Published papers

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.

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

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

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

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

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.

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

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

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

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.

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.

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

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

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

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

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

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

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.