論文 - 大橋　洋士

Tunneling properties of a bound pair of Fermi atoms in an optical lattice

Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  78 （ 6 ） 063617(1-9) 2008年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  10502947

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We investigate the tunneling properties of a bound pair of Fermi atoms in an optical lattice, comparing with results obtained in an attractive Hubbard model. In the strong-coupling regime of the Hubbard model, it has been predicted that the motion of a bound pair between lattice sites is accompanied by virtual dissociation. To explore the possibility of this interesting phenomenon in an optical lattice, we calculate the molecular wave function in a cosine-shaped periodic potential. We show that the molecular tunneling accompanied by dissociation occurs in the intermediate-coupling regime of the optical-lattice system. In the strong-coupling regime, in contrast to the prediction in the Hubbard model, the bound pair is shown to tunnel through the lattice potential without dissociation. As a result, the magnitude of the molecular band mass M remains finite even in the strong-coupling limit, which is in contrast to the diverging molecular mass in the case of the Hubbard model. Including this finite value of the molecular band mass, we evaluate the superfluid phase transition temperature Tc in the Bose-Einstein-condensate limit of the optical-lattice system, where the Hubbard model gives Tc =0 due to the diverging molecular mass. © 2008 The American Physical Society.

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Strong pairing and microscopic inhomogeneity of lattice fermion systems

Yamada S., Machida M., Ohashi Y., Matsumoto H.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  463-465 （ SUPPL. ） 103 - 106 2007年10月

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  09214534

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In order to study an interplay between electronic inhomogeneity and superconductivity as seen in high-Tc cuprates, we numerically examine attractive Hubbard model in the absence of the translation symmetry. We systematically calculate the particle density profile for both 1-D and 2-D attractive Hubbard models with harmonic potential wells using the exact diagonalization and the density-matrix renormalization group methods. The numerical results reveal that fine inhomogeneous zig-zag patterns universally emerge in the 1-D model case and the zig-zag structure becomes checkerboard type in the 2-D one. Moreover, it is numerically and theoretically found that such inhomogeneities are caused by double occupation component, i.e., pairs tightly bound on a site. © 2007 Elsevier B.V. All rights reserved.

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Vortex microscopic structure in BCS to BEC Fermi superfluids

Machida M., Koyama T., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  460-462 I （ SPEC. ISS. ） 275 - 276 2007年09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  09214534

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In order to examine how the vortex core changes its structure when approaching BEC from BCS regime in superfluid fermion-atom gasses we make a systematic numerical study for the core structure of a singly quantized vortex, solving numerically the generalized Bogoliubov-de Gennes equation derived from the fermion-boson model. Numerical calculations reveal that the vortex core can be well characterized by two length scales based on the spatial variation of the fermionic gap functions and those scales show distinctive differences between the BCS and BEC regimes. © 2007 Elsevier B.V. All rights reserved.

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Superfluid density and condensate fraction in the BCS-BEC crossover regime at finite temperatures

Fukushima N., Ohashi Y., Taylor E., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  75 （ 3 ） 033609(1-10) 2007年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

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The superfluid density is a fundamental quantity describing the response to a rotation as well as in two-fluid collisional hydrodynamics. We present extensive calculations of the superfluid density Ïs in the BCS-BEC crossover regime of a uniform superfluid Fermi gas at finite temperatures. We include strong-coupling or fluctuation effects on these quantities within a Gaussian approximation. We also incorporate the same fluctuation effects into the BCS single-particle excitations described by the superfluid order parameter Î" and Fermi chemical potential Î, using the NoziÃresâ€"Schmitt-Rink approximation. This treatment is shown to be necessary for consistent treatment of Ïs over the entire BCS-BEC crossover. We also calculate the condensate fraction Nc as a function of the temperature, a quantity which is quite different from the superfluid density Ïs. We show that the mean-field expression for the condensate fraction Nc is a good approximation even in the strong-coupling BEC regime. Our numerical results show how Ïs and Nc depend on temperature, from the weak-coupling BCS region to the BEC region of tightly bound Cooper pair molecules. In a companion paper, we derive an equivalent expression for Ïs from the thermodynamic potential, which exhibits the role of the pairing fluctuations in a more explicit manner. © 2007 The American Physical Society.

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Ultra Large-scale Exact-diagonalization for Confined Fermion-Hubbard Model on the Earth Simulator: Exploration of Superfluidity in Confined Strongly-Correlated Systems

S. Yamada, T. Imamura, T. Kano, Y. Ohashi, H. Matsumoto, and M Machida

Journal of the Earth Simulator 7   23-35 2007年

研究論文（学術雑誌）, 共著, 査読有り

Spin-polarized Fermi superfluids as Bose-Fermi mixtures

Taylor E., Griffin A., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  76 （ 2 ） 023614(1-12) 2007年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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In the strong-coupling Bose-Einstein condensation (BEC) region where a Feshbach resonance gives rise to tightly bound dimer molecules, we show that a spin-polarized Fermi superfluid reduces to a simple Bose-Fermi mixture of Bose-condensed dimers and the leftover unpaired fermions. Using a many-body functional integral formalism, the Gaussian fluctuations give rise to an induced dimer-dimer interaction mediated by the unpaired fermions, with the dimer-fermion vertex being given by the (mean-field) Born approximation. Treating the pairing fluctuations to quartic order, we show how the action for a spin-polarized Fermi superfluid reduces to one for a Bose-Fermi mixture. This Bose-Fermi action includes an expression for the effective dimer-unpaired fermion interaction in a spin-polarized Fermi superfluid beyond the Born approximation, in the BEC limit. In the low-density limit, we show how this dimer-fermion interaction gives the s -wave scattering length aBF =1.18 aF (aF is the s -wave fermion scattering length), a result first derived by Skorniakov and Ter-Martirosian in 1957 for three interacting fermions. © 2007 The American Physical Society.

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Superfluid density in the BCS-BEC crossover regime of a two-component Fermi gas at finite temperatures

Ohashi Y.

Journal of Superconductivity and Novel Magnetism （Journal of Superconductivity and Novel Magnetism）  20 （ 7-8 ） 609-612 - 612 2007年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  15571939

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We investigate the superfluid carrier density ρ s in the BCS-BEC crossover regime of a superfluid Fermi gas at finite temperatures. Including pairing fluctuations within the gaussian fluctuation approximation, we determine the superfluid order parameter Δ, Fermi chemical potential μ, as well as ρ s , in a consistent manner. In the weak-coupling regime, the normal fluid density ρ n ρ-ρ s is dominated by quasi-particle excitations associated with the dissociation of Cooper pairs. As one approaches the BCS-BEC crossover regime, effects of pairing fluctuations on ρ n become strong. In the strong-coupling BEC regime, ρ n is shown to be dominated by Bogoliubov phonon in a molecular Bose superfluid. © 2007 Springer Science+Business Media, LLC.

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On-site pairing and microscopic inhomogeneneity in confined lattice fermion systems

Machida M., Yamada S., Ohashi Y., Matsumoto H.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  74 （ 5 ） 053621(1-6) 2006年12月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

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In order to explore a universal feature of atomic Fermi gases loaded on optical lattices, we make a systematic study of the particle density profile for one- and two-dimensional attractive Hubbard models with harmonic potential wells. Using the exact diagonalization scheme and the density-matrix renormalization group technique, we show that fine inhomogeneous zigzag patterns universally emerge in the above models. Theoretical and numerical analyses suggest that these structures originate from an effective repulsive interaction between tightly bound pairs and a breakdown of translational invariance. Furthermore, it is emphasized that the pattern obtained numerically in the two-dimensional model is the checkerboard type, which is very similar to results recently observed in a vortex core of high- Tc cuprate superconductor. © 2006 The American Physical Society.

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(LT24 proceedings) BCS-BEC Crossover in a Gas of Fermi Atoms with a P-wave Feshbach Resonance

Y. Ohashi

AIP proceedings (LT24) 850   65-66 2006年12月

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  0094243X

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Itinerant-localized dual character of a strongly correlated superfluid Bose gas in an optical lattice

Ohashi Y., Kitaura M., Matsumoto H.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  73 （ 3 ） 033617(1-5) 2006年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  10502947

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We investigate a strongly correlated Bose gas in an optical lattice. Extending the standard-basis operator method developed by Haley and Erdös to a boson Hubbard model, we calculate excitation spectra in the superfluid phase, as well as in the Mott insulating phase, at T=0. In the Mott phase, the excitation spectrum has a finite energy gap, reflecting the localized character of atoms. In the superfluid phase, the excitation spectrum is shown to have an itinerant-localized dual structure, where the gapless Bogoliubov mode (which describes the itinerant character of superfluid atoms) and a band with a finite energy gap coexist. We also show that the rf-tunneling current measurement would give useful information about the duality of a strongly correlated superfluid Bose gas near the superfluid-insulator transition. © 2006 The American Physical Society.

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Excitations in the BCS-BEC crossover regime of a superfluid Fermi gas with a Feshbach resonance

Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  437-438 （ SPEC. ISS. ） 234-238 - 238 2006年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  09214534

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We investigate single-particle excitations at T = 0 in the BCS-BEC crossover regime of a superfluid Fermi gas. We solve the Bogoliubov-de Gennes equations in a trap, including a tunable pairing interaction associated with a Feshbach resonance. We show that the single-particle energy gap Eg is dominated by the lowest-energy Andreev bound state localized at the surface of the gas, which is much smaller than the superfluid order parameter over(Δ, ̃) (r = 0) at the center of the trap. We also calculate the rf-tunneling current spectrum IF(ω). In a broad Feshbach resonance, the peak energy in IF(ω) is shown to give, not Eg, but over(Δ, ̃) (r = 0). In a narrow resonance, significant spectral weight coming from the low-energy Andreev states hides the appearance of the peak in IF(ω) at ω ≃ over(Δ, ̃) (r = 0). Our results for a broad resonance is in good agreement with the recent rf-tunneling measurement in a superfluid 6Li. © 2006 Elsevier B.V. All rights reserved.

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Vortex structure from BCS to BEC superfluid

Machida M., Koyama T., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  437-438 （ SPEC. ISS. ） 190-194 - 194 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

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In atomic Fermi gases, the pairing character changes from BCS-like to BEC-like when decreasing the threshold energy of the Feshbach resonance. With this crossover, the system enters a strong-coupling regime through the population enhancement of diatom molecules, i.e., tightly bound Cooper pair, and the vortex structure shows much different features in contrast to the well-known core structure in BCS superfluid. In this paper, we study the structure of a single quantized vortex by numerically solving the generalized Bogoliubov-de Gennes equation derived from the boson-fermion model and clarify how the vortex structure changes with the crossover from BCS to BEC. As a result of numerical calculations, we find that the presence of the diatom molecular condensate enhances the matter density depletion inside the vortex core in the crossover regime and the Caroli-de Gennes-Matricon (CdGM) quasi-particle branch almost diminishes in BEC regime. © 2005 Elsevier B.V. All rights reserved.

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Spin correlation and superfluidity of trapped fermi atoms on an optical lattice

Machida M., Yamada S., Ohashi Y., Matsumoto H.

AIP Conference Proceedings （AIP Conference Proceedings）  850   61-62 - 62 2006年

研究論文（国際会議プロシーディングス）, 共著, 査読有り

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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.

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Effect of quantum correction in the Bose-Hubbard model

Matsumoto H., Takahashi K., Ohashi Y.

AIP Conference Proceedings （AIP Conference Proceedings）  850   53-54 - 54 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

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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.

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Reflectivity in the multi-Josephson junction model with Josephson vortex lattice

Matsumoto H., Suzuki Y., Ohashi Y.

AIP Conference Proceedings （AIP Conference Proceedings）  850   925-926 - 926 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り

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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.

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Microscopic structure of a quantized vortex core in atomic Fermi gases

Machida M., Ohashi Y., Koyama T.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  74 （ 2 ） 023621(1-5) 2006年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

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In order to understand what kind of information can be extracted from the vortex-core structure in superfluid fermion-atom gases we make a systematic study for the core structure of a singly quantized vortex in the range of broad Feshbach resonance, solving numerically the generalized Bogoliubov-de Gennes equation derived from the fermion-boson model. Numerical calculations reveal that the vortex core can be well characterized by three length scales from the spatial variation of the fermionic and bosonic gap functions. We also predict that the profile of matter depletion and the quasiparticle spectrum show distinctive differences between the BCS and Bose-Einstein Condensate regimes. © 2006 The American Physical Society.

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Novel pairing in the Hubbard model with confinement potential

Machida M., Yamada S., Ohashi Y., Matsumoto H.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  445-448 （ 1-2 ） 90-93 - 93 2006年

研究論文（国際会議プロシーディングス）, 共著, 査読有り,  ISSN  09214534

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We explore a novel type of pairing in the repulsive Hubbard model with a confinement potential. Such an undertaking situation including the confinement potential is realized in a part of intense spatial modulations observed in High-Tc cuprate superconductors, while an atomic Fermi gas loaded on an optical lattice is described by the Hubbard model with the confinement potential. In this paper, applying the exact diagonalization method to a one-dimensional Hubbard model with the confinement 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 between two fermions. In this case, a 'Mott core' appears in the confinement center, where each site is occupied by one atom, and the Cooper-pair function strongly develops between fermions in the left and right hand sides of this core. © 2006 Elsevier B.V. All rights reserved.

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Vortex structure in weak to strong coupling superconductors: Crossover from BCS to BEC

Machida M., Koyama T., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  445-448 （ 1-2 ） 194-197 - 197 2006年

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  09214534

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High-Tc cuprate and MgB2 superconductors are recognized as intermediate or strong coupling superconductors. In such a non-weak regime, the vortex core structure is an interesting issue, since the vortex dissipation is expected to be anomalously reduced due to large level distance between localized core states. In this paper, we systematically clarify the vortex core structure based on the fermion-boson model which can cover a full coupling range from weak BCS to strong BEC superconductors. The mean-field calculations on the model reveal that the distances between the low-lying core levels expand and the low-lying states finally disappear when changing the coupling from weak BCS to strong BEC regime. This result suggests that the strong coupling makes the vortex motion non-dissipative. © 2006 Elsevier B.V. All rights reserved.

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BCS-BEC crossover in a superfluid Fermi gas

Ohashi Y.

International Journal of Modern Physics B （International Journal of Modern Physics B）  20 （ 30-31 ） 5204-5213 - 5213 2006年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り,  ISSN  02179792

　概要を見る

We discuss the superfluid phase transition in a gas of Fermi atoms with a Feshbach resonance. A tunable pairing interaction associated with the Feshbach resonance is shown to naturally lead to the BCS-BEC crossover, where the character of superfluidity continuously changes from the weak-coupling Bardeen-Cooper-Schrieffer (BCS) type to the Bose-Einstein condensation (BEC) of tightly bound molecules, as one decreases the threshold energy 2ν of the Feshbach resonance. We also discuss effects of a trap, as well as the p-wave BCS-BEC crossover adjusted by a p-wave Feshbach resonance. © World Scientific Publishing Company.

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Pairing fluctuations and the superfluid density through the BCS-BEC crossover

Taylor E., Griffin A., Fukushima N., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  74 （ 6 ） 063626(1-15) 2006年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10502947

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We derive an expression for the superfluid density of a uniform two-component Fermi gas through the BCS-BEC crossover in terms of the thermodynamic potential in the presence of an imposed superfluid flow. Treating the pairing fluctuations in a Gaussian approximation following the approach of Nozières and Schmitt-Rink, we use this definition of ρs to obtain an explicit result which is valid at finite temperatures and over the full BCS-BEC crossover. It is crucial that the BCS gap Δ, the chemical potential μ, and ρs all include the effect of fluctuations at the same level in a self-consistent manner. We show that the normal fluid density ρn n- ρs naturally separates into a sum of contributions from Fermi BCS quasiparticles (ρnF) and Bose collective modes (ρnB). The expression for ρnF is just Landau's formula for a BCS Fermi superfluid but now calculated over the BCS-BEC crossover. The expression for the Bose contribution ρnB is more complicated and only reduces to Landau's formula for a Bose superfluid in the extreme BEC limit, where all the fermions have formed stable Bose pairs and the Bogoliubov excitations of the associated molecular Bose condensate are undamped. In a companion paper, we present numerical calculations of ρs using an expression equivalent to the one derived in this paper, over the BCS-BEC crossover, including unitarity, and at finite temperatures. © 2006 The American Physical Society.

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