論文 - 大橋　洋士

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

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© 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

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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

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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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