論文 - 大橋　洋士

Superfluid density of states and pseudogap phenomenon in the BCS-BEC crossover regime of a superfluid Fermi gas

Watanabe R., Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  82 （ 4 ） 043630(1-9) 2010年

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

　概要を見る

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.

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Theoretical proposal of a method to identify the pairing symmetry of Fe-pnictide superconductors using ac-josephson effect

Inotani D., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  470 （ SUPPL.1 ） S499-S501 - S501 2010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

　概要を見る

We theoretically propose a method to detect ±s-wave state in newly discovered Fe-pnictide superconductors, using ac Josephson current through a single-band superconductor/insulator/±s-wave superconductor (S/I/±S) junction. We show that, because of the Riedel anomaly, the ac Josephson current vanishes at some values of biased voltage across the junction. This phenomenon does not occur without the sign difference of order parameters in the two band superconductor, so that the observation of the vanishing ac Josephson current would be a clear signature of ±s-wave pairing symmetry in Fe-pnictides. © 2009 Elsevier B.V. All rights reserved.

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Pseudogap behaviors of atomic Fermi gases above Tc in the BCS-BEC crossover

Tsuchiya S., Watanabe R., Ohashi Y.

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  470 （ SUPPL.1 ） S986-S988 - S988 2010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

　概要を見る

We investigate pseudogap behaviors of an ultracold Fermi gas in the BCS-BEC crossover above the superfluid transition temperature Tc. 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. © 2009 Elsevier B.V. All rights reserved.

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Identification of the ±s -wave pairing state in iron-pnictide superconductors using the Riedel anomaly

Inotani D., Ohashi Y.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  79 （ 22 ） 224527(1-5) 2009年06月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り,  ISSN  10980121

　概要を見る

We theoretically propose a method to identify ±s -wave order parameter in recently discovered Fe-pnictide superconductors. Our idea uses the Riedel anomaly in ac-Josephson current through a SI (±S) (single-band s -wave superconductor/insulator/ ±s -wave two-band superconductor) junction. We show that the Riedel peak effect leads to vanishing ac-Josephson current at some values of biased voltage. This phenomenon does not occur in the case when the ±s -wave superconductor is replaced by a conventional s -wave one so that the observation of this vanishing Josephson current would be a clear signature of ±s -wave pairing state in Fe-pnictide superconductors. © 2009 The American Physical Society.

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Molecular wavefunction and tunneling properties of a bound pair of Fermi atoms in an optical lattice

Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  150 （ 3 ） 032076 2009年03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  17426588

　概要を見る

We investigate tunneling properties of a bound pair of Fermi atoms in an optical lattice. Including a realistic cosine-shape optical lattice potential, we calculate the molecular wavefunction ψ. Starting from the weak-coupling regime, we show how ψ shrinks as one increases the strength of a pairing interaction between atoms. When the molecular size becomes comparable to the lattice constant in the intermediate coupling regime, we find from the spatial structure of ψ that the molecular tunneling between lattice sites is accompanied by virtual dissociation. This phenomenon is shown to be suppressed in the strong-coupling regime due to very strong binding energy. We point out that, while the molecular tunneling accompanied by virtual dissociation can be described by the Hubbard model, it cannot describe the molecular tunneling with no virtual dissociation in the strong-coupling regime. © 2009 IOP Publishing Ltd.

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Enhanced quasi-particle current of Bogoliubov phonons in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series）  150 （ 3 ） 032113 2009年03月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 査読有り,  ISSN  17426588

　概要を見る

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. © 2009 IOP Publishing Ltd.

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Superfluid properties of an ultracold Fermi gas in the BCS-BEC crossover region

Ohashi Y., Fukushima N.

Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008 （Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008）     18 - 23 2009年

研究論文（国際会議プロシーディングス）, 共著, 筆頭著者, 査読有り

　概要を見る

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

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Quasiparticle inducing josephson effect in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008 （Proceedings of the 9th International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, ISQM-Tokyo 2008）     37 - 40 2009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

　概要を見る

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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Single-particle properties and pseudogap effects in the BCS-BEC crossover regime of an ultracold Fermi gas above Tc

Tsuchiya S., Watanabe R., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  80 （ 3 ） 033613(1-9) 2009年

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

　概要を見る

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□ > □□ 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.

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Stripe formation in fermionic atoms on 2-D optical lattice: DMRG studies for n-leg repulsive hubbard ladder

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

Journal of Superconductivity and Novel Magnetism （Journal of Superconductivity and Novel Magnetism）  22 （ 3 ） 275-279 - 279 2009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  15571939

　概要を見る

In order to examine the ground state of the single-band Hubbard model from a slightly doped region to an overdoped one, we use the density renormalization group method extended directly to the ladder model and obtain hole profiles with changing the on-site repulsion and the doping ratio. The calculation results in a heavily underdoped region, in which one and two holes (up and down spins) are added, reveal that an attractive interaction works between holes which are confined inside a stripe shape pattern while strong disorders separate the two holes. When doping holes further within an underdoped range, the stripes formed by two holes merge and various stripe structures appear depending on U/t and the doping ratio. The remarkable finding in the underdoped region is a stripe formation composed of 4 holes analytically predicted by Chang and Affleck. On the other hand, in the overdoped range the stripe-like shape becomes very simple, i.e., only a small oscillating charge density profile emerges. These rich features in the underdoped range are relevant to electronic inhomogeneities experimentally observed in high-T c superconductors, while those are directly observable in atomic Fermi gases loaded on optical lattices. © 2009 Springer Science+Business Media, LLC.

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Supercurrent induced by tunneling Bogoliubov excitations in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  79 （ 6 ） 063619(1-15) 2009年

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

　概要を見る

We study the tunneling of Bogoliubov excitations through a barrier in a Bose-Einstein condensate. We extend our previous work 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. © 2009 The American Physical Society.

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d-Wave spin density wave phase in the attractive hubbard model with spin polarization

Tamaki H., Miyake K., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan）  78 （ 7 ） 073001(1-4) 2009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 possibility of an unconventional spin density wave (SDW) in the attractive Hubbard model with finite spin polarization. We show that pairing and density fluctuations induce a transverse d-wave SDW near the half-filling. This novel SDW is related to the d-wave superfluidity induced by antiferromagnetic spin fluctuations, in the sense that they are connected with each other through Shibas attraction-repulsion transformation. Our results predict the d-wave SDW in real systems, such as cold Fermi atom gases with a population imbalance and compounds involving valence skipper elements. © 2009 The Physical Society of Japan.

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Supercurrent behavior of low-energy Bogoliubov phonons and the anomalous tunneling effect in a Bose-Einstein condensate

Ohashi Y., Tsuchiya S.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  78 （ 4 ） 043601(1-10) 2008年10月

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 tunneling properties of Bogoliubov mode in a Bose-Einstein condensate. Using an exactly solvable model with a δ -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 Ju and Jv 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 Ju or Jv 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. © 2008 The American Physical Society.

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Correlation effects on atom-density profiles of one- and two-dimensional polarized atomic Fermi gases loaded on an optical lattice

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

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  77 （ 5 ） 053614(1-8) 2008年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

　概要を見る

We investigate effects of optical lattice potential in one- and two-dimensional two-component trapped Fermi gases with population imbalances. Using the exact diagonalization and the density matrix renormalization group methods complementarily, we calculate the atom-density profile from the ground state many-body wave function as a function of attractive interaction strength for various population imbalances. The numerical results reveal that although a phase separation between the superfluid core and the shell cloud of excess atoms occurs as observed in experiments without the optical lattice, the population imbalance generally remains in the core region in contrast to the nonlattice cases. The essence of the numerical results in a strong attractive regime can be explained by an effective model composed of Cooper pairs and excess major fermions. © 2008 The American Physical Society.

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Superfluid density in the BCS-BEC crossover regime of a Fermi superfluid

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

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  468 （ 7-10 ） 599-604 - 604 2008年

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

　概要を見る

We investigate the superfluid carrier density ρ s in the BCS-BEC crossover regime of a Fermi superfluid at finite temperatures. Including pairing fluctuations within a Gaussian approximation, we calculate the superfluid order parameter, chemical potential, and ρ s in a consistent manner in the whole BCS-BEC crossover region. In the weak-coupling BCS regime, the temperature dependence of ρ s is dominated by single-particle excitations accompanied by thermal dissociation of Cooper pairs. In the strong-coupling BEC regime, we show that single-particle excitations become less dominant and thermal excitations of collective Bogoliubov mode become important for ρ s . We clarify how the weak-coupling BCS result smoothly changes into the strong-coupling BEC result, as one increases the strength of a pairing interaction. © 2008 Elsevier B.V. All rights reserved.

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BCS-BEC crossover and effects of density fluctuations in a two-component Fermi gas described by the three-dimensional attractive Hubbard model

Tamaki H., Ohashi Y., Miyake K.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  77 （ 6 ） 063616(1-9) 2008年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10502947

　概要を見る

We investigate the superfluid phase transition in a gas of Fermi atoms loaded on a three-dimensional optical lattice. When the lattice potential is strong, this system can be well described by an attractive Hubbard model. In this model, we calculate the superfluid phase transition temperature Tc, including both superfluid and (spin and charge) density fluctuations within the self-consistent t -matrix theory and fluctuation exchange approximation, respectively. Since we treat these fluctuations in a consistent manner, our theory satisfies the required particle-hole symmetry over the entire BCS-Bose-Einstein-condensation (BEC) crossover region. We show that charge density fluctuations compete against superfluid fluctuations near the half-filling, leading to the suppression of Tc. As a result, the maximum Tc is obtained away from the half-filling. Since the strong density fluctuations originate from the nesting property of the Fermi surface at the half-filling (which is absent in a uniform gas with no lattice potential), our results would be useful in considering lattice effects on strong-coupling superfluidity. © 2008 The American Physical Society.

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Anomalous enhancement of quasiparticle current near a potential barrier in a Bose-Einstein condensate

Tsuchiya S., Ohashi Y.

Physical Review A - Atomic, Molecular, and Optical Physics （Physical Review A - Atomic, Molecular, and Optical Physics）  78 （ 1 ） 013628 2008年

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

　概要を見る

We investigate tunneling properties of Bogoliubov phonons in a Bose-Einstein condensate. We find the anomalous enhancement of the quasiparticle current Jq 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 We also show that the quasiparticle current twists the phase of the condensate wave function across the barrier, leading to a finite Josephson supercurrent Js through the barrier. This induced supercurrent flows in the opposite direction to the quasiparticle current so as to cancel out the enhancement of Jq and conserve the total current J= Jq + Js. © 2008 The American Physical Society.

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Stripe formation in repulsive 4-leg Hubbard ladder: Directly-extended DMRG studies

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

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  468 （ 15-20 ） 1141-1144 - 1144 2008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

　概要を見る

In order to study the ground state of the Hubbard model from a slightly doped region to an over-doped one, we directly apply the density renormalization group method to 4-leg × 20-ladder repulsive Hubbard model and calculate hole distribution profiles with changing the on-site repulsion U / t from 0 to 15 and the doping ratio from 0.025 to 0.250. The calculation results in the underdoped range reveal that various stripe structures formed by doped holes merge or separate depending on U / t and the doping ratio. We suggest that these rich features are relevant to modulated hole distributions as experimentally observed in underdoped high-T c superconductors. © 2008.

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Vortex core structure in strongly correlated superfluidity

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

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications）  468 （ 15-20 ） 1237-1240 - 1240 2008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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In order to study the vortex core electronic structure in high-T c superconductors, we examine 1-D Hubbard model with trap potential by using the density matrix renormalization group method. Instead of directly treating the vortex, the approach mimics the carrier density depression inside the vortex core via the trap potential and exactly calculate strong correlation effects on the depressed region. Consequently, we find that the Mott state emerges in the central region and the metallic edge surrounds the Mott region. Furthermore, when adding spin imbalance, the calculations reveal that a local antiferromagnetic order covers the Mott state region, and moreover, the antiferromagnetic order modulates with a long periodicity. We expect that these results closely relate to the vortex core electronic structure while their calculation results can be directly compared with atomic Fermi gases loaded on an optical lattice. © 2008 Elsevier B.V. All rights reserved.

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Mott phase in polarized two-component atomic Fermi lattice gas

Machida M., Okumura M., Yamada S., Deguchi T., Ohashi Y., Matsumoto H.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics）  78 （ 23 ） 235117(1-5) 2008年

研究論文（学術雑誌）, 共著, 査読有り,  ISSN  10980121

　概要を見る

We investigate effects of pseudospin population imbalance on Mott phases in one-dimensional trapped two-component atomic Fermi gases loaded on optical lattices based on the repulsive Hubbard model in harmonic traps. By using the density-matrix renormalization-group method, we numerically calculate density profiles of each component and clarify the pseudospin magnetism. Consequently, we find that all the features from weakly imbalance to fully polarized cases are well described by S=1/2 antiferromagnetic Heisenberg chain under magnetic field. These results indicate that the Mott phases offer experimental stages for studying various interacting spin systems. © 2008 The American Physical Society.

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