研究者詳細 - 大橋　洋士

論文 - 大橋　洋士

分割表示 208 件中 1 - 20 件目／全件表示 >>

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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

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.

Access to Document (DOI)

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

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

© 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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

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

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.

Access to Document (DOI)

　前のページ - 次のページ　

大橋 洋士

論文 - 大橋 洋士

大橋　洋士

論文 - 大橋　洋士