論文 - 大橋　洋士

BCS-BEC crossover in a gas of Fermi atoms with a p-wave Feshbach resonance

大橋 洋士

Physical Review Letters （Physical Review Letters）  94 （ 5 ） 050403(1-4) 2005年02月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319007

　概要を見る

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.

• Access to Document (DOI)

Single-particle excitations in a trapped gas of Fermi atoms in the BCS-BEC crossover region

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  72 （ 1 ） 013601(1-25) 2005年

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

　概要を見る

We investigate the single-particle properties at T=0 of a trapped superfluid gas of Fermi atoms with a Feshbach resonance. A tunable pairing interaction associated with the Feshbach resonance leads to the BCS Bose-Einstein condensate (BEC) crossover, where the character of superfluidity continuously changes from the BCS-type to a BEC of composite bosons. In this paper, we extend our previous work for a uniform superfluid Fermi gas [Y. Ohashi and A. Griffin, Phys. Rev. A 67, 063612 (2003)] to include the effect of a harmonic trap. We do not use the local density approximation (LDA), but directly solve the Bogoliubov de Gennes (BdG) coupled equations. While our explicit numerical solutions are for a weak (narrow) Feshbach resonance, we argue that the single-particle BdG excitation spectrum will exhibit the same features for a strong (broad) Feshbach resonance. Using these equations, we find self-consistent values for the spatially dependent local density n(r) as well as the composite BCS order parameter, the latter describing both the Cooper-pair and molecular condensate contributions. Using these results, we calculate the single-particle density of states in the crossover region, and from this determine the true single-particle energy gap (Eg) of the trapped Fermi superfluid. This is associated with the in-gap (or Andreev) states in the low-density region at the edge of the trap. We calculate the laser-induced tunneling current I into another hyperfine state, as measured in recent rf spectroscopy experiments. This rf spectrum gives a direct probe of the quasiparticle spectrum. We show how the high-energy part of I gives information about (r=0) at the center of the trap (which is comparable to the Fermi energy F in the crossover region). We show that I is very dependent on the spatial profile of the pair potential (r) that is used. We also emphasize that the narrow cunpaired atom" peak in the rf data gives information about Eg and the low-energy F in-gap states of a Fermi superfluid. While our calculations are limited to T=0, we use them to discuss the recent data of Chin et al. and the LDA calculations of Trm and co-workers. The LDA, while useful, can lead to an incorrect physical picture of the low-density surface region of the Fermi superfluid. © 2005 The American Physical Society.

• Access to Document (DOI)

Single-particle excitations in a trapped gas of Fermi atoms in the BCS-BEC crossover region. II. Broad Feshbach resonance

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  72 （ 6 ） 063606(1-8) 2005年

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

　概要を見る

We apply the formulation developed in a recent paper [Y. Ohashi and A. Griffin, Phys. Rev. A 72, 013601 (2005)] for single-particle excitations in the BCS-BEC crossover to the case of a broad Feshbach resonance. At T=0, we solve the Bogoliubov-de Gennes coupled equations taking into account a Bose condensate of bound states (molecules). In the case of a broad resonance, the density profile n(r), as well as the profile of the superfluid order parameter Δ(r), are spatially spread out to the Thomas-Fermi radius, even in the crossover region. This order parameter Δ(r) suppresses the effects of low-energy Andreev bound states on the rf tunneling current. As a result, the peak energy in the rf spectrum is found to occur at an energy equal to the superfluid order parameter Δ(r=0) at the center of the trap, in contrast to the case of a narrow resonance, and in agreement with recent measurements. The local density approximation is found to give a good approximation for the rf-tunneling spectrum. © 2005 The American Physical Society.

• Access to Document (DOI)

Effective interaction between molecules in the strong-coupling BEC regime of a superfluid fermi gas

Yoji Ohashi

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  74 （ 10 ） 2659-2662 - 2662 2005年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the effective interaction between Cooper-pair molecules in the strong-coupling BEC regime of a superfluid Fermi gas with a Feshbach resonance. Our work uses a path integral formulation and a renormalization group (RG) analysis of fluctuations in a single-channel model. We show that a physical cutoff energy ωc originating from the finite molecular binding energy is the key to understanding the interaction between molecules in the BEC regime. Our work thus clari.es recent results by showing that a M = 2aF is a bare molecular scattering length while a M = (0.6-0.75)aF is the low-energy molecular scattering length renormalized to include high-energy scattering up to ωc (here, aF is the scattering length between Fermi atoms). We also include many-body effects at finite temperatures. We find that aM is strongly dependent on temperature, vanishing at Tc, consistent with the earlier Bose gas results of Bijlsma and Stoof. ©2005 The Physical Society of Japan.

• Access to Document (DOI)

フェルミ原子ガスにおける新しい超流動

大橋 洋士

素粒子論研究 111   C21 2005年

研究論文（大学，研究機関等紀要）, 単著, 筆頭著者

New Fermion Superfluidity and the BCS-BEC crossover in a trapped gas of Fermi atoms with a Feshbach resonance

Y. Ohashi

素粒子論研究 112   B54 2005年

研究論文（大学，研究機関等紀要）, 単著

Theory of superconductor-insulator transition in organic superconductor λ -(BETS)2FexGa1-xCl4

Terao M., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  412-414 （ SPEC. ISS. ） 324 - 328 2004年10月

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

　概要を見る

We investigate superconductivity and magnetism in the organic conductor λ-(BETS)2FexGa1-xCl4. In this material, it is known that the d-electrons on Fe-ions are ordered antiferromagnetically (TN<8.5-9 K) for 0.4<x ≤ 1, while superconductivity (TSC<6 K) of π-electrons appears when 0 ≤ x<0.4. In this paper, including the detailed band structure of this material, we discuss the exchange interaction between the d- and π-electrons, focusing on its role in the antiferromagnetism and superconductivity. We show that the inclusion of only this π-d exchange interaction is difficult to explain both the observed x-dependence of TN and TSC. To understand the phase diagram of λ-(BETS)2FexGa 1-xCl4, we point out that the Coulomb interaction between π-electrons is also important. © 2004 Elsevier B.V. All rights reserved.

• Access to Document (DOI)

Effects of interband impurity scattering on superconducting density of states in a two-band superconductor

大橋 洋士

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  412-414 （ SPEC. ISS. ） 41-45 - 45 2004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

　概要を見る

We investigate interband impurity scattering effects on a double gap structure in the superconducting density of states (SDOS) in a two-band superconductor. Using the self-consistent t-matrix theory, we show how the double gap structure gradually disappears/revives when impurity concentration and the magnitude of impurity potential are varied. The increase of impurity concentration leads to an averaged single gap SDOS due to the mixing of two bands by interband scattering. However, this mixing effect is shown to disappear in the strong interband scattering limit. We also show that the intraband impurity scattering suppresses the interband scattering effect because it tends to exclude electrons away from impurity sites. © 2004 Elsevier B.V. All rights reserved.

• Access to Document (DOI)

Novel superfluidity in a trapped gas of fermi atoms with repulsive interaction loaded on an optical lattice

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

Physical Review Letters （Physical Review Letters）  93 （ 20 ） 200402(1-4) - 200402 2004年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  00319007

　概要を見る

The effects of the trap potential and the on-site repulsion on the pairing correlation in the confined gas of Fermi atoms placed in the optical lattice were investigated using exact diagonalization method. It was found that the local Mott insulation core develops due to the strong repulsion and the singlet pairing correlation grows remotely between the core edges. It was also found that the 'Mott insulation core' appeared in the center of the trap, where each site was occupied by one atom. The results show that the superfluidity can locally develop at the surrounding of the Mott core.

• Access to Document (DOI)

Kohn's theorem in a superfluid Fermi gas with a Feshbach resonance

大橋 洋士

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  70 （ 6 ） 063613(1-13) - 63613 2004年

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

　概要を見る

The dipole mode in a superfluid gas of Fermi atoms trapped in a harmonic potential was analyzed. The frequency of this collective mode was not affected by an interaction between the atoms and was always equal to the trap frequency, according to Kohn's theorem. The effect of a Feshbach resonance and a condensate of the associated molecular bound states was also included. The dipole frequency was shown to depend on the strength of effective interaction associated with the Feshbach resonance, when the molecular bosons feel a different trap frequency from the Fermi atoms.

• Access to Document (DOI)

トラップされたフェルミ原子気体の研究の現状と超流動の可能性

大橋 洋士

素粒子論研究 108   E28 2004年

研究論文（大学，研究機関等紀要）, 単著, 筆頭著者

Superfluid transition temperature in a trapped gas of Fermi atoms with a Feshbach resonance

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  67 （ 3 ） 033603(1-18) 2003年

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

　概要を見る

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.

• Access to Document (DOI)

Superfluidity and collective modes in a uniform gas of Fermi atoms with a Feshbach resonance

Ohashi Y., Griffin A.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  67 （ 6 ） 063612(1-24) - 6361224 2003年

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

　概要を見る

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.

• Access to Document (DOI)

BCS-BEC crossover in a trapped gas of Fermi atoms with a Feshbach resonance

Ohashi Y., Griffin A.

Physica B: Condensed Matter （Physica B: Condensed Matter）  329-333   40-41 - 41 2003年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  09214526

　概要を見る

We present a theoretical study of the BCS-BEC (Bose-Einstein condensation) crossover in a trapped gas of Fermi atoms including the effect of a Feshbach resonance. Going past the simple mean field approximation, we include the effect of fluctuations in the strong coupling limit involving two kinds of Bose molecules, i.e., Feshbach molecules associated with the resonance and preformed Cooper-pairs in which the pairing interaction is mediated by Feshbach quasi-molecules. The superfluid phase transition is shown to change continuously from the BCS-type to a BEC of these two kinds of Bosons, as the threshold energy of the Feshbach resonance is lowered. © 2003 Elsevier Science B.V. All rights reserved.

• Access to Document (DOI)

Nuclear magnetic resonance in atomic-scale superconductor/magnet multilayered systems

Kanegae Y., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  72 （ 7 ） 1631-1634 - 1634 2003年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate the nuclear spin-lattice relaxation rate (T 1T)-1 in atomic-scale superconductor/magnet multilayered systems and discuss the discrepancy between two recent (T1T) -1 experiments on Ru in RuSr2YCu2O8. When the magnetic layers are in the antiferromagnetic state, (T 1T)-1 in the magnetic layers is shown to decrease with decreasing temperature due to the excitation gap associated with the magnetic ordering. The proximity effect of superconductivity on (T1T) -1 in the magnetic layer is negligibly small. Our result indicates that the temperature dependence of (T1T)-1 on Ru in RuSr2YCu2O8 likely originates from the antiferromagnetism in the RuO2 layers, but not from the superconductivity in the CuO2 layers. © 2003 The Physical Society of Japan.

• Access to Document (DOI)

Ni-substituted sites and the effect on Cu electron spin dynamics of YBa2Cu3-xNixO7-δ

Itoh Y., Adachi S., Machi T., Ohashi Y., Koshizuka N.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  66 （ 13 ） 1345111 - 1345115 2002年10月

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  01631829

　概要を見る

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).

• Access to Document (DOI)

Anomalous temperature dependence of spin susceptibility around a nonmagnetic impurity in high-T<inf>c</inf> cuprate superconductors

Ohashi Y.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  66 （ 5 ） 545221 - 545228 2002年08月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  01631829

　概要を見る

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.

• Access to Document (DOI)

BCS-BEC Crossover in a Gas of Fermi Atoms with a Feshbach Resonance

Ohashi Y., Griffin A.

Physical Review Letters （Physical Review Letters）  89 （ 13 ） 130402(1-4) 2002年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り,  ISSN  00319007

　概要を見る

We discuss the BCS-BEC crossover in a degenerate Fermi gas of two hyperfine states interacting close to a Feshbach resonance. We show that, by including fluctuation contributions to the free energy similar to that considered by Nozières and Schmitt-Rink, the character of the superfluid phase transition continuously changes from the BCS-type to the BEC-type, as the threshold of the quasimolecular band is lowered. In the BEC regime, the superfluid phase transition is interpreted in terms of molecules associated with both the Feshbach resonance and Cooper pairing. © 2002 The American Physical Society.

• Access to Document (DOI)

Bound State and Order Parameter Mixing Effect by Nonmagnetic Impurity Scattering in Two-band Superconductors

大橋 洋士

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  71 （ 8 ） 1978-1992 - 1992 2002年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

We investigate nonmagnetic impurity effects in two-band superconductors, focusing on the effects of interband impurity scatterings. Within the Born approximation, it is known that interband scatterings mix order parameters in the two bands. In particular, only one averaged energy gap appears in the excitation spectrum in the dirty limit [G. Gusman: J. Phys. Chem. Solids 28 (1967) 2327]. In this paper, we take into account the interband scattering within the t-matrix approximation beyond the Born approximation in the previous work. We show that, although the interband scattering is responsible for the mixing effect, this effect becomes weak when the interband scattering becomes very strong. In the strong interband scattering limit, a two-gap structure corresponding to two order parameters recovers in the superconducting density of states. We also show that a bound state appears around a nonmagnetic impurity depending on the phase of interband scattering potential.

• Access to Document (DOI)

On the Fulde-Ferrell State in Spatially Isotropic Superconductors

大橋 洋士

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  71 （ 11 ） 2625-2628 - 2628 2002年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り,  ISSN  00319015

　概要を見る

Effects of superconducting fluctuations on the Fulde-Ferrell (FF) state are discussed in a spatially isotropic three-dimensional superconductor under a magnetic field. For this system, Shimahara recently showed that within the phenomenological Ginzburg-Landau theory, the long-range order of the FF state is suppressed by the phase fluctuation of the superconducting order parameter [H. Shimahara: J. Phys. Soc. Jpn. 67 (1998) 1872; Physica B 259-261 (1999) 492]. In this letter, we investigate this instability of the FF state against superconducting fluctuations from the microscopic viewpoint, employing the theory developed by Nozières and Schmitt-Rink in the BCS-BEC crossover field. Besides the absence of the second-order phase transition associated with the FF state, we show that even if the pairing interaction is weak, the shift of the chemical potential from the Fermi energy due to the fluctuations is crucial near the critical magnetic field of the FF state obtained within the mean-field theory.

• Access to Document (DOI)