論文 - 大橋　洋士

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

　概要を見る

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

　概要を見る

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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