論文 - 大橋　洋士

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

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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

　概要を見る

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

　概要を見る

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

　概要を見る

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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