Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s10909-022-02794-w

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Other Link： https://link.springer.com/article/10.1007/s10909-022-02794-w/fulltext.html

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

DOI： 10.1103/physreva.106.012405

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.106.012405/fulltext

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.91.054601

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

We study collective modes and superfluidity of spin-1 bosons with antiferromagnetic interactions in an optical lattice based on the time-dependent Ginzburg-Landau (TDGL) equation derived from the spin-1 Bose-Hubbard model. Specifically, we examine the stability of supercurrents in the polar phase in the vicinity of the Mott insulating phase with even filling factors. Solving the linearized TDGL equation, we obtain gapless spin-nematic modes and gapful spin-wave modes in the polar phase that arise due to the breaking of S-2 symmetry in spin space. Supercurrents exhibit dynamical instabilities induced by growing collective modes. In contrast to the second-order phase transition, the critical momentum for mass currents is finite at the phase boundary of the first-order superfluid-Mott insulator (SF-MI) phase transition. Furthermore, the critical momentum remains finite throughout the metastable SF phase and approaches zero towards the phase boundary, at which the metastable SF state disappears. We also study the stability of spin currents motivated by recent experiments for spinor gases. The critical momentum for spin currents is found to be zero, where a spin-nematic mode causes the dynamical instability. We investigate the origin of the zero critical momentum for spin currents and find it attributed to the fact that the polar state becomes energetically unstable even in the presence of an infinitesimal spin current. We discuss implications of the zero critical momentum for spin currents for the stability of the polar state.

DOI： 10.1103/physreva.103.043305

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.103.043305/fulltext

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

DOI： 10.1103/physreva.100.063612

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Language：English   Publishing type：Research paper (scientific journal)  

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.98.094503

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Other Link： https://link.aps.org/article/10.1103/PhysRevB.98.094503

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study effects of a potential barrier on collective modes of superfluid Bose gases in optical lattices. We assume that the barrier is created by local suppression of the hopping amplitude. When the system is in a close vicinity of the Mott transition at commensurate fillings, where an approximate particle-hole symmetry emerges, there exist bound states of Higgs amplitude mode that are localized around the barrier. By applying the Gutzwiller mean-field approximation to the Bose-Hubbard model, we analyze properties of normal modes of the system with a special focus on the Higgs bound states. We show that when the system moves away from the Mott transition point, the Higgs bound states turn into quasibound states due to inevitable breaking of the particle-hole symmetry. We use a stabilization method to compute the resonance energy and linewidth of the quasibound states. We compare the results obtained by the Gutzwiller approach with those by the Ginzburg-Landau theory. We find that the Higgs bound states survive even in a parameter region far from the Mott transition, where the Ginzburg-Landau theory fails.

DOI： 10.1103/PhysRevA.96.043606

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Motivated by recent developments in the experimental study of superconducting graphene and transition metal dichalcogenides, we investigate superconductivity of the Kane-Mele (KM) model with short-range attractive interactions on the two-dimensional honeycomb lattice. We show that intravalley spin-triplet pairing arises from nearest-neighbor (NN) attractive interaction and the intrinsic spin-orbit coupling. We demonstrate this in two independent approaches: We study superconducting instability driven by condensation of Cooperons, which are in-gap bound states of two conduction electrons, within the T-matrix approximation and also study the superconducting ground state within the mean-field theory. We find that Cooperons with antiparallel spins condense at the K and K' points. This leads to the emergence of an intravalley spin-triplet pairing state belonging to the irreducible representation A(1) of the point group C-6v. The fact that this pairing state has opposite chirality for K and K' identifies this state as a "helical" valley-triplet state, the valley analog to the He-3-B phase in two dimensions. Because of the finite center of mass momentum of Cooper pairs, the pair amplitude in NN bonds exhibits spatial modulation on the length scale of lattice constant, such that this pairing state may be viewed as a pair-density wave state. We find that the pair amplitude spontaneously breaks the translational symmetry and exhibits a p-Kekule pattern. We also discuss the selection rule for pairing states focusing the characteristic band structure of the KM model and the Berry phase effects to the emergence of the intravalley pairing state.

DOI： 10.1103/PhysRevB.94.104508

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We find the angular Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states (or the twisted kink crystals) in which a phase and an amplitude of a pair potential modulate simultaneously in a quasi-one-dimensional superconducting ring with a static Zeeman magnetic field applied on the ring and static Aharonov-Bohm magnetic flux penetrating the ring. The superconducting ring with magnetic flux produces a persistent current, whereas the Zeeman split of Fermi energy results in the spatial modulation of the pair potential. We show that these two magnetic fields stabilize the FFLO phase in a large parameter region of the magnetic fields. We further draw the phase diagram with the two kinds of first-order phase transitions; one corresponds to phase slips separating the Aharonov-Bohm magnetic flux, and the other separates the number of peaks of the pair amplitude for the Zeeman magnetic field.

DOI： 10.1103/PhysRevB.92.224512

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study collective modes of superfluid Bose gases in optical lattices combined with potential barriers. We assume that the system is in the vicinity of the quantum phase transition to a Mott insulator at a commensurate filling, where emergent particle-hole symmetry gives rise to two types of collective mode, namely a gapless Nambu-Goldstone (NG) phase mode and a gapful Higgs amplitude mode. We consider two kinds of potential barrier: One does not break the particle-hole symmetry while the other does. In the presence of the former barrier, we find Higgs bound states that have binding energies lower than the bulk Higgs gap and are localized around the barrier. We analyze tunneling properties of the NG mode incident to both barriers to show that the latter barrier couples the Higgs bound states with the NG mode, leading to Fano resonance mediated by the bound states. Thanks to the universality of the underlying field theory, it is expected that Higgs bound states may be present also in other condensed-matter systems with a particle-hole symmetry and spontaneous breaking of a continuous symmetry, such as quantum dimer antiferromagnets, superconductors, and charge-density-wave materials.

DOI： 10.1103/PhysRevA.92.043610

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

We present an analytic method to approach Eilenberger equation and the associated Bogoliubov-de Gennes equation for quasi-1D fermionic gases. The problem of finding self-consistent inhomogeneous condensates is reduced to solving a certain class of nonlinear Schrodinger equations, whose most general solitonic solution is indeed available. Previously known solutions can be retrieved by taking appropriate limits in the parameters. The applicability of the method extends to systems with population imbalance and subject to external potential. In particular we show that fermionic zero-modes are robust against population imbalance.

DOI： 10.1007/s10909-013-0982-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

We study elementary excitations of spin-1 bosons with antiferromagnetic interaction in an optical lattice by applying the Gutzwiller approximation to the spin-1 Bose-Hubbard model. There appear various excitations associated with spin degrees of freedom in the Mott-insulator (MI) phase as well as in the superfluid (SF) phase. In this system, the ground state in the MI phase is known to exhibit a remarkable effect of even-odd parity of particle filling, in which even fillings stabilize the MI state due to formation of spin-singlet pairs. We find that excitation spectra in the MI phase exhibit characteristic features that reflect the even-odd parity effect of the ground state. We clarify evolution of elementary excitations across the quantum critical point of the SF-MI transition.

DOI： 10.1007/s10909-013-0960-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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 K-40 Fermi gas [Feld et al., Nature 480, 75 (2011)]. 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.

DOI： 10.1103/PhysRevA.88.013637

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the Higgs amplitude mode in the s-wave superfluid state on the honeycomb lattice inspired by recent cold atom experiments. We consider the attractive Hubbard model and focus on the vicinity of a quantum phase transition between semimetal and superfluid phases. On either side of the transition, we find collective mode excitations that are stable against decay into quasiparticle pairs. In the semimetal phase, the collective modes have "Cooperon" and exciton character. These modes smoothly evolve across the quantum phase transition, and become the Anderson-Bogoliubov mode and the Higgs mode of the superfluid phase. The collective modes are accommodated within a window in the quasiparticle-pair continuum, which arises as a consequence of the linear Dirac dispersion on the honeycomb lattice, and allows for sharp collective excitations. Bragg scattering can be used to measure these excitations in cold atom experiments, providing a rare example wherein collective modes can be tracked across a quantum phase transition.

DOI： 10.1103/PhysRevB.88.014527

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

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 T (c). 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.

DOI： 10.1007/s10909-012-0691-7

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Language：English   Publishing type：Research paper (scientific journal)  

We study the superfluid state of atomic fermions in a tunable optical kagome lattice motivated by recent experiments. We show that the imposed superflow induces spatial modulations in the density and order parameter of the pair condensate and leads to a charge modulated superfluid state analogous to a supersolid state. The spatial modulations in the superfluid emerge due to the geometric effect of the kagome lattice that introduces anisotropy in hopping amplitudes of fermion pairs in the presence of superflow. We also study superflow instabilities and find that the critical current limited by the dynamical instability is quite enhanced due to the large density of states associated with the flatband. The charge modulated superfluid state can sustain high temperatures close to the transition temperature that is also enhanced due to the flatband and is therefore realizable in experiments. © 2013 American Physical Society.

DOI： 10.1103/PhysRevLett.110.145304

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The chiral Gross-Neveu model or equivalently the linearized Bogoliubov-de Gennes equation has been mapped to the nonlinear Schrodinger (NLS) hierarchy in the Ablowitz-Kaup-Newell-Segur formalism by Correa, Dunne and Plyushchay. We derive the general expression for exact fermionic solutions for all gap functions in the arbitrary order of the NLS hierarchy. We also find that the energy spectrum of the n-th NLS hierarchy generally has n + 1 gaps. As an illustration, we present the self-consistent two-complex-kink solution with four real parameters and two fermion bound states. The two kinks can be placed at any position and have phase shifts. When the two kinks are well separated, the fermion bound states are localized around each kink in most parameter region. When two kinks with phase shifts close to each other are placed at distance as short as possible, the both fermion bound states have two peaks at the two kinks, i.e., the delocalization of the bound states occurs. (c) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physletb.2012.10.058

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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 Delta(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 T-c, 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 T-c. 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.

DOI： 10.1103/PhysRevA.86.063603

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Motivated by recent experiments on atomic Dirac fermions in a tunable honeycomb optical lattice, we study the attractive Hubbard model of superfluidity in the anisotropic honeycomb lattice. At weak coupling, we find that the maximum mean-field pairing transition temperature, as a function of density and interaction strength, occurs for the case with isotropic hopping amplitudes. In this isotropic case, we go beyond mean-field theory and study collective fluctuations, treating both pairing and density fluctuations for interaction strengths ranging from weak to strong coupling. We find evidence for a sharp sound mode, together with a well-defined Leggett mode over a wide region of the phase diagram. We also calculate the superfluid order parameter and collective modes in the presence of nonzero superfluid flow. The flow-induced softening of these collective modes leads to dynamical instabilities involving stripelike density modulations as well as a Leggett-mode instability associated with the natural sublattice symmetry-breaking charge-ordered state on the honeycomb lattice. The latter provides a nontrivial test for the experimental realization of the one-band Hubbard model. We delineate regimes of the phase diagram where the critical current is limited by depairing or by such collective instabilities, and discuss experimental implications of our results. © 2012 American Physical Society.

DOI： 10.1103/PhysRevA.86.033604

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Motivated by recent experiments on atomic Dirac fermions in a tunable honeycomb optical lattice, we study the attractive Hubbard model of superfluidity in the anisotropic honeycomb lattice. At weak coupling, we find that the maximum mean-field pairing transition temperature, as a function of density and interaction strength, occurs for the case with isotropic hopping amplitudes. In this isotropic case, we go beyond mean-field theory and study collective fluctuations, treating both pairing and density fluctuations for interaction strengths ranging from weak to strong coupling. We find evidence for a sharp sound mode, together with a well-defined Leggett mode over a wide region of the phase diagram. We also calculate the superfluid order parameter and collective modes in the presence of nonzero superfluid flow. The flow-induced softening of these collective modes leads to dynamical instabilities involving stripelike density modulations as well as a Leggett-mode instability associated with the natural sublattice symmetry-breaking charge-ordered state on the honeycomb lattice. The latter provides a nontrivial test for the experimental realization of the one-band Hubbard model. We delineate regimes of the phase diagram where the critical current is limited by depairing or by such collective instabilities, and discuss experimental implications of our results.

DOI： 10.1103/PhysRevA.86.033604

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Publisher：Physical Review A - Atomic, Molecular, and Optical Physics  

DOI： 10.1103/PhysRevA.85.039908

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Language：English   Publisher：AMER PHYSICAL SOC  

DOI： 10.1103/PhysRevA.85.039908

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Language：Japanese   Publisher：素粒子論グループ 素粒子論研究 編集部  

DOI： 10.24532/soken.119.4C_F77

CiNii Books

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

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 up arrow-spin atoms are localized around it (where the up arrow-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.

DOI： 10.1088/1742-6596/400/1/012027

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

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 T-c. We show that the spatial region where single-particle excitations are dominated by the pseudogap may still exist even below T-c, 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 Li-6 Fermi gas.

DOI： 10.1088/1742-6596/400/1/012080

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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 T(c), 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.

DOI： 10.1103/PhysRevA.84.043647

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We discuss an idea to realize a spontaneous current in a superfluid Fermi gas. When a polarized Fermi superfluid (N(up arrow) > N(down arrow), where N(sigma) is the number of atoms in the hyperfine state described by pseudospin sigma = up arrow, down arrow ) is loaded onto a ring-shaped trap with a weak potential barrier, some excess atoms (Delta N = N(up arrow) - N(down arrow)) are localized around the barrier. As shown in our previous paper [T. Kashimura, S. Tsuchiya, and Y. Ohashi, Phys. Rev. A 82, 033617 (2010)], this polarized potential barrier works as a pi 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 pi 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.

DOI： 10.1103/PhysRevA.84.013609

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study spin imbalance effects on the Larkin-Ovchinnikov-Fulde-Ferrel (LOFF) state relevant for superconductors under a strong magnetic field and spin polarized ultracold Fermi gas. We obtain the exact solution for the condensates with arbitrary spin imbalance and the fermion spectrum perturbatively in the presence of small spin imbalance. We also obtain fermion zero mode exactly without perturbation theory.

DOI： 10.1103/PhysRevB.84.024503

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Language：English   Publishing type：Research paper (international conference proceedings)  

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.

DOI： 10.1109/ISAS.2011.5960915

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Language：Japanese  

DOI： 10.24532/soken.118.4_D64

CiNii Books

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Language：Japanese  

DOI： 10.24532/soken.118.4_D54

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Language：English   Publishing type：Research paper (international conference proceedings)  

We investigate the stability of π-phase in a polarized superfluid Fermi gas (N↑ &gt
N↓, where Nσ is 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.

DOI： 10.1109/ISAS.2011.5960916

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We investigate pseudogap behaviors of an ultracold Fermi gas in the BCS-BEC crossover above the super-fluid transition temperature T-c. We calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS-BEC crossover region, including strong pairing fluctuations within a T-matrix approximation. We show that the pseudogap structure in DOS develops from the weak-coupling BCS region, and continuously changes into a fully gapped one in the strong-coupling BEC region. We determine the pseudogap temperature T+ at which the dip structure in DOS disappears, and identify the pseudogap region in the phase diagram in terms of temperatures and pairing interactions. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.11.043

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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 T(c). We show how the pseudogap appearing in the normal phase above T(c) changes into the superfluid gap over the entire BCS-BEC crossover region, as one passes through T(c). (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.10.037

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We investigate single-particle excitations and strong-coupling effects in the BCS-BEC crossover regime of a superfluid Fermi gas. Including phase and amplitude fluctuations of the superfluid order parameter within a T-matrix theory, we calculate the superfluid density of states (DOS), as well as single-particle spectral weight, over the entire BCS-BEC crossover region below the superfluid transition temperature Tc. We clarify how the pseudogap in the normal state evolves into the superfluid gap, as one passes through Tc. While the pseudogap in DOS continuously evolves into the superfluid gap in the weak-coupling BCS regime, the superfluid gap in the crossover region is shown to appear in DOS after the pseudogap disappears below Tc. In the phase diagram with respect to the temperature and interaction strength, we determine the region where strong pairing fluctuations dominate over single-particle properties of the system. Our results would be useful for the study of strong-coupling phenomena in the BCS-BEC crossover regime of a superfluid Fermi gas. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevA.82.043630

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate single-particle excitations and strong-coupling effects in the BCS-BEC crossover regime of a superfluid Fermi gas. Including phase and amplitude fluctuations of the superfluid order parameter within a T -matrix theory, we calculate the superfluid density of states (DOS), as well as single-particle spectral weight, over the entire BCS-BEC crossover region below the superfluid transition temperature T-c. We clarify how the pseudogap in the normal state evolves into the superfluid gap, as one passes through T-c. While the pseudogap in DOS continuously evolves into the superfluid gap in the weak-coupling BCS regime, the superfluid gap in the crossover region is shown to appear in DOS after the pseudogap disappears below T-c. In the phase diagram with respect to the temperature and interaction strength, we determine the region where strong pairing fluctuations dominate over single-particle properties of the system. Our results would be useful for the study of strong-coupling phenomena in the BCS-BEC crossover regime of a superfluid Fermi gas.

DOI： 10.1103/PhysRevA.82.043630

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the photoemission-type spectrum in a cold Fermi gas which was recently measured by the JILA group [Stewart et al., Nature (London) 454, 744 (2008)]. 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.

DOI： 10.1103/PhysRevA.82.033629

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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-up arrow > N-down arrow, where N-sigma is the number of atoms with pseudospin sigma = up arrow, down arrow), this barrier is shown to be magnetized in the sense that excess up arrow-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 pi 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.

DOI： 10.1103/PhysRevA.82.033617

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Language：Japanese   Publisher：物性研究刊行会  

この論文は国立情報学研究所の電子図書館事業により電子化されました。

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10942835

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

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.

DOI： 10.1007/s10909-009-9953-4

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Publishing type：Research paper (scientific journal)  

We study the stability of superfluid Fermi gases in deep optical lattices in the BCS-Bose-Einstein condensation (BEC) crossover at zero temperature. Within the tight-binding attractive Hubbard model, we calculate the spectrum of the low-energy Anderson-Bogoliubov (AB) mode as well as the single-particle excitations in the presence of superfluid flow in order to determine the critical velocities. To obtain the spectrum of the AB mode, we calculate the density response function in the generalized random-phase approximation applying the Green's function formalism developed by Côté and Griffin to the Hubbard model. We find that the spectrum of the AB mode is separated from the particle-hole continuum having the characteristic rotonlike minimum at short wavelength due to the strong charge-density-wave fluctuations. The energy of the rotonlike minimum decreases with increasing the lattice velocity and it reaches zero at the critical velocity which is smaller than the pair-breaking velocity. This indicates that the superfluid state is energetically unstable due to the spontaneous emission of the short-wavelength rotonlike excitations of the AB mode instead due to pair breaking. We determine the critical velocities as functions of the interaction strength across the BCS-BEC crossover regime. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevA.80.063627

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the stability of superfluid Fermi gases in deep optical lattices in the BCS-Bose-Einstein condensation (BEC) crossover at zero temperature. Within the tight-binding attractive Hubbard model, we calculate the spectrum of the low-energy Anderson-Bogoliubov (AB) mode as well as the single-particle excitations in the presence of superfluid flow in order to determine the critical velocities. To obtain the spectrum of the AB mode, we calculate the density response function in the generalized random-phase approximation applying the Green's function formalism developed by Cote and Griffin to the Hubbard model. We find that the spectrum of the AB mode is separated from the particle-hole continuum having the characteristic rotonlike minimum at short wavelength due to the strong charge-density-wave fluctuations. The energy of the rotonlike minimum decreases with increasing the lattice velocity and it reaches zero at the critical velocity which is smaller than the pair-breaking velocity. This indicates that the superfluid state is energetically unstable due to the spontaneous emission of the short-wavelength rotonlike excitations of the AB mode instead due to pair breaking. We determine the critical velocities as functions of the interaction strength across the BCS-BEC crossover regime.

DOI： 10.1103/PhysRevA.80.063627

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Language：English   Publishing type：Research paper (scientific journal)  

We investigate strong-coupling effects on normal-state properties of an ultracold Fermi gas. Within the framework of T -matrix approximation in terms of pairing fluctuations, we calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS-Bose-Einstein condensate (BEC) crossover region above the superfluid phase-transition temperature Tc. Starting from the weak-coupling BCS regime, we show that the so-called pseudogap develops in DOS above Tc, which becomes remarkable in the crossover region. The pseudogap structure continuously changes into a fully gapped one in the strong-coupling BEC regime, where the gap energy is directly related to the binding energy of tightly bound molecules. We determine the pseudogap temperature Tc where the dip structure in DOS vanishes. The value of Tc is shown to be very different from another characteristic temperature T□□ where a BCS-type double-peak structure disappears in the spectral weight. While one finds T□ &gt
□□ in the BCS regime, T□ becomes higher than T□ in the crossover region and BEC regime. Including this, we determine the pseudogap region in the phase diagram of ultracold Fermi gases. Our results would be useful in the search for the pseudogap region in ultracold L6 i and K 40 Fermi gases. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevA.80.033613

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate strong-coupling effects on normal-state properties of an ultracold Fermi gas. Within the framework of T-matrix approximation in terms of pairing fluctuations, we calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS-Bose-Einstein condensate (BEC) crossover region above the superfluid phase-transition temperature T-c. Starting from the weak-coupling BCS regime, we show that the so-called pseudogap develops in DOS above T-c, which becomes remarkable in the crossover region. The pseudogap structure continuously changes into a fully gapped one in the strong-coupling BEC regime, where the gap energy is directly related to the binding energy of tightly bound molecules. We determine the pseudogap temperature T* where the dip structure in DOS vanishes. The value of T* is shown to be very different from another characteristic temperature T** where a BCS-type 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 region and BEC regime. Including this, we determine the pseudogap region in the phase diagram of ultracold Fermi gases. Our results would be useful in the search for the pseudogap region in ultracold Li-6 and K-40 Fermi gases.

DOI： 10.1103/PhysRevA.80.033613

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the tunneling of Bogoliubov excitations through a barrier in a Bose-Einstein condensate. We extend our previous work [S. Tsuchiya and Y. Ohashi, Phys. Rev. A 78, 013628 (2008)] to the case when condensate densities are different between the left and right of the barrier potential. In the framework of the Bogoliubov mean-field theory, we calculate the transmission probability and phase shift, as well as the energy flux and quasiparticle current carried by Bogoliubov excitations. We find that Bogoliubov phonons twist the condensate phase due to a back-reaction effect, which induces the Josephson supercurrent. While the total current given by the sum of quasiparticle current and induced supercurrent is conserved, the quasiparticle current flowing through the barrier potential is shown to be remarkably enhanced in the low-energy region. When the condensate densities are different between the left and right of the barrier, the excess quasiparticle current, as well as the induced supercurrent, remains finite far away from the barrier. We also consider the tunneling of excitations and atoms through the boundary between the normal and superfluid regions. We show that supercurrent can be generated inside the condensate by injecting free atoms from outside. On the other hand, atoms are emitted when the Bogoliubov phonons propagate toward the phase boundary from the superfluid region.

DOI： 10.1103/PhysRevA.79.063619

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We discuss tunneling properties of Bogoliubov excitations through a potential barrier. We find that the quasi-particle current carried by Bogoliubov excitations is remarkably enhanced near the potential barrier at low incident energies. This explains the increase of the transmission probability of Bogoliubov phonons in the low energy region. In addition, taking into account of the backreaction of Bogoliubov excitations on Bose condensates, we show that the quasi-particle current twists the phase of the condensate wavefunction, leading to a Josephson supercurrent through the barrier.

DOI： 10.1088/1742-6596/150/3/032113

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Publishing type：Research paper (international conference proceedings)  

Critical velocities of superfluid Fermi gases in optical lattices are theoretically investigated across the BCS-BEC crossover. We calculate the excitation spectra in the presence of a superfluid flow in one- and two-dimensional optical lattices. It is found that the spectrum of low-lying Anderson-Bogoliubov (AB) mode exhibits a roton-like structure in the short-wavelength region due to the strong charge density wave fluctuations, and with increasing the superfluid velocity one of the roton-like minima reaches zero before the single-particle spectrum does. This means that superfluid Fermi gases in optical lattices are destabilized due to spontaneous emission of the roton-like AB mode instead of due to Cooper pair breaking.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

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.

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Publishing type：Research paper (international conference proceedings)  

Critical velocities of superfluid Fermi gases in optical lattices are theoretically investigated across the BCS-BEC crossover. We calculate the excitation spectra in the presence of a superfluid flow in one- and two-dimensional optical lattices. It is found that the spectrum of low-lying Anderson-Bogoliubov (AB) mode exhibits a roton-like structure in the short-wavelength region due to the strong charge density wave fluctuations, and with increasing the superfluid velocity one of the roton-like minima reaches zero before the single-particle spectrum does. This means that superfluid Fermi gases in optical lattices are destabilized due to spontaneous emission of the roton-like AB mode instead of due to Cooper pair breaking.

DOI： 10.1088/1742-6596/150/3/032128

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate tunneling properties of Bogoliubov mode in a Bose-Einstein condensate. Using an exactly solvable model with a delta-functional barrier, we show that each component in the two-component wave function (u,v) of low-energy Bogoliubov phonon has the same form as the condensate wave function in the supercurrent state. As a result, the currents J(u) and J(v) associated with u and v, respectively, have the same tunneling properties as those of supercurrent carried by condensate. Thus, the tunneling of low-energy Bogoliubov phonon described by the tunneling of these two currents shows perfect transmission. We also show that the supercurrent behaviors of Bogoliubov phonon still exist in the presence of supercurrent carried by condensate, except in the critical supercurrent state. In the critical current state, the perfect transmission is absent, because J(u) or J(v) exceeds their upper limit given by the critical value of the supercurrent associated with the condensate. Our results consistently explain the recently proposed two tunneling phenomena associated with Bogoliubov phonon, namely, the anomalous tunneling effect (perfect transmission in the low-energy limit) and the breakdown of the perfect transmission in the critical supercurrent state.

DOI： 10.1103/PhysRevA.78.043601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate tunneling properties of Bogoliubov phonons in a Bose-Einstein condensate. We find the anomalous enhancement of the quasiparticle current J(q) carried by Bogoliubov phonons near a potential barrier, due to the supply of the excess current from the condensate. This effect leads to the increase of quasiparticle transmission probability in the low energy region found by Kovrizhin et al. We also show that the quasiparticle current twists the phase of the condensate wave function across the barrier, leading to a finite Josephson supercurrent J(s) through the barrier. This induced supercurrent flows in the opposite direction to the quasiparticle current so as to cancel out the enhancement of J(q) and conserve the total current J=J(q)+J(s).

DOI： 10.1103/PhysRevA.78.013628

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The excitations of a two-dimensional (2D) Bose-Einstein condensate in the presence of a soliton are studied by solving the Kadomtsev-Petviashvili equation which is valid when the velocity of the soliton approaches the speed of sound. The excitation spectrum is found to contain states which are localized near the soliton and have a dispersion law similar to the one of the stable branch of transverse oscillations of a 1D gray soliton in a 2D condensate. By using the stabilization method we show that these localized excitations behave as resonant states coupled to the continuum of free excitations of the condensate.

DOI： 10.1103/PhysRevA.77.045601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

The small oscillations of solitons in 2D Bose-Einstein condensates are investigated by solving the Kadomtsev-Petviashvili equation which is valid when the velocity of the soliton approaches the speed of sound. We show that the soliton is stable and that the lowest excited states obey the same dispersion law as the one of the stable branch of excitations of a 1D gray soliton in a 2D condensate. The role of these states in thermodynamics is discussed.

DOI： 10.1007/s10909-007-9400-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

We study elementary excitations of Bose-Einstein condensates in a one-dimensional periodic potential and discuss the stability of superfluid flow based on the Kronig-Penney model. We analytically solve the Bogoliubov equations and calculate the excitation spectrum. The Landau and dynamical instabilities occur in the first condensate band when the superfluid velocity exceeds certain critical values as in a sinusoidal potential. It is found that the onset of the Landau instability coincides with the point where the perfect transmission of low-energy excitations is forbidden, while the dynamical instability occurs when the effective mass is negative. The condensate band has a swallow-tail structure when the periodic potential is shallow compared to the mean-field energy. We find that the upper side of a swallow-tail is dynamically unstable although the excitation spectrum has a linear dispersion reflecting the positive effective mass.

DOI： 10.1007/s10909-007-9394-x

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Language：English   Publisher：AMER PHYSICAL SOC  

In their recent paper [Phys. Rev. A 71, 033622 (2005)], Seaman studied Bloch states of the condensate wave function in a Kronig-Penney potential and calculated the band structure. They argued that the effective mass is always positive when a swallowtail energy loop is present in the band structure. In this Comment, we reexamine their argument by actually calculating the effective mass. It is found that there exists a region where the effective mass is negative even when a swallowtail is present. Based on this fact, we discuss the interpretation of swallowtails in terms of superfluidity.

DOI： 10.1103/PhysRevA.76.017601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the stability of Bose-Einstein condensates with superfluid currents in a one-dimensional periodic potential. By using the Kronig-Penney model, the condensate and Bogoliubov bands are analytically calculated and the stability of condensates in a periodic potential is discussed. The Landau and dynamical instabilities occur in a Kronig-Penney potential when the quasimomentum of the condensate exceeds certain critical values as in a sinusoidal potential. It is found that the onsets of the Landau and dynamical instabilities coincide with the point where the perfect transmission of low energy excitations through each potential barrier is forbidden. The Landau instability is caused by the excitations with small q and the dynamical instability is caused by the excitations with q=pi/a at their onsets, where q is the quasimomentum of excitation and a is the lattice constant. A swallow-tail energy loop appears at the edge of the first condensate band when the mean-field energy is sufficiently larger than the strength of the periodic potential. We find that the upper portion of the swallow-tail is always dynamically unstable, but the second Bogoliubov band has a phonon spectrum reflecting the positive effective mass.

DOI： 10.1103/PhysRevA.75.033612

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MAIK NAUKA/INTERPERIODICA/SPRINGER  

We study spin-1 bosons in an optical lattice under a magnetic field with the Gutzwiller approximation for the Bose-Hubbard model. Phase boundary curves between superfluids and Mott insulators depend continuously on the magnetic field, and this provides better results than those obtained with the perturbative mean-field approximation. The phase boundary curve as a function of magnetic field has a sharp cusp structure under certain circumstances. In superfluid phases, both the spin magnetizations and fluctuations in the total number of bosons show strong magnetic field dependence, which is related to the fact that both first-and second-order transitions appear on the phase boundary curve according to the magnetic field.

DOI： 10.1134/S1054660X07010112

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We study the first- and second-order superfluid-Mott insulator transitions of spin-1 bosons in an optical lattice under a magnetic field by the Gutzwiller approximation. We find that, in contrast to previous perturbative studies, the phase boundary curve continuously changes as a function of magnetic field. We also find a sharp cusp structure on the phase boundary curve under some circumstances. When the phase boundary curve deviates from that obtained with the perturbative studies, the phase transition is a first-order one. We further clarify the specific features of the phase transition in the present system. The order parameters exactly at the boundary point, where the transition changes from first to second order, remain finite under a finite magnetic field and continuously vanish as the magnetic field approaches zero.

DOI： 10.1143/JPJS.75.074601

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

We investigate the excitation spectrum of a Bose-Einstein condensate in a Kronig-Penney potential. We solve the Bogoliubov equations analytically and obtain the band structure of the excitation spectrum. We find that the excitation spectrum is gapless and linear at low energies. This property is found to be attributed to the anomalous tunneling behavior of low energy excitations, which has been predicted by Kagan et al.

DOI： 10.1063/1.2354597

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

With use of the Kronig-Penney model, we study the excitation spectrum of a Bose-Einstein condensate in a one-dimensional periodic potential. We solve the Bogoliubov equations analytically and obtain the band structure of the excitation spectrum for arbitrary values of the lattice depth. We find that the excitation spectrum is gapless and linear at low energies, and that it is due to the anomalous tunneling of low-energy excitations, predicted by Kagan et al.

DOI： 10.1103/PhysRevA.72.053611

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the Landau damping of Bogoliubov excitations in two- and three-dimensional optical lattices at finite temperatures, extending our recent work on one-dimensional (1D) optical lattices. We use a Bose-Hubbard tight-binding model and the Popov approximation to calculate the temperature dependence of the number of condensate atoms n(c0)(T) in each lattice well. As with 1D optical lattices, damping only occurs if the Bogoliubov excitations exhibit anomalous dispersion (i.e., the excitation energy bends upward at low momentum), analogous to the case of phonons in superfluid (4)He. This leads to the disappearance of all damping processes in a D-dimensional simple cubic optical lattice when Un(c0)>= 6DJ, where U is the on-site interaction, and J is the hopping matrix element.

DOI： 10.1103/PhysRevA.72.053621

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We study the superfluid-Mott-insulator transition of antiferromagnetic spin-1 bosons in an optical lattice described by a Bose-Hubbard model. Our variational study with the Gutzwiller variational wave function determines that the superfluid-Mott-insulator transition is a first-order one at a part of the phase boundary curve, contrary to the spinless case.

DOI： 10.1103/PhysReVLett.94.110403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

The antiferromagnetic spin-1 bosons in an optical lattice are studied by the Gutzwiller approximation. We find that the superfluid-Mott insulator transition is of a first-order on a part of the phase boundary.

DOI： 10.1007/s10909-005-2277-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the superfluid-Mott insulator (SF-MI) transition of spin-1 bosons interacting antiferromagnetically in an optical lattice. Starting from a Bose-Hubbard tight-binding model for spin-1 bosons, we obtain the zero-temperature phase diagram by a mean-field approximation. We find that the MI phase with an even number of atoms per site is a spin singlet state, while the MI phase with an odd number of atoms per site has spin 1 at each site in the limit of t=0, where t is the hopping matrix element. We also show that the superfluid phase is a polar state as in the case for a spin-1 Bose condensate in a harmonic trap. It is found that the MI phase is strongly stabilized against the SF-Ml transition when the number of atoms per site is even, due to the formation of singlet pairs. We derive the effective spin Hamiltonian for the MI phase with one atom per site and briefly discuss the spin order in the MI phase.

DOI： 10.1103/PhysRevA.70.043628

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Extending recent work to finite temperatures, we calculate the Landau damping of a Bogoliubov excitation in an optical lattice, due to the coupling to a thermal cloud of such excitations. For simplicity, we consider a one-dimensional Bose-Hubbard model and restrict ourselves to the first energy band. For energy conservation to be satisfied, the excitations in the collision processes must exhibit "anomalous dispersion," analogous to phonons in superfluid (4)He. This leads to the disappearance of all damping processes when Un(c0)greater than or equal to6J, where U is the on-site interaction, J is the hopping matrix element, and n(c0)(T) is the number of condensate atoms at a lattice site. This phenomenon also occurs in two-dimensional and three-dimensional optical lattices. The disappearance of Beliaev damping above a threshold wave vector is noted.

DOI： 10.1103/PhysRevA.70.023611

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We investigate the properties of skyrmion in the ferromagnetic state of spin-1 Bose-Einstein condensates by means of the mean-field theory and show that the size of a skyrmion is fixed to the order of the healing length. It is shown that the interaction between two skyrmions with oppositely rotating spin textures is attractive when their separation is large, following a unique power-law behavior with a power of -7/2.

DOI： 10.1143/JPSJ.70.1182

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.1.0_669

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DOI： 10.11316/jpsgaiyo.70.1.0_566

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DOI： 10.11316/jpsgaiyo.69.2.2.0_53_1

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DOI： 10.11316/jpsgaiyo.69.1.4.0_720_4

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Language：Japanese   Publisher：一般社団法人 日本物理学会  

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DOI： 10.11316/jpsgaiyo.67.2.2.0_177_2

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