordinary pseudogap regime where the pseudogap appears in both the component, and the other case is that the light mass component only exhibits the pseudogap phenomenon. As the origin of these component- dependent pseudogap phenomena, we point out the importance of different Fermi temperatures between the two components. Since the formation of hetero-Cooper pairs and their condensation are expected in various systems, such as a 6Li-40K Fermi gas mixture, an exciton (polariton) gas, as well as color superconductivity, our results would be useful for the understanding of strong-coupling properties of these novel Fermi condensates.

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Heteropairing and component-dependent pseudogap phenomena in an ultracold Fermi gas with different species with different masses

Hanai R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  90 （ 4 ） 043622(1-11) 2014年10月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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© 2014 American Physical Society. We investigate the superfluid phase transition and single-particle excitations in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an unpolarized ultracold Fermi gas with mass imbalance (where two different species contributing to Cooper pairs have different atomic masses). In our recent paper [R. Hanai, Phys. Rev. A 88, 053621 (2013)PLRAAN1050-294710.1103/PhysRevA.88.053621], we showed that an extended T-matrix approximation (ETMA) can overcome the serious problem known in the ordinary (non-self-consistent) T-matrix approximation that it unphysically gives double-valued superfluid phase transition temperature Tc in the presence of mass imbalance. However, at the same time, the ETMA was also found to give the vanishing Tc in the weak-coupling and highly mass-imbalanced case. In this paper, we inspect the correctness of this ETMA result, using the self-consistent T-matrix approximation (SCTMA). We show that the vanishing Tc is an artifact of the ETMA, coming from an internal inconsistency of this theory. The superfluid phase transition actually always occurs, irrespective of the ratio of mass imbalance. We also apply the SCTMA to the pseudogap problem in a mass-imbalanced Fermi gas. We show that pairing fluctuations induce different pseudogap phenomena between the light component and heavy component. We also point out that a Li6-K40 mixture is a useful system for the realization of a heteropairing state, as well as for the study of component-dependent pseudogap phenomena.

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Green's-function formalism for a condensed Bose gas consistent with infrared-divergent longitudinal susceptibility and Nepomnyashchii-Nepomnyashchii identity

Watabe S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  90 （ 1 ） 013603(1-12) 2014年07月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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We present a Green's-function formalism for an interacting Bose-Einstein condensate (BEC) satisfying the two required conditions: (i) the infrared-divergent longitudinal susceptibility with respect to the BEC order parameter, and (ii) the Nepomnyashchii-Nepomnyashchii identity stating the vanishing off-diagonal self-energy in the low-energy and low-momentum limit. These conditions cannot be described by the ordinary mean-field Bogoliubov theory, the many-body T-matrix theory, or the random-phase approximation with the vertex correction. In this paper, we show that these required conditions can be satisfied, when we divide many-body corrections into singular and nonsingular parts, and separately treat them as different self-energy corrections. The resulting Green's function may be viewed as an extension of the Popov's hydrodynamic theory to the region at finite temperatures. Our results would be useful in constructing a consistent theory of BECs satisfying various required conditions, beyond the mean-field level. © 2014 American Physical Society.

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Self-consistent T-matrix approach to an interacting ultracold fermi gas with mass imbalance

Hanai R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  175 （ 1-2 ） 272-278 - 278 2014年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We investigate the superfluid phase transition in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas with mass imbalance. In the presence of mass imbalance, it is known that the strong-coupling Gaussian fluctuation theory, as well as the ordinary non-self-consistent T-matrix theory, that have been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically give double valued superfluid phase transition temperature T c in the crossover region. In our previous paper (R. Hanai et al., J. Low Temp. Phys. 171:389, 2013), this difficulty was shown to be eliminated by an extended T-matrix approximation (ETMA). However, it was also found that this improved theory still gives vanishing T c in the BCS regime, when the mass imbalance ratio remarkably deviates from unity. In this paper, further extending ETMA to include higher order pairing fluctuations to the fully self-consistent T-matrix level, we clarify whether the vanishing T c obtained in ETMA is an intrinsic phenomenon or an artifact of this approximation. We show that the self-consistent T-matrix theory always gives a finite T c, even in the region where ETMA predicts the absence of superfluid instability. The key to the recovery of T c is found to be a consistent treatment with respect to the Fermi surface sizes of the light mass and heavy mass components, which ETMA lacks in. Since Fermi condensates with mass imbalance have been recently discussed in various systems, such as a 40K-6Li Fermi gas, exciton-polariton condensate, as well as color superconductivity, our results would be useful in constructing a reliable strong-coupling theory to examine physical properties of these novel Fermi superfluids. © 2013 Springer Science+Business Media New York.

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Spin susceptibility and strong coupling effects in an ultracold Fermi gas

Tajima H., Hanai R., Watanabe R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  175 （ 1-2 ） 279-286 - 286 2014年04月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We investigate magnetic properties and strong coupling corrections in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Within the framework of an extended T-matrix theory, we calculate the spin susceptibility χ above the superfluid phase transition temperature Tc. In the crossover region, the formation of preformed Cooper pairs is shown to cause a non-monotonic temperature dependence of χ, which is similar to the so-called spin-gap phenomenon observed in the under-doped regime of high-T c cuprates. From this behavior of χ, we determine the spin-gap temperature as the temperature at which χ takes a maximum value, in the BCS-BEC crossover region. Since the spin susceptibility is sensitive to the formation of singlet Cooper pairs, our results would be useful in considering the temperature region where pairing fluctuations are important in the BCS-BEC crossover regime of an ultracold Fermi gas. © 2013 Springer Science+Business Media New York.

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Uniform spin susceptibility and spin-gap phenomenon in the BCS-BEC-crossover regime of an ultracold Fermi gas

Tajima H., Kashimura T., Hanai R., Watanabe R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  89 （ 3 ） 033167(1-9) 2014年03月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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We investigate the uniform spin susceptibility χs in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Including pairing fluctuations within the framework of an extended T-matrix approximation, we show that χs exhibits nonmonotonic temperature dependence in the normal state. In particular, χs is suppressed near the superfluid phase transition temperature Tc due to strong pairing fluctuations. To characterize this anomalous behavior, we introduce the spin-gap temperature Ts as the temperature at which χs takes a maximum value. Determining Ts in the whole BCS-BEC crossover region, we identify the spin-gap regime in the phase diagram of a Fermi gas in terms of the temperature and the strength of a pairing interaction. We also clarify how the spin-gap phenomenon is related to the pseudogap phenomenon appearing in the single-particle density of states. Our results indicate that an ultracold Fermi gas in the BCS-BEC crossover region is a very useful system to examine the pseudogap phenomenon and the spin-gap phenomenon in a unified manner. © 2014 American Physical Society.

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Strong-Coupling Effects and Single-Particle Properties in an Ultracold Fermi Gas with Mass Imbalance

R. Hanai, and Y. Ohashi

JPS Conference Proceedings 1   012094(1-4) 2014年03月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

Spin-gap temperature in the BCS-BEC crossover regime of an ultracold Fermi gas

H. Tajima, T. Kashimura, R. Watanabe, and Y. Ohashi

JPS Conference Proceedings 1   012096(1-5) 2014年03月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

Unconventional triplet pairing and effects of Fermi surface deformation in a gas of polarized dipolar Fermi molecules

Y. Endo, D. Inotani, and Y. Ohashi

JPS Conference Proceedings 1   012095(1-4) 2014年03月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

Pseudogap phenomenon and effects of population imbalance in the normal state of a unitary Fermi gas

Kashimura T., Watanabe R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  89 （ 1 ） 013618(1-7) 2014年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate strong-coupling corrections to single-particle excitations in the normal state of a spin-polarized unitary Fermi gas. Within the framework of an extended T-matrix approximation, we calculate the single-particle density of states, as well as the single-particle spectral weight, to show that the so-called pseudogap phenomenon gradually disappears with increasing magnitude of an effective magnetic field. In the highly spin-polarized regime, the calculated spin polarization as a function of the effective magnetic field agrees well with the recent experiment on a 6Li Fermi gas. Although this experiment has been considered to be incompatible with the existence of the pseudogap in an unpolarized Fermi gas, our result clarifies that the observed spin polarization in the highly spin-polarized regime and the pseudogap in the unpolarized limit can be explained in a consistent manner when one correctly includes effects of population imbalance on single-particle excitations. Since it is a crucial issue to clarify whether the pseudogap exists or not in the BCS-BEC crossover regime of an ultracold Fermi gas, our results would be useful for the understanding of this strongly interacting fermion system. © 2014 American Physical Society.

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Comparative studies of many-body corrections to an interacting Bose-Einstein condensate

Watabe S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  88 （ 5 ） 053633(1-9) 2013年11月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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We compare many-body theories describing fluctuation corrections to the mean-field theory in a weakly interacting Bose-condensed gas. Using a generalized random-phase approximation, we include both density fluctuations and fluctuations in the particle-particle scattering channel in a consistent manner. We also separately examine effects of the fluctuations within the framework of the random-phase approximation. Effects of fluctuations in the particle-particle scattering channel are also separately examined by using the many-body T-matrix approximation. We assess these approximations with respect to the transition temperature Tc, the order of phase transition, as well as the so-called Nepomnyashchii-Nepomnyashchii identity, which states the vanishing off-diagonal self-energy in the low-energy and low-momentum limit. Since the construction of a consistent theory for interacting bosons which satisfies various required conditions is a long-standing problem in cold atom physics, our results would be useful for this important challenge. © 2013 American Physical Society.

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Excitation properties and effects of mass imbalance in the BCS-BEC crossover regime of an ultracold Fermi gas

Hanai R., Kashimura T., Watanabe R., Inotani D., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  88 （ 5 ） 053621(1-12) 2013年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate single-particle properties of a mass-imbalanced Fermi gas in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. In the presence of mass imbalance, we point out that the ordinary T-matrix approximation, which has been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically gives a double-valued solution in terms of the superfluid phase transition temperature Tc in the crossover region. To overcome this serious problem, we include higher order strong-coupling corrections beyond the T-matrix level. Using this extended T-matrix theory, we calculate single-particle excitations in the normal state above Tc. The so-called pseudogap phenomena originating from pairing fluctuations are shown to be different between the light-mass component and heavy-mass component, which becomes more remarkable at higher temperatures. Since Fermi condensates with hetero-Cooper pairs have recently been discussed in various fields, such as exciton (polariton) condensates as well as color superconductivity, our results would be useful for the further development of Fermi superfluid physics, beyond the conventional superfluid state with homo-Cooper pairs. © 2013 American Physical Society.

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Superfluid properties of one-component fermi gas with an anisotropic p-wave interaction

Inotani D., Sigrist M., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  171 （ 3-4 ） 376-381 - 381 2013年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We investigate superfluid properties and strong-coupling effects in a onecomponent Fermi gas with an anisotropic p-wave interaction. Within the framework of the Gaussian fluctuation theory, we determine the superfluid transition temperature Tc, as well as the temperature T0 at which the phase transition from the px-wave pairing state to the px + ipy-wave state occurs below Tc. We also show that while the anisotropy of the p-wave interaction enhances Tc in the strong-coupling regime, it suppresses T0. © Springer Science+Business Media, LLC 2012.

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Two-dimensional pseudogap effects of an ultracold fermi gas in the BCS-BEC crossover region

Watanabe R., Tsuchiya S., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  171 （ 3-4 ） 341-347 - 347 2013年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We investigate pseudogap phenomena originating from pairing fluctuations in the BCS-BEC crossover regime of a two-dimensional Fermi gas in a harmonic trap. Including pairing fluctuations within a T -matrix theory and effects of a trap within the local density approximation, we calculate the local density of states (LDOS) at the superfluid phase transition temperature Tc. In the weak-coupling regime, we show that the pseudogap already appears in LDOS around the trap center. The spatial region where the pseudogap can be seen in LDOS becomes wider for a strong pairing interaction. We also discuss how the pseudogap affects the spectrum of the photoemission-type experiment developed by JILA group. © Springer Science+Business Media, LLC 2012.

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Superfluid phase transition and strong-coupling effects in an ultracold fermi gas with mass imbalance

Hanai R., Kashimura T., Watanabe R., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  171 （ 3-4 ） 389-396 - 396 2013年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We investigate the superfluid phase transition and effects of mass imbalance in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an cold Fermi gas.We point out that the Gaussian fluctuation theory developed by Nozières and Schmitt-Rink and the T -matrix theory, that are now widely used to study strong-coupling physics of cold Fermi gases, give unphysical results in the presence of mass imbalance. To overcome this problem, we extend the T -matrix theory to include higher-order pairing fluctuations. Using this, we examine how the mass imbalance affects the superfluid phase transition. Since the mass imbalance is an important key in various Fermi superfluids, such as 40K-6Li Fermi gas mixture, exciton condensate, and color superconductivity in a dense quark matter, our results would be useful for the study of these recently developing superfluid systems. © Springer Science+Business Media, LLC 2012.

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Magnetic properties and strong-coupling corrections in an ultracold fermi gas with population imbalance

Kashimura T., Watanabe R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  171 （ 3-4 ） 355-361 - 361 2013年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We investigate magnetic properties of an ultracold Fermi gas with population imbalance. In the presence of population imbalance, the strong-coupling theory developed by Nozières and Schmitt-Rink (which is frequently referred to as the NSR theory, or Gaussian fluctuation theory) is known to give unphysical results in the BCS-BEC crossover region. We point out that this problem comes from how to treat pseudogap effects originating from pairing fluctuations and many-body corrections to the spin susceptibility. We also clarify how to overcome this problem by including higher order fluctuations beyond the ordinary T -matrix approximation. Calculated spin susceptibility based on our extended T -matrix approximation agrees well with the recent experiment on a 6Li Fermi gas. © Springer Science+Business Media, LLC 2012.

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Low-dimensional pairing fluctuations and pseudogapped photoemission spectrum in a trapped two-dimensional Fermi gas

Watanabe R., Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  88 （ 1 ） 013637(1-8) 2013年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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We investigate strong-coupling properties of a trapped two-dimensional normal Fermi gas. Within the framework of a combined T-matrix theory with the local density approximation, we calculate the local density of states, as well as the photoemission spectrum, to see how two-dimensional pairing fluctuations affect these single-particle quantities. In the BCS (Bardeen-Cooper-Schrieffer)- BEC (Bose-Einstein condensation) crossover region, we show that the local density of states exhibits a dip structure in the trap center, which is more remarkable than the three-dimensional case. This pseudogap phenomenon is found to naturally lead to a double-peak structure in the photoemission spectrum. The peak-to-peak energy of the spectrum at p=0 agrees well with a recent experiment on a two-dimensional 40K Fermi gas. Since pairing fluctuations are sensitive to the dimensionality of a system, our results would be useful for the study of many-body physics in the BCS-BEC crossover regime of a two-dimensional Fermi gas. © 2013 American Physical Society.

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Superfluid theory of a gas of polarized dipolar Fermi molecules

Y. Endo, D. Inotani, and Y. Ohashi

ArXiv:13072151 2013年

研究論文（学術雑誌）, 共著, 最終著者

Coexistence of superfluid gap and pseudogap in the BCS-BEC crossover regime of a trapped Fermi gas below Tc

Watanabe R., Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  86 （ 6 ） 063603(1-11) 2012年12月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate strong pairing fluctuations and effects of a harmonic trap in the superfluid phase of an ultracold Fermi gas. Including amplitude and phase fluctuations of the inhomogeneous superfluid order parameter Δ(r) in a trap within a combined T-matrix theory with the local density approximation, we examine local properties of single-particle excitations and a thermodynamic quantity in the BCS-BEC crossover region. Below the superfluid phase transition temperature Tc, we show that inhomogeneous pairing fluctuations lead to a shell structure of the gas cloud in which the spatial region where the ordinary BCS-type superfluid density of states appears surrounded by the region where the pseudogap associated with strong pairing fluctuations dominates single-particle excitations. The former spatial region enlarges to eventually cover the whole gas cloud far below Tc. We also examine how this shell structure affects the photoemission spectrum, as well as the local pressure. Since a cold Fermi gas is always trapped in a harmonic potential, our results would be useful for the study of strong-coupling superfluid physics, including this realistic situation. © 2012 American Physical Society.

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π-phase and Spontaneous Supercurrent induced by Pseudo-ferromagnetic junction in a spin-polarized superfluid Fermi gas

Kashimura T., Tsuchiya S., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  400 （ PART 1 ）  2012年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

We theoretically investigate a possible spontaneous current state in a polarized superfluid Fermi gas confined in a toroidal trap. When one puts a weak nonmagnetic potential barrier in this system, this barrier is known to be magnetized in the sense that some of excess ↑-spin atoms are localized around it (where the ↑-spin describes the majority component of the polarized Fermi gas). Using this unique property, we show that this magnetized barrier, or the pseudo-ferromagnetic junction, induces a spontaneous current circulating along the toroidal trap. While the ordinary supercurrent state appears as a metastable state, this spontaneous current state is realized as the most stable state, originating from the phase twist of the superfluid order parameter by the magnetized potential barrier. © Published under licence by IOP Publishing Ltd.

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Pseudogap phenomena of an ultracold Fermi gas with a P-wave feshbach resonance

Inotani D., Watanabe R., Sigrist M., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  400 （ PART 1 ）  2012年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

We investigate pseudogap phenomena in an ultracold Fermi gas with a p-wave tunable pairing interaction. Including p-wave pairing fluctuations within a strong-coupling T-matrix theory, we calculate the single-particle density of states (DOS) above the superfluid transition temperature Tc. Starting from the weak-coupling regime, we show that, at first, a dip structure gradually appears to become remarkable in DOS with increasing the strength of the p-wave pairing interaction at Tc. However, this pseudogap structure again becomes less remarkable to eventually disappear, as one passes through the intermediate coupling regime to enter the strong-coupling region. This non-monotonic interaction dependence of the pseudogap is quite different from the s-wave case, where the pseudogap size simply increases with increasing a s-wave interaction strength. This difference is found to originate from the momentum dependence of the p-wave interaction. We also determine the pseudogap temperature below which the dip structure in DOS appears and identify the pseudogap regime in the phase diagram of a Fermi gas with respect to the temperature and p-wave interaction strength. © Published under licence by IOP Publishing Ltd.

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Inhomogeneous pseudogap phenomenon in the BCS-BEC crossover regime of a trapped superfluid Fermi gas

Watanabe R., Tsuchiya S., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  400 （ PART 1 ） 012080(1-4) 2012年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

We investigate pseudogap phenomena in the unitarity limit of a trapped superfluid Fermi gas. Including effect of strong pairing fluctuations within a T-matrix approximation, as well as effects of a harmonic trap within the local density approximation (LDA), we calculate the local superfluid density of states below the superfluid phase transition temperature Tc. We show that the spatial region where single-particle excitations are dominated by the pseudogap may still exist even below Tc, due to inhomogeneous pairing fluctuations caused by the trap potential. From the temperature dependence of the pseudogapped density of states, we identify the pseudogap regime of the unitarity Fermi gas with respect to the temperature and spatial position. We also show that the combined T-matrix theory with the LDA can quantitatively explain the local pressure which was recently observed in the unitarity limit of a 6Li Fermi gas. © Published under licence by IOP Publishing Ltd.

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Pseudogap phenomenon in an ultracold Fermi gas with a p-wave pairing interaction

Inotani D., Watanabe R., Sigrist M., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  85 （ 5 ） 053628(1-9) 2012年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate single-particle properties of a one-component Fermi gas with a tunable p-wave interaction. Including pairing fluctuations associated with this anisotropic interaction within a T-matrix theory, we calculate the single-particle density of states, as well as the spectral weight, above the superfluid transition temperature T c. Starting from the weak-coupling regime, we show that the so-called pseudogap first develops in these quantities with increasing the interaction strength. However, when the interaction becomes strong to some extent, the pseudogap becomes obscure to eventually disappear in the intermediate coupling regime. In the strong-coupling regime, the excitation gap associated with the formation of tightly bound molecules again opens. This nonmonotonic interaction dependence of the excitation gap is quite different from the case of an s-wave interaction, where the pseudogap simply develops with increasing the interaction strength, which continuously changes into the molecular excitation gap. The difference between the two cases is shown to originate from the momentum dependence of the p-wave interaction, which vanishes in the low momentum limit. We also identify the pseudogap regime in the phase diagram with respect to the temperature and the p-wave interaction strength. Since the pseudogap is a precursor phenomenon of the superfluid phase transition, our results would be useful for the research toward the realization of p-wave superfluid Fermi gases. © 2012 American Physical Society.

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Pseudogap phenomenon of an Ultracold Fermi gas with a p-wave Feshbach resonance

Daisuke Inotani, Ryota Watanabe, Manfred Sigrist, Yoji Ohashi

Journal of Physics, conference series 400   012020(1-4) 2012年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

pi-phase and spontaneous supercurrent induced by pseudo-ferromagnetic junction in a spin polarized superfluid Fermi gas

T. Kashimura, S. Tsuchiya, and Y. Ohashi

Journal of Physics, conference series 400   012027(104) 2012年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

Anomalous tunneling of spin wave in polar state of spin-1 BEC

Watabe S., Kato Y., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  400 （ PART 1 ） 012079(1-4) 2012年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

We investigate tunneling properties of collective spin-wave excitations in the polar state of a spin-1 spinor Bose-Einstein condensate. Within the mean-field theory at T = 0, we show that when the condensate is in the critical supercurrent state, the spin wave mode exhibits perfect transmission through a nonmagnetic potential barrier in the low energy limit, unless the strength of a spin-independent interaction co equals that of a spin-dependent interaction co Such an anomalous tunneling behavior is absent in the case of a magnetic barrier. We also clarify a scaling law of the transmission probability as a function of the mode energy. © Published under licence by IOP Publishing Ltd.

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Anomalous tunneling of spin wave in heisenberg ferromagnet

Kato Y., Watabe S., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  400 （ PART 3 ） 032036(1-4) 2012年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

The ferromagnetic spin wave (FSW) in classical Heisenberg chain exhibits the perfect transmission in the long-wavelength limit in the transmission-reflection problem with an inhomogeneity of exchange integral. In the presence of local magnetic field, on the other hand, FSW undergoes the perfect reflection in the long-wavelength limit. This difference in the long-wavelength limit is attributed to the symmetry property of the scatterers; it is crucial whether the potential preserves or breaks the spin rotation symmetry. Our result implies that the anomalous tunneling (i.e., perfect transmission in the low-energy limit) found both in scalar and spinor BECs is not specific to gapless modes in superfluids but is a common property shared with generic Nambu-Goldstone modes in the presence of a symmetry-preserving potential scatterer.

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(ERRATUM) Superfluid density of states and pseudogap phenomenon in the BCS-BEC crossover regime of a superfluid Fermi gas

R. Watanabe, S. Tsuchiya, and Y. Ohashi

Physical Review A 85   039908(1-2) 2012年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

Excitation transport through a domain wall in a Bose-Einstein condensate

Watabe S., Kato Y., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  86 （ 2 ） 023622(1-9) 2012年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate the tunneling properties of collective excitations through a domain wall in the ferromagnetic phase of a spin-1 spinor Bose-Einstein condensate. Within the mean-field theory at T=0, we show that the transverse spin wave undergoes perfect reflection in the low-energy limit. This reflection property differs considerably from that of a domain wall in a Heisenberg ferromagnet where spin-wave excitations exhibit perfect transmission at arbitrary energy. When the Bogoliubov mode is scattered from this domain wall soliton, the transmission and reflection coefficients exhibit pronounced nonmonotonicity. In particular, we find perfect reflection of the Bogoliubov mode at energies where bound states appear. This is in stark contrast to the perfect transmission of the Bogoliubov mode with arbitrary energy through a dark soliton in a scalar Bose-Einstein condensate. © 2012 American Physical Society.

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Spin susceptibility and fluctuation corrections in the BCS-BEC crossover regime of an ultracold Fermi gas

Kashimura T., Watanabe R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  86 （ 4 ） 043622(1-8) 2012年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate magnetic properties and effects of pairing fluctuations in the Bardeen-Cooper-Schrieffer-Bose-Einstein condensation (BCS-BEC) crossover regime of an ultracold Fermi gas. Recently, Liu and Hu, and Parish, pointed out that the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR), which has been extensively used to successfully clarify various physical properties of cold Fermi gases, unphysically gives negative spin susceptibility in the BCS-BEC crossover region. The same problem also exists in the ordinary non-self-consistent T-matrix approximation. In this paper, we clarify that this serious problem comes from incomplete treatment in term of pseudogap phenomena originating from strong pairing fluctuations, as well as effects of spin fluctuations on the spin susceptibility. Including these two key issues, we construct an extended T-matrix theory which can overcome this problem. The resulting positive spin susceptibility agrees well with the recent experiment on a 6Li Fermi gas done by Sanner. We also apply our theory to a polarized Fermi gas to examine the superfluid phase transition temperature Tc, as a function of the polarization rate. Since the spin susceptibility is an important physical quantity, especially in singlet Fermi superfluids, our results would be useful in considering how singlet pairs appear above and below Tcin the BCS-BEC crossover regime of cold Fermi gases. © 2012 American Physical Society.

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π junction and spontaneous current state in a superfluid Fermi gas

Kashimura T., Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  84 （ 1 ） 013609(1-8) 2011年07月

研究論文（学術雑誌）, 共著, 最終著者,  ISSN  10502947

　概要を見る

We discuss an idea to realize a spontaneous current in a superfluid Fermi gas. When a polarized Fermi superfluid (NN, where Nσ is the number of atoms in the hyperfine state described by pseudospin σ=) is loaded onto a ring-shaped trap with a weak potential barrier, some excess atoms (ΔN=N-N) are localized around the barrier. As shown in our previous paper, this polarized potential barrier works as a π junction in the sense that the superfluid order parameter changes its sign across the barrier. Because of this, the phase of the superfluid order parameter outside the junction is shown to be twisted by π along the ring, which naturally leads to a circulating supercurrent. While the ordinary supercurrent state is obtained as a metastable state, this spontaneous current state is shown to be more stable than the case with no current. Our results indicate that localized excess atoms would be useful for the manipulation of the superfluid order parameter in cold Fermi gases. © 2011 American Physical Society.

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Pseudogap temperature and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas

Tsuchiya S., Watanabe R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  84 （ 4 ） 043647(1-11) 2011年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We theoretically investigate excitation properties in the pseudogap regime of a trapped Fermi gas. Using a combined T-matrix theory with the local density approximation, we calculate strong-coupling corrections to single-particle local density of states (LDOS), as well as the single-particle local spectral weight (LSW). Starting from the superfluid phase transition temperature Tc, we clarify how the pseudogap structures in these quantities disappear with increasing the temperature. As in the case of a uniform Fermi gas, LDOS and LSW give different pseudogap temperatures T* and T ** at which the pseudogap structures in these quantities completely disappear. Determining T* and T ** over the entire BCS (Bardeen-Cooper- Schrieffer)-BEC (Bose-Einstein condensation) crossover region, we identify the pseudogap regime in the phase diagram with respect to the temperature and the interaction strength. We also show that the so-called back-bending peak recently observed in the photoemission spectra by the JILA group may be explained as an effect of pseudogap phenomenon in the trap center. Since strong pairing fluctuations, spatial inhomogeneity, and finite temperatures are important keys in considering real cold Fermi gases, our results would be useful for clarifying normal-state properties of this strongly interacting Fermi system. © 2011 American Physical Society.

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フェルミ原子ガス超流動における超流動/強磁性/超流動接合とπ-phaseの実現可能性

柏村孝、土屋俊二、大橋 洋士

素粒子論研究(電子版） 7   64 2011年

研究論文（大学，研究機関等紀要）, 共著, 最終著者

超流動Fermi原子気体の１粒子状態とphotoemissionスペクトル

渡邉亮太、土屋俊二、大橋 洋士

素粒子論研究(電子版） 7   54 2011年

研究論文（大学，研究機関等紀要）, 共著, 最終著者

ｐ波相互作用するフェルミ気体における強結合効果と擬ギャップ構造

猪谷太輔、渡邉亮太、Manfred Sigrist、大橋 洋士

素粒子論研究(電子版） 7   76 2011年

研究論文（大学，研究機関等紀要）, 共著, 最終著者

Tunneling properties of Bogoliubov mode and spin-wave modes in supercurrent states of a spin-1 ferromagnetic spinor Bose-Einstein condensate

Watabe S., Kato Y., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  83 （ 3 ） 033627(1-12) 2011年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate the tunneling properties of collective excitations in the ferromagnetic phase of a spin-1 spinor Bose-Einstein condensate (BEC). In addition to the Bogoliubov mode, this superfluid phase has two spin excitations, namely, the gapless transverse spin wave and the quadrupolar mode with a finite excitation gap. In the mean-field theory at T=0, we examine how these collective modes tunnel through a barrier potential that couples to the local density of particles. In the presence of supercurrent with a finite momentum q, while the Bogoliubov mode shows the so-called anomalous tunneling behavior (which is characterized by perfect transmission) in the low-energy limit, the transverse spin wave transmits perfectly only when the momentum k of this mode coincides with ±q. At k=±q, the wave function of this spin wave has the same form as the condensate wave function in the current-carrying state, so that the mechanism of this perfect transmission is found to be the same as tunneling of the supercurrent. Using this fact, the perfect transmission of the spin wave is proved for a generic barrier potential. We show that such perfect transmission does not occur in the quadrupolar mode. Further, we consider the effects of potentials breaking U(1) and spin-rotation symmetries on the transmission properties of excitations. Our results would be useful for understanding excitation properties of spinor BEC's, as well as the anomalous tunneling phenomenon in Bose superfluids. © 2011 American Physical Society.

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Formation of magnetic impurities and pair-breaking effect in a superfluid Fermi gas

Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  83 （ 6 ） 063611(1-12) 2011年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

We theoretically investigate the introduction of magnetic impurities into a superfluid Fermi gas. In the presence of a population imbalance (NN, where Nσ is the number of Fermi atoms with pseudospin σ), we show that nonmagnetic potential scatterers embedded in the system are magnetized in the sense that some of the excess spin- atoms are localized around them. They destroy the superfluid order parameter around them, as in the case of the magnetic impurity effect discussed in the superconductivity literature. This pair-breaking effect naturally leads to localized excited states below the superfluid excitation gap. To confirm our idea in a simple manner, we consider an attractive Fermi-Hubbard model within the mean-field theory at T=0. We determine consistent superfluid properties around a nonmagnetic impurity, such as the superfluid order parameter, local population imbalance, as well as the single-particle density of states, in the presence of a population imbalance. Since competition between superconductivity and magnetism is one of the most fundamental problems in condensed-matter physics, our results would be useful for the study of this important issue in cold Fermi gases. © 2011 American Physical Society.

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Anomalous tunneling of collective excitations and effects of superflow in the polar phase of a spin-1 spinor Bose-Einstein condensate

Watabe S., Kato Y., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  84 （ 1 ） 013616(1-8) 2011年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate tunneling properties of collective modes in the polar phase of a spin-1 spinor Bose-Einstein condensate (BEC). This spinor BEC state has two kinds of gapless modes (i.e., Bogoliubov and spin-wave). Within the framework of mean-field theory at T=0, we show that these Goldstone modes exhibit perfect transmission in the low-energy limit. Their anomalous tunneling behavior still holds in the presence of superflow, except in the critical current state. In the critical current state, while the tunneling of Bogoliubov mode is accompanied by finite reflection, the spin wave still exhibits perfect transmission, unless the strengths of spin-dependent and spin-independent interactions take the same value. We discuss the relation between perfect transmission of a spin wave and underlying superfluidity through a comparison of wave functions of the spin wave and the condensate. © 2011 American Physical Society.

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Pseudogap phenomenon in an ultracold Fermi gas with a p-wave interaction

Inotani D., Watanabe R., Sigrist M., Ohashi Y.

Proceedings of 2011 1st International Symposium on Access Spaces, ISAS 2011 （Proceedings of 2011 1st International Symposium on Access Spaces, ISAS 2011）     23-25 - 25 2011年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

　概要を見る

We investigate fluctuation effects in a single-component ultracold Fermi gas with a p-wave interaction. Within the framework of a strong-coupling T-matrix theory, we calculate the single-particle density of states (DOS) at the superfluid phase transition temperature Tc. Starting from the weak-coupling regime, we show that the so-called pseudogap structure develops in DOS, with increasing the strength of a pairing interaction. However, in contrast to the case of s-wave interaction (where the pseudogap size in DOS monotonically increases with increasing the interaction strength), the pseudogap structure is found to gradually disappear in the strong-coupling regime. We point out that this non-monotonic behavior of pseudogap originates from the momentum dependence of p-wave interaction. Our results clarify how the symmetry of pairing interaction affects pseudogap phenomena in strongly correlated fermion systems. © 2011 IEEE.

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Theory of photoemission-type experiment in the BCS-BEC crossover regime of a superfluid Fermi gas

Watanabe R., Tsuchiya S., Ohashi Y.

Proceedings of 2011 1st International Symposium on Access Spaces, ISAS 2011 （Proceedings of 2011 1st International Symposium on Access Spaces, ISAS 2011）     26-31 - 31 2011年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

　概要を見る

We theoretically investigate the recent photoemission-type experiment on 40K Fermi gases done by JILA group. Including pairing fluctuations within a strong-coupling T-matrix theory, as well as effects of a harmonic trap within the local density approximation, we calculate photoemission spectra in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. We show that the energy resolution of the current photoemission experiment is enough to detect the pseudogap phenomenon. We also show how the pseudogap in single-particle excitations continuously changes into the superfluid gap, as one decreases the temperature below the superfluid phase transition temperature. Our results would be useful for the study of single-particle properties of ultracold Fermi gases in the BCS-BEC crossover. © 2011 IEEE.

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Superfluid/ferromagnet/superfluid-junction and π-phase in a superfluid Fermi gas at finite temperatures

Kashimura T., Tsuchiya S., Ohashi Y.

Proceedings of 2011 1st International Symposium on Access Spaces, ISAS 2011 （Proceedings of 2011 1st International Symposium on Access Spaces, ISAS 2011）     32-35 - 35 2011年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

　概要を見る

We investigate the stability of π-phase in a polarized superfluid Fermi gas (N↑> N↓, where Nσis the number of atoms in the hyperfine state described by pseudospin-σ). In our previous paper [T. Kashimura, S. Tsuchiya, and Y. Ohashi, Phys. Rev. A 82, 033617 (2010)], we showed that excess atoms (ΔN = N↑- N↓) localized around a potential barrier embedded in the system induces the π-phase at T = 0, where the phase of superfluid order parameter differ by π across the junction. In this paper, we extend our previous work to include temperature effects within the mean-field theory. We show that the π-phase is stable even at finite temperatures, although transition from the π-phase to 0-phase eventually occurs at a certain temperature. Our results indicate that the π-phase is experimentally accessible in cold Fermi gases. © 2011 IEEE.

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Photoemission spectrum and effect of inhomogeneous pairing fluctuations in the BCS-BEC crossover regime of an ultracold Fermi gas

Tsuchiya S., Watanabe R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  82 （ 3 ） 033629(1-5) 2010年09月

研究論文（学術雑誌）, 共著, 最終著者,  ISSN  10502947

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We investigate the photoemission-type spectrum in a cold Fermi gas which was recently measured by the JILA group. This quantity gives us very useful information about single-particle properties in the BCS-BEC crossover. In this paper, including pairing fluctuations within a T-matrix theory, as well as effects of a harmonic trap within the local density approximation, we show that spatially inhomogeneous pairing fluctuations due to the trap potential are an important key to understanding the observed spectrum. In the crossover region, while strong pairing fluctuations lead to the so-called pseudogap phenomenon in the trap center, such strong-coupling effects are found to be weak around the edge of the gas. Our results including this effect are shown to agree well with the recent photoemission data of the JILA group. © 2010 The American Physical Society.

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Superfluid-ferromagnet-superfluid junction and the π phase in a superfluid Fermi gas

Kashimura T., Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  82 （ 3 ） 32-35 2010年09月

研究論文（学術雑誌）, 共著, 最終著者,  ISSN  10502947

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We investigate the possibility of a superfluid-ferromagnet-superfluid (SFS) junction in a superfluid Fermi gas. To examine this possibility in a simple manner, we consider an attractive Hubbard model at T=0 within the mean-field theory. When a potential barrier is embedded in a superfluid Fermi gas with population imbalance (N↑>N↓, where Nσ is the number of atoms with pseudospin σ= ↑, ↓), this barrier is shown to be magnetized in the sense that excess ↑-spin atoms are localized around it. The resulting superfluid Fermi gas is spatially divided into two by this ferromagnet, so that one obtains a junction similar to the superconductor- ferromagnet-superconductor junction discussed in superconductivity. Indeed, we show that the so-called π phase, which is a typical phenomenon in the SFS junction, is realized, where the superfluid order parameter changes its sign across the junction. Our results would be useful for the study of magnetic effects on fermion superfluidity using an ultracold Fermi gas. © 2010 The American Physical Society.

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Ambegaokar-Baratoff relations for Josephson critical current in heterojunctions with multigap superconductors

Ota Y., Nakai N., Nakamura H., Machida M., Inotani D., Ohashi Y., Koyama T., Matsumoto H.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  81 （ 21 ） 214511(1-6) 2010年06月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10980121

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An extension of the Ambegaokar-Baratoff relation to a superconductor- insulator-superconductor (SIS) Josephson junction with multiple tunneling channels is derived. Applying the resultant relation to a SIS Josephson junction formed by an iron-based (five-band) and a single-band Bardeen-Cooper-Schrieffer (BCS) type superconductors, a theoretical bound of the Josephson critical current (I c ) multiplied by the resistance of the junction (R n ) is given. We reveal that such a bound is useful for identifying the pairing symmetry of iron-pnictide superconductors. One finds that if a measured value of I c R n is smaller than the bound then the symmetry is ±s wave, and otherwise s wave without any sign changes. In addition, we stress that temperature dependence of I c R n is sensitive to the difference of the gap functions from the BCS type gap formula in the above heterojunction. © 2010 The American Physical Society.

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Pseudogap in fermionic density of states in the BCS-BEC crossover of atomic fermi gases

Tsuchiya S., Watanabe R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics）  158 （ 1-2 ） 29 - 35 2010年01月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  00222291

　概要を見る

We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC crossover region. We calculate the density of states (DOS), as well as the single-particle spectral weight, above the superfluid transition temperature T c including pairing fluctuations within a T-matrix approximation. We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover region, which is most remarkable near the unitarity limit. We determine the pseudogap temperature T * at which the pseudogap structure in DOS disappears. We also introduce another temperature T** at which the BCS-like double-peak structure disappears in the spectral weight. While one finds T* >T** in the BCS regime, T ** becomes higher than T* in the crossover and BEC regime. We also determine the pseudogap region in the phase diagram in terms of temperature and pairing interaction. © 2009 Springer Science+Business Media, LLC.

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Pairing fluctuations and pseudogap effects in the BCS-BEC crossover regime of a superfluid Fermi gas

Watanabe R., Tsuchiya S., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  470 （ SUPPL.1 ） S980-S981 - S981 2010年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

　概要を見る

We study effects of strong pairing fluctuations on single-particle properties of a superfluid Fermi gas in the BCS-BEC crossover region. Including phase and amplitude fluctuations of the order parameter within the T-matrix approximation, we calculate the superfluid density of states below the superfluid phase transition temperature Tc. We show how the pseudogap appearing in the normal phase above Tc changes into the superfluid gap over the entire BCS-BEC crossover region, as one passes through Tc. © 2009 Elsevier B.V. All rights reserved.

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Superfluid density of states and pseudogap phenomenon in the BCS-BEC crossover regime of a superfluid Fermi gas

Watanabe R., Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  82 （ 4 ） 043630(1-9) 2010年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate single-particle excitations and strong-coupling effects in the BCS-BEC crossover regime of a superfluid Fermi gas. Including phase and amplitude fluctuations of the superfluid order parameter within a T-matrix theory, we calculate the superfluid density of states (DOS), as well as single-particle spectral weight, over the entire BCS-BEC crossover region below the superfluid transition temperature Tc. We clarify how the pseudogap in the normal state evolves into the superfluid gap, as one passes through Tc. While the pseudogap in DOS continuously evolves into the superfluid gap in the weak-coupling BCS regime, the superfluid gap in the crossover region is shown to appear in DOS after the pseudogap disappears below Tc. In the phase diagram with respect to the temperature and interaction strength, we determine the region where strong pairing fluctuations dominate over single-particle properties of the system. Our results would be useful for the study of strong-coupling phenomena in the BCS-BEC crossover regime of a superfluid Fermi gas. © 2010 The American Physical Society.

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Theoretical proposal of a method to identify the pairing symmetry of Fe-pnictide superconductors using ac-josephson effect

Inotani D., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  470 （ SUPPL.1 ） S499-S501 - S501 2010年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

　概要を見る

We theoretically propose a method to detect ±s-wave state in newly discovered Fe-pnictide superconductors, using ac Josephson current through a single-band superconductor/insulator/±s-wave superconductor (S/I/±S) junction. We show that, because of the Riedel anomaly, the ac Josephson current vanishes at some values of biased voltage across the junction. This phenomenon does not occur without the sign difference of order parameters in the two band superconductor, so that the observation of the vanishing ac Josephson current would be a clear signature of ±s-wave pairing symmetry in Fe-pnictides. © 2009 Elsevier B.V. All rights reserved.

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Pseudogap behaviors of atomic Fermi gases above Tc in the BCS-BEC crossover

Tsuchiya S., Watanabe R., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  470 （ SUPPL.1 ） S986-S988 - S988 2010年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

　概要を見る

We investigate pseudogap behaviors of an ultracold Fermi gas in the BCS-BEC crossover above the superfluid transition temperature Tc. We calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS-BEC crossover region, including strong pairing fluctuations within a T-matrix approximation. We show that the pseudogap structure in DOS develops from the weak-coupling BCS region, and continuously changes into a fully gapped one in the strong-coupling BEC region. We determine the pseudogap temperature T at which the dip structure in DOS disappears, and identify the pseudogap region in the phase diagram in terms of temperatures and pairing interactions. © 2009 Elsevier B.V. All rights reserved.

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Identification of the ±s -wave pairing state in iron-pnictide superconductors using the Riedel anomaly

Inotani D., Ohashi Y.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  79 （ 22 ） 224527(1-5) 2009年06月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10980121

　概要を見る

We theoretically propose a method to identify ±s -wave order parameter in recently discovered Fe-pnictide superconductors. Our idea uses the Riedel anomaly in ac-Josephson current through a SI (±S) (single-band s -wave superconductor/insulator/ ±s -wave two-band superconductor) junction. We show that the Riedel peak effect leads to vanishing ac-Josephson current at some values of biased voltage. This phenomenon does not occur in the case when the ±s -wave superconductor is replaced by a conventional s -wave one so that the observation of this vanishing Josephson current would be a clear signature of ±s -wave pairing state in Fe-pnictide superconductors. © 2009 The American Physical Society.

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Molecular wavefunction and tunneling properties of a bound pair of Fermi atoms in an optical lattice

Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  150 （ 3 ） 032076 2009年03月

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  17426588

　概要を見る

We investigate tunneling properties of a bound pair of Fermi atoms in an optical lattice. Including a realistic cosine-shape optical lattice potential, we calculate the molecular wavefunction ψ. Starting from the weak-coupling regime, we show how ψ shrinks as one increases the strength of a pairing interaction between atoms. When the molecular size becomes comparable to the lattice constant in the intermediate coupling regime, we find from the spatial structure of ψ that the molecular tunneling between lattice sites is accompanied by virtual dissociation. This phenomenon is shown to be suppressed in the strong-coupling regime due to very strong binding energy. We point out that, while the molecular tunneling accompanied by virtual dissociation can be described by the Hubbard model, it cannot describe the molecular tunneling with no virtual dissociation in the strong-coupling regime. © 2009 IOP Publishing Ltd.

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Enhanced quasi-particle current of Bogoliubov phonons in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  150 （ 3 ） 032113 2009年03月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

We discuss tunneling properties of Bogoliubov excitations through a potential barrier. We find that the quasi-particle current carried by Bogoliubov excitations is remarkably enhanced near the potential barrier at low incident energies. This explains the increase of the transmission probability of Bogoliubov phonons in the low energy region. In addition, taking into account of the backreaction of Bogoliubov excitations on Bose condensates, we show that the quasi-particle current twists the phase of the condensate wavefunction, leading to a Josephson supercurrent through the barrier. © 2009 IOP Publishing Ltd.

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Superfluid properties of an ultracold Fermi gas in the BCS-BEC crossover region

Ohashi Y., Fukushima N.

Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008 （Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008）     18 - 23 2009年

研究論文（国際会議プロシーディングス）, 共著, 筆頭著者, 査読有り

　概要を見る

We investigate superfluid properties of a two-component Fermi gas in the BCS-BEC crossover region. Including strong-coupling effects within a Gaussian fluctuation approximation, we self-consistently determine the superfluid order parameter and chemical potential at finite temperatures. Using these self-consistent solutions, we calculate single-particle excitations, as well as collective mode, over the entire BCS-BEC crossover region. As one increases the strength of a pairing interaction, we show that, while the single-particle excitation gap becomes large, the velocity of collective Goldstone mode becomes small. As a result, the origin of temperature dependence of a physical quantity continuously changes from single-particle excitations to collective excitations, as one passes through the BCS-BEC crossover region. To see this, we examine the superfluid density.

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Quasiparticle inducing josephson effect in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008 （Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008）     37 - 40 2009年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

　概要を見る

We investigate tunneling properties of Bogoliubov phonons in a Bose-Einstein condensate. We find that the quasiparticle current of Bogoliubov phonon is remarkably enhanced near the potential barrier at low energies. This leads to the increase of the transmission probability in the low energy region. We show that the quasiparticle current twists the relative phase of the condensate wavefunctions and induces a Josephson supercurrent through the barrier. The Josephson critical current is estimated by calculating the induced supercurrent and phase difference.

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Single-particle properties and pseudogap effects in the BCS-BEC crossover regime of an ultracold Fermi gas above Tc

Tsuchiya S., Watanabe R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  80 （ 3 ） 033613(1-9) 2009年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate strong-coupling effects on normal-state properties of an ultracold Fermi gas. Within the framework of T -matrix approximation in terms of pairing fluctuations, we calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS-Bose-Einstein condensate (BEC) crossover region above the superfluid phase-transition temperature Tc. Starting from the weak-coupling BCS regime, we show that the so-called pseudogap develops in DOS above Tc, which becomes remarkable in the crossover region. The pseudogap structure continuously changes into a fully gapped one in the strong-coupling BEC regime, where the gap energy is directly related to the binding energy of tightly bound molecules. We determine the pseudogap temperature Tc where the dip structure in DOS vanishes. The value of Tc is shown to be very different from another characteristic temperature T□□ where a BCS-type double-peak structure disappears in the spectral weight. While one finds T□ > □□ in the BCS regime, T□ becomes higher than T□ in the crossover region and BEC regime. Including this, we determine the pseudogap region in the phase diagram of ultracold Fermi gases. Our results would be useful in the search for the pseudogap region in ultracold L6 i and K 40 Fermi gases. © 2009 The American Physical Society.

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Stripe formation in fermionic atoms on 2-D optical lattice: DMRG studies for n-leg repulsive hubbard ladder

Machida M., Okumura M., Yamada S., Ohashi Y., Matsumoto H.

Journal of Superconductivity and Novel Magnetism （Journal of Superconductivity and Novel Magnetism）  22 （ 3 ） 275-279 - 279 2009年

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  15571939

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In order to examine the ground state of the single-band Hubbard model from a slightly doped region to an overdoped one, we use the density renormalization group method extended directly to the ladder model and obtain hole profiles with changing the on-site repulsion and the doping ratio. The calculation results in a heavily underdoped region, in which one and two holes (up and down spins) are added, reveal that an attractive interaction works between holes which are confined inside a stripe shape pattern while strong disorders separate the two holes. When doping holes further within an underdoped range, the stripes formed by two holes merge and various stripe structures appear depending on U/t and the doping ratio. The remarkable finding in the underdoped region is a stripe formation composed of 4 holes analytically predicted by Chang and Affleck. On the other hand, in the overdoped range the stripe-like shape becomes very simple, i.e., only a small oscillating charge density profile emerges. These rich features in the underdoped range are relevant to electronic inhomogeneities experimentally observed in high-T c superconductors, while those are directly observable in atomic Fermi gases loaded on optical lattices. © 2009 Springer Science+Business Media, LLC.

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Supercurrent induced by tunneling Bogoliubov excitations in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  79 （ 6 ） 063619(1-15) 2009年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We study the tunneling of Bogoliubov excitations through a barrier in a Bose-Einstein condensate. We extend our previous work to the case when condensate densities are different between the left and right of the barrier potential. In the framework of the Bogoliubov mean-field theory, we calculate the transmission probability and phase shift, as well as the energy flux and quasiparticle current carried by Bogoliubov excitations. We find that Bogoliubov phonons twist the condensate phase due to a back-reaction effect, which induces the Josephson supercurrent. While the total current given by the sum of quasiparticle current and induced supercurrent is conserved, the quasiparticle current flowing through the barrier potential is shown to be remarkably enhanced in the low-energy region. When the condensate densities are different between the left and right of the barrier, the excess quasiparticle current, as well as the induced supercurrent, remains finite far away from the barrier. We also consider the tunneling of excitations and atoms through the boundary between the normal and superfluid regions. We show that supercurrent can be generated inside the condensate by injecting free atoms from outside. On the other hand, atoms are emitted when the Bogoliubov phonons propagate toward the phase boundary from the superfluid region. © 2009 The American Physical Society.

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d-Wave spin density wave phase in the attractive hubbard model with spin polarization

Tamaki H., Miyake K., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  78 （ 7 ） 073001(1-4) 2009年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the possibility of an unconventional spin density wave (SDW) in the attractive Hubbard model with finite spin polarization. We show that pairing and density fluctuations induce a transverse d-wave SDW near the half-filling. This novel SDW is related to the d-wave superfluidity induced by antiferromagnetic spin fluctuations, in the sense that they are connected with each other through Shibas attraction-repulsion transformation. Our results predict the d-wave SDW in real systems, such as cold Fermi atom gases with a population imbalance and compounds involving valence skipper elements. © 2009 The Physical Society of Japan.

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Supercurrent behavior of low-energy Bogoliubov phonons and the anomalous tunneling effect in a Bose-Einstein condensate

Ohashi Y., Tsuchiya S.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  78 （ 4 ） 043601(1-10) 2008年10月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10502947

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We investigate tunneling properties of Bogoliubov mode in a Bose-Einstein condensate. Using an exactly solvable model with a δ -functional barrier, we show that each component in the two-component wave function (u,v) of low-energy Bogoliubov phonon has the same form as the condensate wave function in the supercurrent state. As a result, the currents Ju and Jv associated with u and v, respectively, have the same tunneling properties as those of supercurrent carried by condensate. Thus, the tunneling of low-energy Bogoliubov phonon described by the tunneling of these two currents shows perfect transmission. We also show that the supercurrent behaviors of Bogoliubov phonon still exist in the presence of supercurrent carried by condensate, except in the critical supercurrent state. In the critical current state, the perfect transmission is absent, because Ju or Jv exceeds their upper limit given by the critical value of the supercurrent associated with the condensate. Our results consistently explain the recently proposed two tunneling phenomena associated with Bogoliubov phonon, namely, the anomalous tunneling effect (perfect transmission in the low-energy limit) and the breakdown of the perfect transmission in the critical supercurrent state. © 2008 The American Physical Society.

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Correlation effects on atom-density profiles of one- and two-dimensional polarized atomic Fermi gases loaded on an optical lattice

Machida M., Yamada S., Okumura M., Ohashi Y., Matsumoto H.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  77 （ 5 ） 053614(1-8) 2008年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

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We investigate effects of optical lattice potential in one- and two-dimensional two-component trapped Fermi gases with population imbalances. Using the exact diagonalization and the density matrix renormalization group methods complementarily, we calculate the atom-density profile from the ground state many-body wave function as a function of attractive interaction strength for various population imbalances. The numerical results reveal that although a phase separation between the superfluid core and the shell cloud of excess atoms occurs as observed in experiments without the optical lattice, the population imbalance generally remains in the core region in contrast to the nonlattice cases. The essence of the numerical results in a strong attractive regime can be explained by an effective model composed of Cooper pairs and excess major fermions. © 2008 The American Physical Society.

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Superfluid density in the BCS-BEC crossover regime of a Fermi superfluid

Ohashi Y., Fukushima N., Matsumoto H., Taylor E., Griffin A.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  468 （ 7-10 ） 599-604 - 604 2008年

研究論文（国際会議プロシーディングス）, 共著, 筆頭著者, 査読有り,  ISSN  09214534

　概要を見る

We investigate the superfluid carrier density ρ s in the BCS-BEC crossover regime of a Fermi superfluid at finite temperatures. Including pairing fluctuations within a Gaussian approximation, we calculate the superfluid order parameter, chemical potential, and ρ s in a consistent manner in the whole BCS-BEC crossover region. In the weak-coupling BCS regime, the temperature dependence of ρ s is dominated by single-particle excitations accompanied by thermal dissociation of Cooper pairs. In the strong-coupling BEC regime, we show that single-particle excitations become less dominant and thermal excitations of collective Bogoliubov mode become important for ρ s . We clarify how the weak-coupling BCS result smoothly changes into the strong-coupling BEC result, as one increases the strength of a pairing interaction. © 2008 Elsevier B.V. All rights reserved.

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BCS-BEC crossover and effects of density fluctuations in a two-component Fermi gas described by the three-dimensional attractive Hubbard model

Tamaki H., Ohashi Y., Miyake K.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  77 （ 6 ） 063616(1-9) 2008年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

　概要を見る

We investigate the superfluid phase transition in a gas of Fermi atoms loaded on a three-dimensional optical lattice. When the lattice potential is strong, this system can be well described by an attractive Hubbard model. In this model, we calculate the superfluid phase transition temperature Tc, including both superfluid and (spin and charge) density fluctuations within the self-consistent t -matrix theory and fluctuation exchange approximation, respectively. Since we treat these fluctuations in a consistent manner, our theory satisfies the required particle-hole symmetry over the entire BCS-Bose-Einstein-condensation (BEC) crossover region. We show that charge density fluctuations compete against superfluid fluctuations near the half-filling, leading to the suppression of Tc. As a result, the maximum Tc is obtained away from the half-filling. Since the strong density fluctuations originate from the nesting property of the Fermi surface at the half-filling (which is absent in a uniform gas with no lattice potential), our results would be useful in considering lattice effects on strong-coupling superfluidity. © 2008 The American Physical Society.

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Anomalous enhancement of quasiparticle current near a potential barrier in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  78 （ 1 ） 013628 2008年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate tunneling properties of Bogoliubov phonons in a Bose-Einstein condensate. We find the anomalous enhancement of the quasiparticle current Jq carried by Bogoliubov phonons near a potential barrier, due to the supply of the excess current from the condensate. This effect leads to the increase of quasiparticle transmission probability in the low energy region found by Kovrizhin We also show that the quasiparticle current twists the phase of the condensate wave function across the barrier, leading to a finite Josephson supercurrent Js through the barrier. This induced supercurrent flows in the opposite direction to the quasiparticle current so as to cancel out the enhancement of Jq and conserve the total current J= Jq + Js. © 2008 The American Physical Society.

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Stripe formation in repulsive 4-leg Hubbard ladder: Directly-extended DMRG studies

Machida M., Yamada S., Okumura M., Ohashi Y., Matsumoto H.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  468 （ 15-20 ） 1141-1144 - 1144 2008年

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  09214534

　概要を見る

In order to study the ground state of the Hubbard model from a slightly doped region to an over-doped one, we directly apply the density renormalization group method to 4-leg × 20-ladder repulsive Hubbard model and calculate hole distribution profiles with changing the on-site repulsion U / t from 0 to 15 and the doping ratio from 0.025 to 0.250. The calculation results in the underdoped range reveal that various stripe structures formed by doped holes merge or separate depending on U / t and the doping ratio. We suggest that these rich features are relevant to modulated hole distributions as experimentally observed in underdoped high-T c superconductors. © 2008.

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Vortex core structure in strongly correlated superfluidity

Yamada S., Okumura M., Machida M., Ohashi Y., Matsumoto H.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  468 （ 15-20 ） 1237-1240 - 1240 2008年

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  09214534

　概要を見る

In order to study the vortex core electronic structure in high-T c superconductors, we examine 1-D Hubbard model with trap potential by using the density matrix renormalization group method. Instead of directly treating the vortex, the approach mimics the carrier density depression inside the vortex core via the trap potential and exactly calculate strong correlation effects on the depressed region. Consequently, we find that the Mott state emerges in the central region and the metallic edge surrounds the Mott region. Furthermore, when adding spin imbalance, the calculations reveal that a local antiferromagnetic order covers the Mott state region, and moreover, the antiferromagnetic order modulates with a long periodicity. We expect that these results closely relate to the vortex core electronic structure while their calculation results can be directly compared with atomic Fermi gases loaded on an optical lattice. © 2008 Elsevier B.V. All rights reserved.

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Mott phase in polarized two-component atomic Fermi lattice gas

Machida M., Okumura M., Yamada S., Deguchi T., Ohashi Y., Matsumoto H.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  78 （ 23 ） 235117(1-5) 2008年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10980121

　概要を見る

We investigate effects of pseudospin population imbalance on Mott phases in one-dimensional trapped two-component atomic Fermi gases loaded on optical lattices based on the repulsive Hubbard model in harmonic traps. By using the density-matrix renormalization-group method, we numerically calculate density profiles of each component and clarify the pseudospin magnetism. Consequently, we find that all the features from weakly imbalance to fully polarized cases are well described by S=1/2 antiferromagnetic Heisenberg chain under magnetic field. These results indicate that the Mott phases offer experimental stages for studying various interacting spin systems. © 2008 The American Physical Society.

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Tunneling properties of a bound pair of Fermi atoms in an optical lattice

Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  78 （ 6 ） 063617(1-9) 2008年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate the tunneling properties of a bound pair of Fermi atoms in an optical lattice, comparing with results obtained in an attractive Hubbard model. In the strong-coupling regime of the Hubbard model, it has been predicted that the motion of a bound pair between lattice sites is accompanied by virtual dissociation. To explore the possibility of this interesting phenomenon in an optical lattice, we calculate the molecular wave function in a cosine-shaped periodic potential. We show that the molecular tunneling accompanied by dissociation occurs in the intermediate-coupling regime of the optical-lattice system. In the strong-coupling regime, in contrast to the prediction in the Hubbard model, the bound pair is shown to tunnel through the lattice potential without dissociation. As a result, the magnitude of the molecular band mass M remains finite even in the strong-coupling limit, which is in contrast to the diverging molecular mass in the case of the Hubbard model. Including this finite value of the molecular band mass, we evaluate the superfluid phase transition temperature Tc in the Bose-Einstein-condensate limit of the optical-lattice system, where the Hubbard model gives Tc =0 due to the diverging molecular mass. © 2008 The American Physical Society.

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Strong pairing and microscopic inhomogeneity of lattice fermion systems

Yamada S., Machida M., Ohashi Y., Matsumoto H.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  463-465 （ SUPPL. ） 103 - 106 2007年10月

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  09214534

　概要を見る

In order to study an interplay between electronic inhomogeneity and superconductivity as seen in high-Tc cuprates, we numerically examine attractive Hubbard model in the absence of the translation symmetry. We systematically calculate the particle density profile for both 1-D and 2-D attractive Hubbard models with harmonic potential wells using the exact diagonalization and the density-matrix renormalization group methods. The numerical results reveal that fine inhomogeneous zig-zag patterns universally emerge in the 1-D model case and the zig-zag structure becomes checkerboard type in the 2-D one. Moreover, it is numerically and theoretically found that such inhomogeneities are caused by double occupation component, i.e., pairs tightly bound on a site. © 2007 Elsevier B.V. All rights reserved.

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Vortex microscopic structure in BCS to BEC Fermi superfluids

Machida M., Koyama T., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  460-462 I （ SPEC. ISS. ） 275 - 276 2007年09月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

　概要を見る

In order to examine how the vortex core changes its structure when approaching BEC from BCS regime in superfluid fermion-atom gasses we make a systematic numerical study for the core structure of a singly quantized vortex, solving numerically the generalized Bogoliubov-de Gennes equation derived from the fermion-boson model. Numerical calculations reveal that the vortex core can be well characterized by two length scales based on the spatial variation of the fermionic gap functions and those scales show distinctive differences between the BCS and BEC regimes. © 2007 Elsevier B.V. All rights reserved.

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Superfluid density and condensate fraction in the BCS-BEC crossover regime at finite temperatures

Fukushima N., Ohashi Y., Taylor E., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  75 （ 3 ） 033609(1-10) 2007年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

　概要を見る

The superfluid density is a fundamental quantity describing the response to a rotation as well as in two-fluid collisional hydrodynamics. We present extensive calculations of the superfluid density Ïs in the BCS-BEC crossover regime of a uniform superfluid Fermi gas at finite temperatures. We include strong-coupling or fluctuation effects on these quantities within a Gaussian approximation. We also incorporate the same fluctuation effects into the BCS single-particle excitations described by the superfluid order parameter Î" and Fermi chemical potential Î, using the NoziÃresâ€"Schmitt-Rink approximation. This treatment is shown to be necessary for consistent treatment of Ïs over the entire BCS-BEC crossover. We also calculate the condensate fraction Nc as a function of the temperature, a quantity which is quite different from the superfluid density Ïs. We show that the mean-field expression for the condensate fraction Nc is a good approximation even in the strong-coupling BEC regime. Our numerical results show how Ïs and Nc depend on temperature, from the weak-coupling BCS region to the BEC region of tightly bound Cooper pair molecules. In a companion paper, we derive an equivalent expression for Ïs from the thermodynamic potential, which exhibits the role of the pairing fluctuations in a more explicit manner. © 2007 The American Physical Society.

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Ultra Large-scale Exact-diagonalization for Confined Fermion-Hubbard Model on the Earth Simulator: Exploration of Superfluidity in Confined Strongly-Correlated Systems

S. Yamada, T. Imamura, T. Kano, Y. Ohashi, H. Matsumoto, and M Machida

Journal of the Earth Simulator 7   23-35 2007年

研究論文（学術雑誌）, 共著, 査読有り

Spin-polarized Fermi superfluids as Bose-Fermi mixtures

Taylor E., Griffin A., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  76 （ 2 ） 023614(1-12) 2007年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

In the strong-coupling Bose-Einstein condensation (BEC) region where a Feshbach resonance gives rise to tightly bound dimer molecules, we show that a spin-polarized Fermi superfluid reduces to a simple Bose-Fermi mixture of Bose-condensed dimers and the leftover unpaired fermions. Using a many-body functional integral formalism, the Gaussian fluctuations give rise to an induced dimer-dimer interaction mediated by the unpaired fermions, with the dimer-fermion vertex being given by the (mean-field) Born approximation. Treating the pairing fluctuations to quartic order, we show how the action for a spin-polarized Fermi superfluid reduces to one for a Bose-Fermi mixture. This Bose-Fermi action includes an expression for the effective dimer-unpaired fermion interaction in a spin-polarized Fermi superfluid beyond the Born approximation, in the BEC limit. In the low-density limit, we show how this dimer-fermion interaction gives the s -wave scattering length aBF =1.18 aF (aF is the s -wave fermion scattering length), a result first derived by Skorniakov and Ter-Martirosian in 1957 for three interacting fermions. © 2007 The American Physical Society.

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Superfluid density in the BCS-BEC crossover regime of a two-component Fermi gas at finite temperatures

Ohashi Y.

Journal of Superconductivity and Novel Magnetism （Journal of Superconductivity and Novel Magnetism）  20 （ 7-8 ） 609-612 - 612 2007年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  15571939

　概要を見る

We investigate the superfluid carrier density ρ s in the BCS-BEC crossover regime of a superfluid Fermi gas at finite temperatures. Including pairing fluctuations within the gaussian fluctuation approximation, we determine the superfluid order parameter Δ, Fermi chemical potential μ, as well as ρ s , in a consistent manner. In the weak-coupling regime, the normal fluid density ρ n ρ-ρ s is dominated by quasi-particle excitations associated with the dissociation of Cooper pairs. As one approaches the BCS-BEC crossover regime, effects of pairing fluctuations on ρ n become strong. In the strong-coupling BEC regime, ρ n is shown to be dominated by Bogoliubov phonon in a molecular Bose superfluid. © 2007 Springer Science+Business Media, LLC.

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On-site pairing and microscopic inhomogeneneity in confined lattice fermion systems

Machida M., Yamada S., Ohashi Y., Matsumoto H.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  74 （ 5 ） 053621(1-6) 2006年12月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

　概要を見る

In order to explore a universal feature of atomic Fermi gases loaded on optical lattices, we make a systematic study of the particle density profile for one- and two-dimensional attractive Hubbard models with harmonic potential wells. Using the exact diagonalization scheme and the density-matrix renormalization group technique, we show that fine inhomogeneous zigzag patterns universally emerge in the above models. Theoretical and numerical analyses suggest that these structures originate from an effective repulsive interaction between tightly bound pairs and a breakdown of translational invariance. Furthermore, it is emphasized that the pattern obtained numerically in the two-dimensional model is the checkerboard type, which is very similar to results recently observed in a vortex core of high- Tc cuprate superconductor. © 2006 The American Physical Society.

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(LT24 proceedings) BCS-BEC Crossover in a Gas of Fermi Atoms with a P-wave Feshbach Resonance

Y. Ohashi

AIP proceedings (LT24) 850   65-66 2006年12月

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  0094243X

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Itinerant-localized dual character of a strongly correlated superfluid Bose gas in an optical lattice

Ohashi Y., Kitaura M., Matsumoto H.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  73 （ 3 ） 033617(1-5) 2006年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate a strongly correlated Bose gas in an optical lattice. Extending the standard-basis operator method developed by Haley and Erdös to a boson Hubbard model, we calculate excitation spectra in the superfluid phase, as well as in the Mott insulating phase, at T=0. In the Mott phase, the excitation spectrum has a finite energy gap, reflecting the localized character of atoms. In the superfluid phase, the excitation spectrum is shown to have an itinerant-localized dual structure, where the gapless Bogoliubov mode (which describes the itinerant character of superfluid atoms) and a band with a finite energy gap coexist. We also show that the rf-tunneling current measurement would give useful information about the duality of a strongly correlated superfluid Bose gas near the superfluid-insulator transition. © 2006 The American Physical Society.

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Excitations in the BCS-BEC crossover regime of a superfluid Fermi gas with a Feshbach resonance

Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  437-438 （ SPEC. ISS. ） 234-238 - 238 2006年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  09214534

　概要を見る

We investigate single-particle excitations at T = 0 in the BCS-BEC crossover regime of a superfluid Fermi gas. We solve the Bogoliubov-de Gennes equations in a trap, including a tunable pairing interaction associated with a Feshbach resonance. We show that the single-particle energy gap Eg is dominated by the lowest-energy Andreev bound state localized at the surface of the gas, which is much smaller than the superfluid order parameter over(Δ, ̃) (r = 0) at the center of the trap. We also calculate the rf-tunneling current spectrum IF(ω). In a broad Feshbach resonance, the peak energy in IF(ω) is shown to give, not Eg, but over(Δ, ̃) (r = 0). In a narrow resonance, significant spectral weight coming from the low-energy Andreev states hides the appearance of the peak in IF(ω) at ω ≃ over(Δ, ̃) (r = 0). Our results for a broad resonance is in good agreement with the recent rf-tunneling measurement in a superfluid 6Li. © 2006 Elsevier B.V. All rights reserved.

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Vortex structure from BCS to BEC superfluid

Machida M., Koyama T., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  437-438 （ SPEC. ISS. ） 190-194 - 194 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

　概要を見る

In atomic Fermi gases, the pairing character changes from BCS-like to BEC-like when decreasing the threshold energy of the Feshbach resonance. With this crossover, the system enters a strong-coupling regime through the population enhancement of diatom molecules, i.e., tightly bound Cooper pair, and the vortex structure shows much different features in contrast to the well-known core structure in BCS superfluid. In this paper, we study the structure of a single quantized vortex by numerically solving the generalized Bogoliubov-de Gennes equation derived from the boson-fermion model and clarify how the vortex structure changes with the crossover from BCS to BEC. As a result of numerical calculations, we find that the presence of the diatom molecular condensate enhances the matter density depletion inside the vortex core in the crossover regime and the Caroli-de Gennes-Matricon (CdGM) quasi-particle branch almost diminishes in BEC regime. © 2005 Elsevier B.V. All rights reserved.

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Spin correlation and superfluidity of trapped fermi atoms on an optical lattice

Machida M., Yamada S., Ohashi Y., Matsumoto H.

AIP Conference Proceedings （AIP Conference Proceedings）  850   61-62 - 62 2006年

研究論文（国際会議プロシーディングス）, 共著, 査読有り

　概要を見る

We investigate a possibility of superfluidity in a trapped gas of Fermi atoms with repulsive interaction U in the presence of an optical lattice. Applying the exact diagonalization method to a one-dimensional fermion-Hubbard model including the trap potential, we find that, when U exceeds a critical value, the binding energy of two Fermi atoms becomes negative indicating that an attractive interaction effectively works. In this case, a "Mott core" appears in the trap center, where each site is occupied by one atom, and a large Cooper-pair amplitude develops between atoms in the left and right hand sides of this core. In order to clarify the origin of the attractive force, we compute a pseudo spin correlation between neighboring sites given by 〈SiSi+1〉. As a result, we find that a creation of a singlet pair between both sides of the Mott core is strongly coupled with a pseudo-spin singlet re-formation leading to an energy gain process. © 2006 American Institute of Physics.

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Effect of quantum correction in the Bose-Hubbard model

Matsumoto H., Takahashi K., Ohashi Y.

AIP Conference Proceedings （AIP Conference Proceedings）  850   53-54 - 54 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

　概要を見る

Effects of quantum correction in the Bose-Hubbard model at finite temperature are investigated for a homogeneous atomic Bose gas in an optical lattice near its superfluid-insulator transition. Starting from a strong coupling limit, higher order quantum corrections due to the hopping interaction is included in a local approximation (a dynamical mean field approximation) of the non-crossing approximation. When the upper or lower Hubbard band approaches zero energy, there appears a shallow band in the middle of the Hubbard gap due to a strong correlation in the system. © 2006 American Institute of Physics.

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Reflectivity in the multi-Josephson junction model with Josephson vortex lattice

Matsumoto H., Suzuki Y., Ohashi Y.

AIP Conference Proceedings （AIP Conference Proceedings）  850   925-926 - 926 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

　概要を見る

In the intrinsic multi-Josephson model, we calculate reflectivity in order to investigate a coupling between external electromagnetic fields and Josephson plasma oscillations with an influence of a Josephson vortex lattice. We treat electromagnetic fields in the vacuum and the system simultaneously in numerical simulation. With increasing the applied magnetic field, there appear three regions; absorption region in low frequency as the result of Josephson vortices motion, forbidden region in intermediate frequency and plasma edge and above region. The effect of the interlayer coupling induces a sharp change in the boundary of absorption and forbidden regions. © 2006 American Institute of Physics.

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Microscopic structure of a quantized vortex core in atomic Fermi gases

Machida M., Ohashi Y., Koyama T.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  74 （ 2 ） 023621(1-5) 2006年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

　概要を見る

In order to understand what kind of information can be extracted from the vortex-core structure in superfluid fermion-atom gases we make a systematic study for the core structure of a singly quantized vortex in the range of broad Feshbach resonance, solving numerically the generalized Bogoliubov-de Gennes equation derived from the fermion-boson model. Numerical calculations reveal that the vortex core can be well characterized by three length scales from the spatial variation of the fermionic and bosonic gap functions. We also predict that the profile of matter depletion and the quasiparticle spectrum show distinctive differences between the BCS and Bose-Einstein Condensate regimes. © 2006 The American Physical Society.

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Novel pairing in the Hubbard model with confinement potential

Machida M., Yamada S., Ohashi Y., Matsumoto H.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  445-448 （ 1-2 ） 90-93 - 93 2006年

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  09214534

　概要を見る

We explore a novel type of pairing in the repulsive Hubbard model with a confinement potential. Such an undertaking situation including the confinement potential is realized in a part of intense spatial modulations observed in High-Tc cuprate superconductors, while an atomic Fermi gas loaded on an optical lattice is described by the Hubbard model with the confinement potential. In this paper, applying the exact diagonalization method to a one-dimensional Hubbard model with the confinement potential, we find that, when U exceeds a critical value, the binding energy of two Fermi atoms becomes 'negative, indicating that an attractive interaction effectively works between two fermions. In this case, a 'Mott core' appears in the confinement center, where each site is occupied by one atom, and the Cooper-pair function strongly develops between fermions in the left and right hand sides of this core. © 2006 Elsevier B.V. All rights reserved.

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Vortex structure in weak to strong coupling superconductors: Crossover from BCS to BEC

Machida M., Koyama T., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  445-448 （ 1-2 ） 194-197 - 197 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

　概要を見る

High-Tc cuprate and MgB2 superconductors are recognized as intermediate or strong coupling superconductors. In such a non-weak regime, the vortex core structure is an interesting issue, since the vortex dissipation is expected to be anomalously reduced due to large level distance between localized core states. In this paper, we systematically clarify the vortex core structure based on the fermion-boson model which can cover a full coupling range from weak BCS to strong BEC superconductors. The mean-field calculations on the model reveal that the distances between the low-lying core levels expand and the low-lying states finally disappear when changing the coupling from weak BCS to strong BEC regime. This result suggests that the strong coupling makes the vortex motion non-dissipative. © 2006 Elsevier B.V. All rights reserved.

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BCS-BEC crossover in a superfluid Fermi gas

Ohashi Y.

International Journal of Modern Physics B （International Journal of Modern Physics B）  20 （ 30-31 ） 5204-5213 - 5213 2006年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  02179792

　概要を見る

We discuss the superfluid phase transition in a gas of Fermi atoms with a Feshbach resonance. A tunable pairing interaction associated with the Feshbach resonance is shown to naturally lead to the BCS-BEC crossover, where the character of superfluidity continuously changes from the weak-coupling Bardeen-Cooper-Schrieffer (BCS) type to the Bose-Einstein condensation (BEC) of tightly bound molecules, as one decreases the threshold energy 2ν of the Feshbach resonance. We also discuss effects of a trap, as well as the p-wave BCS-BEC crossover adjusted by a p-wave Feshbach resonance. © World Scientific Publishing Company.

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Pairing fluctuations and the superfluid density through the BCS-BEC crossover

Taylor E., Griffin A., Fukushima N., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  74 （ 6 ） 063626(1-15) 2006年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

　概要を見る

We derive an expression for the superfluid density of a uniform two-component Fermi gas through the BCS-BEC crossover in terms of the thermodynamic potential in the presence of an imposed superfluid flow. Treating the pairing fluctuations in a Gaussian approximation following the approach of Nozières and Schmitt-Rink, we use this definition of ρs to obtain an explicit result which is valid at finite temperatures and over the full BCS-BEC crossover. It is crucial that the BCS gap Δ, the chemical potential μ, and ρs all include the effect of fluctuations at the same level in a self-consistent manner. We show that the normal fluid density ρn n- ρs naturally separates into a sum of contributions from Fermi BCS quasiparticles (ρnF) and Bose collective modes (ρnB). The expression for ρnF is just Landau's formula for a BCS Fermi superfluid but now calculated over the BCS-BEC crossover. The expression for the Bose contribution ρnB is more complicated and only reduces to Landau's formula for a Bose superfluid in the extreme BEC limit, where all the fermions have formed stable Bose pairs and the Bogoliubov excitations of the associated molecular Bose condensate are undamped. In a companion paper, we present numerical calculations of ρs using an expression equivalent to the one derived in this paper, over the BCS-BEC crossover, including unitarity, and at finite temperatures. © 2006 The American Physical Society.

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BCS-BEC crossover in a gas of Fermi atoms with a p-wave Feshbach resonance

大橋 洋士

Physical Review Letters （Physical Review Letters）  94 （ 5 ） 050403(1-4) 2005年02月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319007

　概要を見る

We investigate unconventional superfluidity in a gas of Fermi atoms with an anisotropic p-wave Feshbach resonance. Including the p-wave Feshbach resonance as well as the associated three kinds of quasimolecules with finite orbital angular momenta Lz = ± 1, 0, we calculate the transition temperature of the superfluid phase. As one passes through the p-wave Feshbach resonance, we find the usual BCS-BEC crossover phenomenon. The p-wave BCS state continuously changes into the BEC of bound molecules with L = 1. Our calculation includes the effect of fluctuations associated with Cooper pairs and molecules which are not Bose condensed. © 2005 The American Physical Society.

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Single-particle excitations in a trapped gas of Fermi atoms in the BCS-BEC crossover region

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  72 （ 1 ） 013601(1-25) 2005年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

We investigate the single-particle properties at T=0 of a trapped superfluid gas of Fermi atoms with a Feshbach resonance. A tunable pairing interaction associated with the Feshbach resonance leads to the BCS Bose-Einstein condensate (BEC) crossover, where the character of superfluidity continuously changes from the BCS-type to a BEC of composite bosons. In this paper, we extend our previous work for a uniform superfluid Fermi gas [Y. Ohashi and A. Griffin, Phys. Rev. A 67, 063612 (2003)] to include the effect of a harmonic trap. We do not use the local density approximation (LDA), but directly solve the Bogoliubov de Gennes (BdG) coupled equations. While our explicit numerical solutions are for a weak (narrow) Feshbach resonance, we argue that the single-particle BdG excitation spectrum will exhibit the same features for a strong (broad) Feshbach resonance. Using these equations, we find self-consistent values for the spatially dependent local density n(r) as well as the composite BCS order parameter, the latter describing both the Cooper-pair and molecular condensate contributions. Using these results, we calculate the single-particle density of states in the crossover region, and from this determine the true single-particle energy gap (Eg) of the trapped Fermi superfluid. This is associated with the in-gap (or Andreev) states in the low-density region at the edge of the trap. We calculate the laser-induced tunneling current I into another hyperfine state, as measured in recent rf spectroscopy experiments. This rf spectrum gives a direct probe of the quasiparticle spectrum. We show how the high-energy part of I gives information about (r=0) at the center of the trap (which is comparable to the Fermi energy F in the crossover region). We show that I is very dependent on the spatial profile of the pair potential (r) that is used. We also emphasize that the narrow cunpaired atom" peak in the rf data gives information about Eg and the low-energy F in-gap states of a Fermi superfluid. While our calculations are limited to T=0, we use them to discuss the recent data of Chin et al. and the LDA calculations of Trm and co-workers. The LDA, while useful, can lead to an incorrect physical picture of the low-density surface region of the Fermi superfluid. © 2005 The American Physical Society.

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Single-particle excitations in a trapped gas of Fermi atoms in the BCS-BEC crossover region. II. Broad Feshbach resonance

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  72 （ 6 ） 063606(1-8) 2005年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

We apply the formulation developed in a recent paper [Y. Ohashi and A. Griffin, Phys. Rev. A 72, 013601 (2005)] for single-particle excitations in the BCS-BEC crossover to the case of a broad Feshbach resonance. At T=0, we solve the Bogoliubov-de Gennes coupled equations taking into account a Bose condensate of bound states (molecules). In the case of a broad resonance, the density profile n(r), as well as the profile of the superfluid order parameter Δ(r), are spatially spread out to the Thomas-Fermi radius, even in the crossover region. This order parameter Δ(r) suppresses the effects of low-energy Andreev bound states on the rf tunneling current. As a result, the peak energy in the rf spectrum is found to occur at an energy equal to the superfluid order parameter Δ(r=0) at the center of the trap, in contrast to the case of a narrow resonance, and in agreement with recent measurements. The local density approximation is found to give a good approximation for the rf-tunneling spectrum. © 2005 The American Physical Society.

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Effective interaction between molecules in the strong-coupling BEC regime of a superfluid fermi gas

Yoji Ohashi

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  74 （ 10 ） 2659-2662 - 2662 2005年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the effective interaction between Cooper-pair molecules in the strong-coupling BEC regime of a superfluid Fermi gas with a Feshbach resonance. Our work uses a path integral formulation and a renormalization group (RG) analysis of fluctuations in a single-channel model. We show that a physical cutoff energy ωc originating from the finite molecular binding energy is the key to understanding the interaction between molecules in the BEC regime. Our work thus clari.es recent results by showing that a M = 2aF is a bare molecular scattering length while a M = (0.6-0.75)aF is the low-energy molecular scattering length renormalized to include high-energy scattering up to ωc (here, aF is the scattering length between Fermi atoms). We also include many-body effects at finite temperatures. We find that aM is strongly dependent on temperature, vanishing at Tc, consistent with the earlier Bose gas results of Bijlsma and Stoof. ©2005 The Physical Society of Japan.

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フェルミ原子ガスにおける新しい超流動

大橋 洋士

素粒子論研究 111   C21 2005年

研究論文（大学，研究機関等紀要）, 単著, 筆頭著者

New Fermion Superfluidity and the BCS-BEC crossover in a trapped gas of Fermi atoms with a Feshbach resonance

Y. Ohashi

素粒子論研究 112   B54 2005年

研究論文（大学，研究機関等紀要）, 単著

Theory of superconductor-insulator transition in organic superconductor λ -(BETS)2FexGa1-xCl4

Terao M., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  412-414 （ SPEC. ISS. ） 324 - 328 2004年10月

研究論文（国際会議プロシーディングス）, 共著, 最終著者,  ISSN  09214534

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We investigate superconductivity and magnetism in the organic conductor λ-(BETS)2FexGa1-xCl4. In this material, it is known that the d-electrons on Fe-ions are ordered antiferromagnetically (TN<8.5-9 K) for 0.4<x ≤ 1, while superconductivity (TSC<6 K) of π-electrons appears when 0 ≤ x<0.4. In this paper, including the detailed band structure of this material, we discuss the exchange interaction between the d- and π-electrons, focusing on its role in the antiferromagnetism and superconductivity. We show that the inclusion of only this π-d exchange interaction is difficult to explain both the observed x-dependence of TN and TSC. To understand the phase diagram of λ-(BETS)2FexGa 1-xCl4, we point out that the Coulomb interaction between π-electrons is also important. © 2004 Elsevier B.V. All rights reserved.

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Effects of interband impurity scattering on superconducting density of states in a two-band superconductor

大橋 洋士

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  412-414 （ SPEC. ISS. ） 41-45 - 45 2004年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  09214534

　概要を見る

We investigate interband impurity scattering effects on a double gap structure in the superconducting density of states (SDOS) in a two-band superconductor. Using the self-consistent t-matrix theory, we show how the double gap structure gradually disappears/revives when impurity concentration and the magnitude of impurity potential are varied. The increase of impurity concentration leads to an averaged single gap SDOS due to the mixing of two bands by interband scattering. However, this mixing effect is shown to disappear in the strong interband scattering limit. We also show that the intraband impurity scattering suppresses the interband scattering effect because it tends to exclude electrons away from impurity sites. © 2004 Elsevier B.V. All rights reserved.

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Novel superfluidity in a trapped gas of fermi atoms with repulsive interaction loaded on an optical lattice

Machida M., Yamada S., Ohashi Y., Matsumoto H.

Physical Review Letters （Physical Review Letters）  93 （ 20 ） 200402(1-4) - 200402 2004年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  00319007

　概要を見る

The effects of the trap potential and the on-site repulsion on the pairing correlation in the confined gas of Fermi atoms placed in the optical lattice were investigated using exact diagonalization method. It was found that the local Mott insulation core develops due to the strong repulsion and the singlet pairing correlation grows remotely between the core edges. It was also found that the 'Mott insulation core' appeared in the center of the trap, where each site was occupied by one atom. The results show that the superfluidity can locally develop at the surrounding of the Mott core.

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Kohn's theorem in a superfluid Fermi gas with a Feshbach resonance

大橋 洋士

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  70 （ 6 ） 063613(1-13) - 63613 2004年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

The dipole mode in a superfluid gas of Fermi atoms trapped in a harmonic potential was analyzed. The frequency of this collective mode was not affected by an interaction between the atoms and was always equal to the trap frequency, according to Kohn's theorem. The effect of a Feshbach resonance and a condensate of the associated molecular bound states was also included. The dipole frequency was shown to depend on the strength of effective interaction associated with the Feshbach resonance, when the molecular bosons feel a different trap frequency from the Fermi atoms.

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トラップされたフェルミ原子気体の研究の現状と超流動の可能性

大橋 洋士

素粒子論研究 108   E28 2004年

研究論文（大学，研究機関等紀要）, 単著, 筆頭著者

Superfluid transition temperature in a trapped gas of Fermi atoms with a Feshbach resonance

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  67 （ 3 ） 033603(1-18) 2003年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

The effect of particle-particle fluctuations in conjunction with an atomic Feshbach resonance on the superfluid phase transition in a gas of Fermi atoms in a harmonic trap, was investigated using a simple local density approximation (LDA) approach. It was shown how the BCS-BEC crossover can be observed from characteristic change in the static atomic density profile, easily measured by standard techniques.

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Superfluidity and collective modes in a uniform gas of Fermi atoms with a Feshbach resonance

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  67 （ 6 ） 063612(1-24) - 6361224 2003年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10502947

　概要を見る

A study was performed on superfluidity and collective modes in a uniform gas of Fermi atoms with Feshbach resonance. The Goldstone mode associated with superfluidity was discussed. The results showed that the Goldstone mode appeared as a resonance in the spectrum of the density-density correlation function, which was experimentally accessible.

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BCS-BEC crossover in a trapped gas of Fermi atoms with a Feshbach resonance

Ohashi Y., Griffin A.

Physica B: Condensed Matter （Physica B: Condensed Matter）  329-333   40-41 - 41 2003年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  09214526

　概要を見る

We present a theoretical study of the BCS-BEC (Bose-Einstein condensation) crossover in a trapped gas of Fermi atoms including the effect of a Feshbach resonance. Going past the simple mean field approximation, we include the effect of fluctuations in the strong coupling limit involving two kinds of Bose molecules, i.e., Feshbach molecules associated with the resonance and preformed Cooper-pairs in which the pairing interaction is mediated by Feshbach quasi-molecules. The superfluid phase transition is shown to change continuously from the BCS-type to a BEC of these two kinds of Bosons, as the threshold energy of the Feshbach resonance is lowered. © 2003 Elsevier Science B.V. All rights reserved.

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Nuclear magnetic resonance in atomic-scale superconductor/magnet multilayered systems

Kanegae Y., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  72 （ 7 ） 1631-1634 - 1634 2003年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the nuclear spin-lattice relaxation rate (T 1T)-1 in atomic-scale superconductor/magnet multilayered systems and discuss the discrepancy between two recent (T1T) -1 experiments on Ru in RuSr2YCu2O8. When the magnetic layers are in the antiferromagnetic state, (T 1T)-1 in the magnetic layers is shown to decrease with decreasing temperature due to the excitation gap associated with the magnetic ordering. The proximity effect of superconductivity on (T1T) -1 in the magnetic layer is negligibly small. Our result indicates that the temperature dependence of (T1T)-1 on Ru in RuSr2YCu2O8 likely originates from the antiferromagnetism in the RuO2 layers, but not from the superconductivity in the CuO2 layers. © 2003 The Physical Society of Japan.

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Ni-substituted sites and the effect on Cu electron spin dynamics of YBa2Cu3-xNixO7-δ

Itoh Y., Adachi S., Machi T., Ohashi Y., Koshizuka N.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  66 （ 13 ） 1345111 - 1345115 2002年10月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  01631829

　概要を見る

We report a Cu nuclear quadrupole resonance experiment on magnetic impurity Ni-substituted YBa2Cu3-xNixO7-δ. The distribution of Ni-substituted sites and its effect on the Cu electron spin dynamics are investigated. Two samples with the same Ni concentration x = 0.10 and nearly the same oxygen content but different Tc's were prepared: One is an as-synthesized sample (7-δ=6.93) in an oxygen gas (Tc≈80 K), and the other is a quenched one (7-δ= 6.92) in a reduced oxygen atmosphere (Tc≈70 K). The plane-site 63Cu(2) nuclear spin-lattice relaxation for the quenched sample was faster than that for the as-synthesized sample, in contrast to the 63Cu(1) relaxation that was faster for the as-synthesized sample. This indicates that the density of plane-site Ni(2) is higher in the quenched sample, contrary to the chain-site Ni(1) density, which is lower in the quenched sample. From the analysis in terms of the Ni-induced nuclear spin-lattice relaxation, we suggest that the primary origin of suppression of Tc is associated with the nonmagnetic depairing effect of the plane-site Ni(2).

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Anomalous temperature dependence of spin susceptibility around a nonmagnetic impurity in high-T<inf>c</inf> cuprate superconductors

Ohashi Y.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  66 （ 5 ） 545221 - 545228 2002年08月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  01631829

　概要を見る

Local magnetic properties around a nonmagnetic impurity are investigated in the superconducting state of high-Tc cuprates. We consider a model two-dimensional dx2-y2-wave superconductor with strong antiferromagnetic (AF) spin fluctuations and calculate the spin susceptibility around the impurity. We show that the uniform susceptibility, which is usually suppressed in spin-singlet superconductivity, increases with decreasing temperature around the impurity when the impurity scattering is in the unitarity limit. This anomalous temperature dependence is more remarkable when the impurity potential is extended spatially and the AF spin fluctuations are stronger. Our results agree with the anomalous temperature dependence of the Knight shift on Li that was recently observed in YBa2(Cu1-xLix)3O6+y.

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BCS-BEC Crossover in a Gas of Fermi Atoms with a Feshbach Resonance

Ohashi Y., Griffin A.

Physical Review Letters （Physical Review Letters）  89 （ 13 ） 130402(1-4) 2002年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319007

　概要を見る

We discuss the BCS-BEC crossover in a degenerate Fermi gas of two hyperfine states interacting close to a Feshbach resonance. We show that, by including fluctuation contributions to the free energy similar to that considered by Nozières and Schmitt-Rink, the character of the superfluid phase transition continuously changes from the BCS-type to the BEC-type, as the threshold of the quasimolecular band is lowered. In the BEC regime, the superfluid phase transition is interpreted in terms of molecules associated with both the Feshbach resonance and Cooper pairing. © 2002 The American Physical Society.

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Bound State and Order Parameter Mixing Effect by Nonmagnetic Impurity Scattering in Two-band Superconductors

大橋 洋士

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  71 （ 8 ） 1978-1992 - 1992 2002年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate nonmagnetic impurity effects in two-band superconductors, focusing on the effects of interband impurity scatterings. Within the Born approximation, it is known that interband scatterings mix order parameters in the two bands. In particular, only one averaged energy gap appears in the excitation spectrum in the dirty limit [G. Gusman: J. Phys. Chem. Solids 28 (1967) 2327]. In this paper, we take into account the interband scattering within the t-matrix approximation beyond the Born approximation in the previous work. We show that, although the interband scattering is responsible for the mixing effect, this effect becomes weak when the interband scattering becomes very strong. In the strong interband scattering limit, a two-gap structure corresponding to two order parameters recovers in the superconducting density of states. We also show that a bound state appears around a nonmagnetic impurity depending on the phase of interband scattering potential.

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On the Fulde-Ferrell State in Spatially Isotropic Superconductors

大橋 洋士

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  71 （ 11 ） 2625-2628 - 2628 2002年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

Effects of superconducting fluctuations on the Fulde-Ferrell (FF) state are discussed in a spatially isotropic three-dimensional superconductor under a magnetic field. For this system, Shimahara recently showed that within the phenomenological Ginzburg-Landau theory, the long-range order of the FF state is suppressed by the phase fluctuation of the superconducting order parameter [H. Shimahara: J. Phys. Soc. Jpn. 67 (1998) 1872; Physica B 259-261 (1999) 492]. In this letter, we investigate this instability of the FF state against superconducting fluctuations from the microscopic viewpoint, employing the theory developed by Nozières and Schmitt-Rink in the BCS-BEC crossover field. Besides the absence of the second-order phase transition associated with the FF state, we show that even if the pairing interaction is weak, the shift of the chemical potential from the Fermi energy due to the fluctuations is crucial near the critical magnetic field of the FF state obtained within the mean-field theory.

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Stability of π-Phase in Atomic-Scale Superconductor/Magnet Multilayered System

Kanegae Y., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  70 （ 7 ） 2124 - 2133 2001年07月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

Atomic-scale superconductor/ferromagnet (S/F) and superconductor/antiferromagnet (S/AF) multilayered systems are investigated in order to clarify the stability of the π-phase where the phase difference of the superconducting (SC) order parameter between neighboring SC layers is π. We clarify the stable region of this phase in the phase diagram with respect to temperature and exchange field (B) in magnetic layers. In the phase diagram, we show that the π-phase appears in both the S/F and S/AF cases. In particular, although a previous work reported that the π-phase is not obtained for any reasonable value of B in the S/AF case where the band structure in the normal state of SC and magnetic layers are metallic and identical with each other, we find in this case that the stable region of the π-phase is as wide as that in the S/F case under a certain condition. We also investigate how the π-phase is affected by detailed band structures and the symmetry of SC order parameter, and discuss the possibility of this phase in the layered magnetic superconductor, RuSr2GdCu2O8.

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Local Enhancement of Spin Susceptibility around a Nonmagnetic Impurity in the Normal State of High-Tc Cuprate Superconductors

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  70 （ 7 ） 2054 - 2060 2001年07月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate local characters of spin susceptibility around a nonmagnetic impurity in the normal state of high-Tc cuprate superconductors. In a model two-dimensional system with strong antiferromagnetic (AF) spin fluctuations and with an extended impurity potential which is finite not only at the impurity site but also around it, we find that the AF susceptibility and the uniform one are strongly enhanced around the impurity. In particular, the uniform susceptibility which is almost temperature-independent in the clean system remarkably increases with decreasing temperature. We also show that the local enhancement of the susceptibility is due to (1) an increase of the local density of states at the Fermi level near the impurity site, and (2) a mode-mode coupling effect which is caused by the impurity potential. Our results can explain the recent Knight shift measurements in YBa2(Cu1-xZnx)3O6+y and YBa2(Cu1-xLix)3O6+y showing a Curie-Weiss temperature dependence of the uniform susceptibility at (around) the impurities, and also agree with the recent experiments on the nuclear spin-lattice relaxation rate in Zn-doped YBa2Cu3O6+y and YBa2Cu4O8 which observed anomalous enhancement of the low-energy AF spin fluctuations.

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Possible mechanism of Semiconductive temperature dependence of c-axis resistivity in the normal state of high-Tc superconductors: Renormalization effect of fermi surface by antiferromagnetic spin fluctuations

Terao M., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  70 （ 1 ） 233 - 240 2001年01月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  00319015

　概要を見る

We propose a mechanism of the semiconductive temperature dependence of the c-axis resistivity (ρc) in the normal state of high-Tc superconductors. We show that antiferromagnetic spin fluctuations in the CuO2-layer affect the temperature dependence of the Fermi surface shape and that they decrease (increase) the conductive carrier density (band mass) in the c-direction. Thus, when this effect dominates the temperature dependence of ρc, ρc increases at low temperatures. On the other hand, in the in-plane direction, the AF spin fluctuations do not affect the carrier density while they decrease the so-called k-mass at low temperatures. As a consequence, taking into account the damping rate which is caused by the electron-electron interaction, we obtain the in-plane resistivity (ρab) which decreases with decreasing temperature. Thus, the present mechanism can explain the semiconductive temperature dependence of ρc satisfying the metallic behavior of ρab.

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Nonmagnetic impurity effects on antiferromagnetic spin fluctuations in high-Tc superconductivity

大橋 洋士

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  357-360 （ SUPPL. 2 ） 74-77 - 77 2001年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  09214534

　概要を見る

We investigate nonmagnetic impurity effects on antiferromagnetic (AF) spin fluctuations in dx2-y2-wave superconductivity. We show that, far below the transition temperature (Tc), the low-energy AF spin fluctuations are enhanced by impurity scatterings in the unitarity limit owing to the formation of low-energy bound states around the impurities. As a result, the nuclear-spin lattice relaxation rate on Cu (1/(T1T)Cu) increases at low temperatures even below Tc. On the other hand, the enhancement of the AF spin fluctuations does not occur in the case of the Born scattering because of the absent of the bound states. The result in the unitarity limit can explain the anomalous increase of 1/(T1T)Cuwhich was recently observed in Zn-doped YBa2Cu4O8. © 2001 Elsevier Science B.V. All rights reserved.

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Nonmagnetic impurity effects on spectrum of antiferromagnetic spin fluctuations and nuclear spin-lattice relaxation rate in high-Tc superconductivity

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  69 （ 9 ） 2977 - 2992 2000年09月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We theoretically investigate antiferromagnetic (AF) spin fluctuations in impurity-doped high-Tcsuperconductivity. We assume a dx2-y2-wave superconductor with strong AF spin fluctuations, and take into account nonmagnetic impurities within the t-matrix approximation. In the clean system, the imaginary part of the AF spin susceptibility (Imχ(Q,ω)) is suppressed far below the excitation gap Eg= 2 × Max[Δp] (Δp: superconducting order parameter), while it has a peak slightly below Eg. On the other hand, in the presence of unitarity scatterers, it is shown that Imχ(Q,ω) far below Egincreases with decreasing temperature even in the superconducting state. As a result, at low temperatures, Imχ(Q,ω) has two peaks far and slightly below Egas observed in Zn-doped YBa2Cu3O6.97. In addition, because of the enhancement of the low-energy AF spin fluctuations, the nuclear spin-lattice relaxation rate on Cu ((1/T1T)Cu) also increases at low temperatures in contrast to the case of the clean superconductivity. This result can explain the recent experiment on (1/T1T)Cuin Zn-doped YBa2Cu4O8.

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Theory of Basov correlation between penetration depth and conductivity along the c-axis in high-Tc cuprate superconductors

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  69 （ 3 ） 659 - 662 2000年03月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We present a theoretical explanation for the correlation between the c-axis penetration depth and the c-axis conductivity (σc) in high-Tccuprates which was experimentally discovered by Basov et al. [Phys. Rev. B 50 (1994) 3511]. Our model is based on the BCS theory for the quasi-two-dimensional d-wave superconductivity in which the strength of the incoherency in the c-axis charge dynamics is controlled by potential scatterings. We show that the Basov correlation is realized when the difference in the magnitude of σcamong various high-Tc cuprate materials is dominated not by the damping of electrons but by the carrier density and/or the band mass along the c-axis. It is also shown that there exists a correlation between the c-axis Josephson plasma frequency and σcin high-Tccuprates.

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Temperature dependence of c-axis Josephson plasma oscillation in high superconductors

Ohashi Y., Takada S.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  61 （ 6 ） 4276 - 4288 2000年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10980121

　概要を見る

We present a microscopic theory for the c-axis transverse Josephson plasma oscillation in quasi-two-dimensional d-wave superconductors. In our model, the incoherent motion of electrons along the c axis are introduced by potential scattering with arbitrary scattering strength, and their depairing effect on superconductivity is determined self-consistently. We show that the plasma frequency decreases with increasing temperature in the presence of the scattering. In particular, when the scattering strength exceeds a critical value, it vanishes at the transition temperature (Formula presented) as is observed in (Formula presented) In this case, when the scattering is in the Born limit, the V-shaped superconducting density of states (DOS) can be obtained in spite of the impure system. This result can explain why the V-shaped DOS is observed in this material by tunneling experiment although the temperature-dependent plasma indicates the presence of a scattering. Our model also produces a good fit to experimental data on (Formula presented) From the fit, we find that both the superfluid and the normal electrons contribute to the plasma oscillation in this material. This feature is different from the ordinary Josephson plasma, which is the oscillation of only the superfluid component. © 2000 The American Physical Society.

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Collective phase oscillation in two-dimensional d-wave superconductors

Ohashi Y., Takada S.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  62 （ 9 ） 5971-5983 - 5983 2000年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  01631829

　概要を見る

We theoretically investigate the Carlson-Goldman (CG) mode in two-dimensional dx2-y2-wave superconductivity. In conventional s-wave superconductors, it is known that the CG mode is observed as a peak in the structure function of the pair field susceptibility S(q,ω) only just below the transition temperature Tc and only in dirty systems. On the other hand, in the d-wave state, we show that the peak structure can be observed down to T≁0.2Tc in clean systems. Furthermore, we also show that the peak splits into two when the direction of the momentum q deviates from the qx or qy axes, or nodal directions of the order parameter. These features originate from a singularity in the charge-density fluctuation due to the presence of the nodes of the d-wave order parameter: It enhances the screening of the long-range Coulomb interaction so that the CG mode can exist far below Tc. We also investigate a quasi-two-dimensional system, and show that the Josephson plasma can be observed in S(q,ω) when q is parallel to the c direction.

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Numerical renormalization group studies on single impurity Anderson model in superconductivity: A unified treatment of magnetic, nonmagnetic impurities, and resonance scattering

Yoshioka T., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  69 （ 6 ） 1812-1823 - 1823 2000年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate a single impurity Anderson model in superconducting state focusing on the localized excited states induced around the impurity site with use of the numerical renormalization group method. Using this model, we present a unified treatment of electronic states around various kinds of impurities embedded in conventional s-wave superconductivity that have been investigated independently so far: (1) magnetic impurity, (2) nonmagnetic impurity with potential scattering, and (3) nonmagnetic impurity with resonance scattering. In the case of the symmetric Anderson model, we clarify how the localized excited state induced around (1) is transformed into the one induced around (3) with a decrease of the strength of the on-site Coulomb repulsion at the impurity site. On the other hand, in the case of the asymmetric Anderson model, we show how the bound state induced by the magnetic impurity disappears when the impurity is gradually transformed from (1) into (2) with an increase of the energy of the impurity level. We also clarify the region where competition between the Kondo effect and superconductivity dominates the properties of the ground and the localized excited states in the parameter space of the Anderson model. In this region, we show that the bound state energy is scaled well by the single parameter TK/Δ (TK: Kondo temperature, Δ: superconducting order parameter) as in the case of the antiferromagnetic s-d interaction in superconductivity.

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Local antiferromagnetism in high-Tc cuprate superconductors and the magnetic phases of U-based heavy-fermion compounds

Ohashi Y.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  60 （ 22 ） 15388 - 15393 1999年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  10980121

　概要を見る

We investigate the possibility of the local induction of the antiferromagnetism (AF) in high-(Formula presented) superconductors and U-based heavy fermion compounds. In the former case, assuming the d-wave superconductivity in the bulk system, we find that AF can locally appear around surfaces and impurities and can coexist with the d-wave superconductivity. The induced AF is enhanced near the surface by the presence of superconductivity. Furthermore, the zero-energy bound state near the (110) surface of (Formula presented)-wave superconductors is split by the local AF, as was observed in (Formula presented) We also show in a model that this phenomenon is mathematically equivalent to (1) the surface (Formula presented)-wave superconductivity, which has been also proposed in the high-(Formula presented) cuprates, and (2) the local AF in the unconventional spin-density wave, which has been proposed as a candidate for the magnetism in (Formula presented) In particular, the case (2) can explain the recent experiment for (Formula presented) which observes that the temperature at which the antiferromagnetic moment appears is lower than the phase-transition temperature. © 1999 The American Physical Society.

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Ikeda and ohashi reply

Ikeda H., Ohashi Y.

Physical Review Letters （Physical Review Letters）  82 （ 25 ）  1999年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319007

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Monte Carlo Study on the Nuclear Magnetic Ordering in Scandium Crystal: Possibility of a Ferrimagnetic State

Kanegae Y., Ohashi Y., Ikeda H.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  68 （ 7 ） 2366-2372 - 2372 1999年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

Nuclear magnetic ordering is investigated in scandium crystal by the Monte Carlo simulation. We determine the phase diagram in the ground state with respect to interactions between the nuclear spins. In this phase diagram, we find a ferrimagnetic state with the total magnetization being smaller than the complete ferromagnetism by 1/3: This state changes into the ferromagnetism at a finite external magnetic field, which leads to a threefold jump in the magnetization as was observed by the experiment. Thus, this state is a strong candidate for the nuclear magnetic ordering in scandium below T ≲ 2[μK].

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Effect of the BCS supercurrent on the spontaneous surface flow in unconventional superconductivity with broken time-reversal-symmetry

Kusama Y., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  68 （ 3 ） 987-993 - 993 1999年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

A spontaneous surface current state is investigated in unconventional superconductivity with broken time-reversal-symmetry. It is shown that a BCS supercurrent lowers the energy of the system in this state. The lowest state has zero total current; it satisfies the Bloch theorem which states absence of the total current in any stable state with no external field. We also show that the magnitude and the direction of the spontaneous current are strongly affected by the detail of the Fermi surface. We explain this effect in connection with a low-energy bound state near the surface originating from the anisotropy of the order parameter.

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Theoretical studies on the observation of the low-frequency plasma oscillation and its coupling with the carlson-goldman mode in dirty quasi-two-dimensional superconductors

Ohashi Y., Takada S.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  59 （ 6 ） 4404-441 - 4411 1998年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10980121

　概要を見る

The plasma oscillation in a superconductor is investigated for the case when the plasma energy is smaller than the superconducting gap. We show that the plasma energy has a temperature dependence in dirty superconductors. When the strength of the impurity scattering exceeds a critical value, the plasma disappears at the transition temperature (Formula presented) as observed in high-(Formula presented) cuprates (Formula presented) We also study the coupling of the plasma with the Carlson-Goldman (CG) mode in a dirty superconductor. A pole analysis show that the coupling occurs just below (Formula presented) However, even in this situation, we find that two different modelike peak structures can be still observed in the spectra of the density-density correlation function (Formula presented) and the structure function of the pair field susceptibility (Formula presented) The peak in (Formula presented) is located at a finite energy at (Formula presented) corresponding to the low-frequency plasma, while the CG mode with a linear dispersion is found in (Formula presented) © 1999 The American Physical Society.

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On the Plasma Oscillation in Superconductivity

Ohashi Y., Takada S.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  67 （ 2 ） 551-559 - 559 1998年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

The plasma mode in the superconducting state is studied within the random phase approximation. It is proved that the plasma oscillation in the limit of zero momentum (g) is not affected by the superconductivity. This statement itself has been obtained so far under some special conditions where energies of the plasma frequency and the superconducting order parameter are much different. In this paper, we clarify that it holds true at all temperatures irrespective of (1) the magnitude of the plasma frequency, (2) the symmetry of the order parameter, and (3) details of the system, e.g., dimensionality and the layered structure. We also show that the plasma sum-rule is satisfied even in the superconducting state, which states that the spectrum of the density-density correlation function in the limit q → 0 is dominated by the plasma mode only Although the phase fluctuation of the order parameter couples with the charge one, the collective phase oscillation, so-called the Carlson-Goldman mode, does not contribute to the spectrum of the density-density correlation function in this limit.

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Ground State Properties and Localized Excited States around a Magnetic Impurity Described by the Anisotropic s-d Interaction in Superconductivity

Yoshioka T., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  67 （ 4 ） 1332-1341 - 1341 1998年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the electronic state around a magnetic impurity in the superconductivity in order to clarify how the anisotropy of the s-d interaction works in the presence of the superconducting energy gap. Using the numerical renormalization group method, we obtain regions induced by the anisotropy where two localized excited states with different energies appear at the same time; they cannot obtain as far as the isotropic interaction is considered. This means that the anisotropy of the s-d interaction works relevantly in some cases in the superconducting state. We also examine whether or not the bound state energy for the anisotropic and antiferromagnetic s-d interaction is scaled by TK/Δ (TK: Kondo temperature. Δ: superconducting order parameter), and find that it does not hold in the regions with two bound states.

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Theory of unconventional spin density wave: A possible mechanism of the micromagnetism in u-based heavy fermion compounds

Ikeda H., Ohashi Y.

Physical Review Letters （Physical Review Letters）  81 （ 17 ） 3723-3726 - 3726 1998年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319007

　概要を見る

We propose a novel spin density wave (SDW) state as a possible mechanism of the anomalous antiferromagnetism, the so called micromagnetism, in URu2Si2 below 17.5 K. In this new SDW, the electron-hole pair amplitude changes its sign in the momentum space as in the case of the unconventional superconductivity. It is shown that this state can be realized in an extended Hubbard model within the mean field theory. We also examine some characteristic properties of this SDW to compare with the experimental results. All these properties well explain the unsolved problem of the micromagnetism. © 1998 The American Physical Society.

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Quantum diffusion of the positive muon in superconducting tantalum

Kadono D., Kiefl R., Chakhalian J., Dunsiger S., Hitti B., MacFarlane W., Major J., Schimmele L., Matsumoto M., Ohashi Y.

Physical Review Letters （Physical Review Letters）  79 （ 1 ） 107-110 - 110 1997年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  00319007

　概要を見る

The tunneling probability for a positive muon in high-purity tantalum is significantly enhanced by the onset of superconductivity. This establishes the predominant influence of low energy infrared couplings to the conduction electrons in the quantum tunneling motion of a charged interstitial atom in metals. © 1997 The American Physical Society.

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Goldstone Mode in Charged Superconductivity: Theoretical Studies of the Carlson-Goldman Mode and Effects of the Landau Damping in the Superconducting State

Ohashi Y., Takada S.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  66 （ 8 ） 2437-2458 - 2458 1997年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the collective phase oscillations in superconductivity, i.e., the Carlson-Goldman mode in the charged system and the phason in the neutral, focusing on their velocities and how they appear in the structure function of the phase fluctuation of the order parameter. Our microscopic theory can cover from the clean to the dirty system, and furthermore it can treat the Landau damping correctly. Based on our formulation, we obtain following results: (1) The velocity of the phason is proportional to (TC- T)1/6(T ≲ TC) in the clean system, though it has been believed as (TC- T)1/4(TC: superconducting transition temperature). (2) The structure function has a central peak which grows as T → TC. This phenomenon is related to the central peak problem in the field of the structural phase transition. (3) The Carlson-Goldman mode can be observed by a tunneling experiment in case of the dirty system, while not in the clean. It is in contrast to the previous prediction. All of these results are found to be deeply related to the presence of the Landau damping as well as the screening effect, both of which are caused by the quasi-particles excited thermally.

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Unified Description of the Mid Gap States in Inhomogeneous Superconductors

Ohashi Y., Takada S.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  65 （ 1 ） 246 - 254 1996年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

Condition for the mid gap state to exist in inhomogeneous superconductor is investigated. On the basis of the Andreev equations, a general expression of the wavefunction of this bound state is derived. Then, the mid gap state is shown to exist only if signs of the pair potential is different, which an quasiparticle feels in the initial and final states in the scattering process described by the Andreev equations. It is also studied why some inhomogeneous systems have no mid gap state. Our results present a unified picture that the absence of the mid gap states results from their hybridization due to the spatial variation of the order parameter and the presence of the normal reflection.

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Unusual Proximity Effect of d-Wave Superconductivity

大橋 洋士

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  65 （ 3 ） 823-839 - 839 1996年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

A novel proximity effect, that the d-wave superconductivity induces a different symmetric order parameter in the normal metal side, is investigated for the normal metal (N)-d-wave superconductor (S) junction in the clean limit. The induced superconductivity is obtained self-consistently on the basis of the quasi-classical theory and the condition of this effect to occur is clarified. Our result shows that the deformation of the pair amplitude and the direction of the NS boundary with respect to the phase of the d-wave superconductivity are crucial in this phenomenon. A characteristic structure appearing in the local density of states due to this proximity effect is also examined comparing with the conventional case.

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Resonance State Localized around Nonmagnetic Impurity with Unitarity Scattering in Two-Dimensional d-Wave Superconductor

Onishi Y., Ohashi Y., Shingaki Y., Miyake K.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  65 （ 3 ） 675-678 - 678 1996年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  00319015

　概要を見る

Effects of nonmagnetic impurities with unitarity scattering in two-dimensional d-wave superconductors are investigated by the numerical diagonalization of finite-size systems of up to 35×35 sites. It is shown that a localized state with almost zero excitation energy is formed around an impurity site if it is isolated. However, its localized state is extended in real space along the direction of the node of the d-wave gap, so that the localized states of adjacent impurities easily interfere with each other forming a kind of impurity band, i.e., resonance states around the zero energy. It is also pointed out that there exists a strong association between the resonance density of states due to impurity scattering in the unitarity limit and the so-called zero-bias anomaly of tunneling conductance.

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On the Bloch Theorem Concerning Spontaneous Electric Current

Ohashi Y., Momoi T.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  65 （ 10 ） 3254-3259 - 3259 1996年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We study the Bloch theorem which states absence of the spontaneous current in interacting electron systems. This theorem is shown to be still applicable to the system with the magnetic field induced by the electric current. Application to the spontaneous surface current is also examined in detail. Our result excludes the possibility of the recently proposed d-wave superconductivity having the surface flow and finite total current.

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Theoretical proposal of a method to identify the symmetry of the order parameter in high-Tc superconductivity

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  64 （ 3 ） 887 - 912 1995年03月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

A new experimental method to judge the symmetry of the order parameter in high-Tc superconductivity is proposed. The interference of reflected holes from different directions due to the Andreev reflection causes quite different phenomena among s-,dx2-y2- and dxy-wave superconductivity. It is shown that this interference effect may be observed as a change of the conductance below Tc, which enables us to identify the symmetry of the pairing state in high-Tc superconductivity. Various cases in which the experimental setup deviates from the ideal situation are also studied and experimental condition for this method is clarified.

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Temperature Dependence of Uniform Susceptibility in High-Tc Superconductors

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  62 （ 10 ） 3702 - 3709 1993年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

The uniform susceptibility of high-Tc cuprates is studied by focusing on its anomalous temperature dependence. Our result shows that the uniform susceptibility has a temperature dependence due to the coupling with antiferromagnetic (AF) spin fluctuations. Furthermore, the difference of the temperature dependence between the lightly doped region and the overdoped region is explained with this mechanism as related to the strength of the AF spin fluctuations. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

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Pairing and Depairing Effects of Antiferromagnetic Spin Fluctuations in High-7c Superconductivity

Ohashi Y., Shiba H.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  62 （ 8 ） 2783 - 2802 1993年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

The role of antiferromagnetic (AF) spin fluctuations in high-rc superconductivity is studied by including both pairing and depairing effects. Thereby the possibility and nature of superconductivity mediated by AF spin fluctuations is examined. Since both of them are based on the same electron system, the superconductivity affects also the AF spin fluctuations as a feedback effect. The present paper discusses this mutual influence to find properties which may be a key to identify experimentally the symmetry of the order parameter. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

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Quantum Diffusion of MuonsInteracting with Superconduccting Electrons. I

M. Matsumoto and Y. Ohashi

J. Phys. Soc. Jpn. 62   2088-2097 1993年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

Quantum Diffusion of Muons Interacting with Superconducting Electrons. II

Ohashi Y., Matsumoto M.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  62 （ 10 ） 3532-3538 - 3538 1993年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

The band motion of muons in superconductors is studied in detail. Both the damping rate and the effective mass of the muon due to the interaction with conduction electrons are examined. Then, the average hopping rate of a muon is obtained by combining these quantities. The results are compared with that obtained by the double-well model and the relation between the two is discussed. In addition, as an extention of our theory, the quantum diffusion in some unconventional superconductors is also examined. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

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