研究者詳細 - 大橋　洋士

論文 - 大橋　洋士

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Strong-Coupling Theory for a Non-equilibrium Unitary Fermi Gas

Kawamura T., Kagamihara D., Hanai R., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics） 201 （ 1-2 ） 41 - 48 2019年12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 00222291

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Superfluid Phase Transitions and Effects of Thermal Pairing Fluctuations in Asymmetric Nuclear Matter

Tajima H., Hatsuda T., van Wyk P., Ohashi Y.

Scientific Reports （Scientific Reports） 9 （ 1 ） 1 - 13 2019年12月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 2045-2322

© 2019, The Author(s). We investigate superfluid phase transitions of asymmetric nuclear matter at finite temperature (T) and density (ρ) with a low proton fraction (Yp ≤ 0.2), which is relevant to the inner crust and outer core of neutron stars. A strong-coupling theory developed for two-component atomic Fermi gases is generalized to the four-component case, and is applied to the system of spin-1/2 neutrons and protons. The phase shifts of neutron-neutron (nn), proton-proton (pp) and neutron-proton (np) interactions up to k = 2 fm−1 are described by multi-rank separable potentials. We show that the critical temperature Tcnn of the neutron superfluidity at Yp = 0 agrees well with Monte Carlo data at low densities and takes a maximum value Tcnn= 1.68 MeV at ρ/ρ=0.14 with ρ0 = 0.17 fm−3. Also, the critical temperature Tcnn of the proton superconductivity for Yp ≤ 0.2 is substantially suppressed at low densities due to np-pairing fluctuations, and starts to dominate over Tcnn only above ρ/ρ=0.70(0.77) for Yp = 0.1(0.2), and (iii) the deuteron condensation temperature Tcd is suppressed at Yp ≤ 0.2 due to a large mismatch of the two Fermi surfaces.

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Single-particle properties of a strongly interacting Bose-Fermi mixture with mass and population imbalance

Manabe K., Inotani D., Ohashi Y.

Physical Review A （Physical Review A） 100 （ 6 ） 063609(1-13) 2019年12月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 24699926

We theoretically investigate strong-coupling properties of a Bose-Fermi mixture. In the mass- A nd population-balanced case, two of the authors have shown that a strong heteropairing interaction in this mixture brings about coupling phenomena between Fermi atomic excitations and Bose atomic and composite molecular excitations, that appear as an anomalous multiple peak structure in the single-particle spectral weight (SW) [Kharga, Inotani, Hanai, and Ohashi, J. Phys. Soc. Jpn. 86, 084301 (2017)JUPSAU0031-901510.7566/JPSJ.86.084301]. In this paper, we show that, although these many-body phenomena are sensitive to mass and population imbalances between the Bose and Fermi components, SW still exhibits the multiple peak structure in moderately mass-imbalanced Rb87-K40 and Na23-K40 mixtures. We also point out that the photoemission spectrum is a useful quantity to observe this spectral anomaly. Since a real trapped Bose-Fermi mixture is usually accompanied by mass and (local) population imbalances, our results would contribute to the study of a strongly interacting Bose-Fermi mixture, under realistic imbalanced conditions.

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Shear viscosity and strong-coupling corrections in the BCS–BEC crossover regime of an ultracold Fermi gas

Kagamihara D., Inotani D., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 88 （ 11 ） 114001(1-10) 2019年10月

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

We theoretically investigate the shear viscosity η in the BCS–BEC crossover regime of an ultracold Fermi gas with a Feshbach resonance. Within the framework of the strong-coupling self-consistent T-matrix approximation, we examine how a strong pairing interaction associated with a Feshbach resonance affects this transport coefficient, in the normal state above the superfluid phase transition temperature T . We show that, while η diverges in both the weak-coupling BCS and strong-coupling BEC limits, it becomes small in the unitary regime. The minimum of η is obtained, not at the unitarity, but slightly in the strong-coupling BEC side. This deviation is consistent with the recent experiment on a Li Fermi gas. In the weak-coupling BCS regime, we also find that η exhibits anomalous temperature dependence near T , which is deeply related to the pseudogap phenomenon originating from strong pairing fluctuations. c c 6

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Isothermal Compressibility of an Ultracold Fermi Gas in the BCS–BEC Crossover

Sato R., Kagamihara D., Manabe K., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics） 196 （ 1-2 ） 119 - 125 2019年07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 00222291

© 2018, Springer Science+Business Media, LLC, part of Springer Nature. We theoretically investigate the isothermal compressibility κT in the normal state of an ultracold Fermi gas with a tunable attractive interaction. We calculate this thermodynamic quantity by considering fluctuations in the Cooper channel, within the framework of the self-consistent T-matrix approximation (SCTMA). For comparison, we also evaluate this quantity in a “non”-self-consistent T-matrix approximation (TMA). We show that the calculated κT diverges at Tc in the BCS–BEC crossover region. On the other hand, such a singular behavior is absent when we deal with this quantity in SCTMA. We point out that the origin of this difference is the neglect of an effective inter-pair interaction in the former approximation. We also explicitly show how such an interaction is involved in the theory when one deals with pairing fluctuations in SCTMA. Our results indicate that the isothermal compressibility is a useful quantity in considering how preformed Cooper pairs interact with one another in the BCS–BEC crossover regime of an ultracold Fermi gas.

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Non-Hermitian phase transition from a polariton Bose-Einstein condensate to a photon laser

R. Hanai, A. Edelman, Y. Ohashi, and P. B. Littlewood, Yoji Ohashi

Physical Review Letters （Physical Review Letters） 122 （ 18 ） 185301(1-6) 2019年05月

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

We propose a novel mechanism for a nonequilibrium phase transition in a U(1)-broken phase of an electron-hole-photon system, from a Bose-Einstein condensate of polaritons to a photon laser, induced by the non-Hermitian nature of the condensate. We show that a (uniform) steady state of the condensate can always be classified into two types, namely, arising either from lower or upper-branch polaritons. We prove (for a general model) and demonstrate (for a particular model of polaritons) that an exceptional point where the two types coalesce marks the end point of a first-order-like phase boundary between the two types, similar to a critical point in a liquid-gas phase transition. Since the phase transition found in this paper is not in general triggered by population inversion, our result implies that the second threshold observed in experiments is not necessarily a strong-to-weak-coupling transition, contrary to the widely believed understanding. Although our calculation mainly aims to clarify polariton physics, our discussion is applicable to general driven-dissipative condensates composed of two complex fields.

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Strong-Coupling Effects on Specific Heat in the BCS–BEC Crossover

Inotani D., van Wyk P., Ohashi Y.

Journal of Low Temperature Physics （Journal of Low Temperature Physics） 196 （ 1-2 ） 111 - 118 2019年05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 00222291

© 2019, Springer Science+Business Media, LLC, part of Springer Nature. We theoretically investigate strong-coupling effects on specific heat at constant volume CV in a superfluid Fermi gas with a tunable interaction associated with Feshbach resonance. Including fluctuations of the superfluid order parameter within the strong-coupling theory developed by Nozières and Schmitt-Rink, we calculate the temperature dependence of CV at the unitarity limit in the superfluid phase. We show that, in the low-temperature region, T3-behavior is shown in the temperature dependence of CV. This result indicates that the low-lying excitations are dominated by the gapless Goldstone mode, associated with the phase fluctuations of the superfluid order parameter. Since the Goldstone mode is one of the most fundamental phenomena in the Fermionic superfluidity, our results are useful for further understanding how the pairing fluctuations affect physical properties in the BCS–BEC crossover physics below the superfluid transition temperature.

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Single-Particle Structures in the Normal State of a Strongly Interacting Bose–Fermi Mixture with Mass Imbalance

Manabe K., Kagamihara D., Inotani D., Ohashi Y.

Journal of Low Temperature Physics （Springer） 196 （ 1-2 ） 126 - 132 2018年12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 00222291

We extend some of the authors’ previous work for a mass-balanced Bose–Fermi mixture to include effects of realistic mass imbalance in a Na– K and a Rb– K mixtures. Including effects of pairing fluctuations within the framework of an improved T-matrix theory, we calculate the single-particle spectral weight (SW), as well as the photoemission spectrum (PES) in the unitary regime at the Bose–Einstein condensation temperature T . We show that the characteristic three-peak structure induced by paring fluctuations, being predicted in the mass-balanced case, remains to exist in SW even in the presence of mass difference. We also show that PES is a useful experimental method to observe this phenomenon in a Na– K, as well as Rb– K, Bose–Fermi mixtures. Since this strong-coupling phenomenon is qualitatively different from that in a two-component Fermi gas, our results would be useful in clarifying how the quantum statistics affects Bose and Fermi pairing phenomena in a two-component gas mixture. 23 40 87 40 23 40 87 40 BEC

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Photoemission spectrum in the BCS-BEC Crossover Regime of a Rare-Earth Fermi Gas with an Orbital Feshbach Resonance

S. Mondai, D. Inotani, Yoji Ohashi

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 87 （ 9 ） 094301(1-9) 2018年08月

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

© 2018 The Physical Society of Japan. We theoretically investigate photoemission spectrum (PES) in a 173Yb Fermi gas with a tunable pairing interaction associated with an orbital Feshbach resonance. Including the two-channel character of this rare-earth atomic system, we examine strong-coupling effects on single-particle properties of the open and closed channels, within the framework of an T-matrix approximation in terms of fluctuations in the Cooper channel. In the BCS-BEC crossover region, PES in the closed channel is shown to exhibit non-vanishing intensity in the negative energy region, as an effect of pairing fluctuations. On the other hand, PES in the open channel exhibits a pseudogap phenomenon, as in the case of a broad magnetic Feshbach resonance discussed in 40K and 6Li Fermi gases. We also examine effects of a harmonic trap in the local density approximation, to see how the spatial inhomogeneity affects strong-coupling corrections to PES.

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Strong-Coupling and Finite Temperature Effects on p-wave Contacts

D. Inotani, Yoji Ohashi

Physical Review A （Physical Review A） 98 （ 2 ） 023603(1-12) 2018年08月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 24699926

© 2018 American Physical Society. We investigate theoretically strong-coupling and finite-temperature effects on the p-wave contacts, as well as the asymptotic behavior of the momentum distribution in the large-momentum region in a one-component Fermi gas with a tunable p-wave interaction. Including p-wave pairing fluctuations within a strong-coupling theory, we calculate the p-wave contacts above the superfluid transition temperature Tc from the adiabatic energy relations. We show that, while the p-wave contact related to the scattering volume monotonically increases with increasing interaction strength, the one related to the effective range depends nonmonotonically on interaction strength and its sign changes in the intermediate-coupling regime. The nonmonotonic interaction dependence of these quantities is shown to originate from the competition between the increase of the cutoff momentum and the decrease of the coupling constant of the p-wave interaction with increasing effective range. We also analyze the asymptotic form of the momentum distribution in the large-momentum region. In contrast to the conventional s-wave case, we show that the asymptotic behavior cannot be completely described by only the p-wave contacts, and the extra terms, which are not related to the thermodynamic properties, appear. Furthermore, in the high-temperature region, we find that the extra terms dominate the subleading term of the large-momentum distribution. We also directly compare our results with the recent experimental measurement by including the effects of a harmonic trap potential within the local density approximation. We show that, while our model qualitatively explains the dependence on the interaction strength of the p-wave contacts, there are some quantitative disagreements between our theory and recent experiment; for example, the peak position of the p-wave contact related to the effective range. Since the p-wave contacts connect the microscopic properties to the thermodynamic properties of the system, our results would be helpful for further understanding many-body properties of this anisotropic interacting Fermi gas in the normal phase.

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Single-particle Excitations and Strong Coupling Eﬀects in the BCS-BEC Crossover Regime of a Rare-Earth Fermi Gas with an Orbital Feshbach Resonance

S. Mondal, D. Inotani, Yoji Ohashi

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 87 （ 8 ） 084302(1-10) 2018年08月

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

We theoretically investigate normal-state properties of an ultracold Fermi gas with an orbital Feshbach resonance (OFR). Recently, OFR has attracted much attention as a promising pairing mechanism to realize a superfluid Yb Fermi gas. Including pairing fluctuations within a T-matrix approximation, and removing effects of an experimentally inaccessible deep bound state, we evaluate strong-coupling corrections to single-particle excitations. With increasing the strength of an OFR-induced tunable pairing interaction, the open channel is shown to exhibit the pseudogap phenomenon in the BCS–BEC crossover region, as in the case of a broad magnetic Feshbach resonance (MFR) in Li and K Fermi gases. We also show that the strong pairing interaction affects the closed channel, leading to the coexistence of particle and hole branches in the single-particle spectral weight. Since the latter phenomenon cannot be observed in the conventional MFR case, it may be viewed as a characteristic strong-coupling phenomenon peculiar to the OFR case. 173 6 40

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Photoluminescence and gain/absorption spectra of a driven-dissipative electron-hole-photon condensate

Hanai R., Littlewood P., Ohashi Y.

Physical Review B （Physical Review B） 97 （ 24 ） 245302(1-22) 2018年06月

研究論文（学術雑誌）, 共著, 最終著者, ISSN 24699950

© 2018 American Physical Society. We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a driven-dissipative exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are obtained. We show that the nonequilibrium nature of the exciton-polariton condensate strongly suppresses the visibility of the Bogoliubov dispersion in the negative energy branch (ghost branch) in photoluminescence spectra. We also show that the trace of this branch can be captured as a hole burning effect in gain/absorption spectra. Our results indicate that the nonequilibrium nature of the exciton-polariton condensate strongly reduces quantum depletion, while a scattering channel to the ghost branch is still present.

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Pairing fluctuations in a strongly interacting two-dimensional Fermi gas

Matsumoto M., Hanai R., Inotani D., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series） 969 （ 1 ） 012012(1-6) 2018年04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 17426588

© Published under licence by IOP Publishing Ltd. We theoretically investigate strong-coupling properties of a two-dimensional Fermi gas in the normal phase. Including pairing fluctuations within the framework of a self-consistent T-matrix approximation, we calculate the single-particle density of states (DOS) near the observed Berezinskii-Kosterlitz-Thouless phase transition temperature T BKT. We show that the pseudogap phenomenon associated with strong pairing fluctuations appears as an almost fully gapped single-particle density of states in the strong-coupling regime, indicating that amplitude fluctuations of the order parameter are almost absent and the system is dominated by phase fluctuations of the order parameter, as expected in a two-dimensional system near T BKT. On the other hand, although pairing fluctuations are enhanced by the low-dimensionality of the system, they are found to be still not strong enough to open such a full gap in the intermediate coupling (crossover) regime. In this regime, a dip structure is only seen in DOS around ω = 0, indicating that amplitude of the superfluid order parameter is also fluctuating, in addition to phase fluctuations. In theoretically determining T BKT in a two-dimensional Fermi gas, it is frequently assumed that, while phase fluctuations exist, the amplitude of the superfluid order parameter is fixed. However, our results indicate the necessity of including both amplitude and phase fluctuations of the order parameter in evaluating T BKT in the crossover region. Since the BKT phase transition has recently been reported in a two-dimensional 6Li Fermi gas, our results would be useful for the theoretical study of this observed novel Fermi superfluid and effects of strong low-dimensional pairing fluctuations.

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Strongly Interacting Multi-component Fermions: From Ultracold Atomic Fermi Gas to Asymmetric Nuclear Matter in Neutron Stars

Tajima H., Hatsuda T., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series） 969 （ 1 ） 012003(1-6) 2018年04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

© Published under licence by IOP Publishing Ltd. We investigate an asymmetric nuclear matter consisting of protons and neutrons with spin degrees of freedom (σ = ↑, ↓). By generalizing the Nozières and Schmitt-Rink theory for two-component Fermi gases to the four-component case, we analyze the critical temperature T c of the superfluid phase transition. Although the pure neutron matter exhibits the dineutron condensation in the low-density region, the superfluid instability toward the deuteron condensation is found to take place as the proton fraction increases. We clarify the mechanism of the competition between the deuteron condensation and dineutron condensation. Our results would serve for understanding the properties of asymmetric nuclear matter realized in the interior of neutron stars.

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Closed-channel contribution in the BCS-BEC crossover regime of an ultracold Fermi gas with an orbital Feshbach resonance

Mondal S., Inotani D., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series） 969 （ 1 ） 012017(1-6) 2018年04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

© Published under licence by IOP Publishing Ltd. We theoretically investigate strong-coupling properties of an ultracold Fermi gas with an orbital Feshbach resonance (OFR). Including tunable pairing interaction associated with an OFR within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR), we examine the occupation of the closed channel. We show that, although the importance of the closed channel is characteristic of the system with an OFR, the occupation number of the closed channel is found to actually be very small at the superfluid phase transition temperature T c, in the whole BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region, when we use the scattering parameters for an ultracold 173Yb Fermi gas. The occupation of the closed channel increases with increasing the temperature above T c, which is more remarkable for a stronger pairing interaction. We also present a prescription to remove effects of an experimentally inaccessible deep bound state from the NSR formalism, which we meet when we theoretically deal with a 173Yb Fermi gas with an OFR.

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Specific heat and effects of strong pairing fluctuations in a superfluid Fermi atom gas in the BCS-BEC crossover region

Van Wyk P., Inotani D., Ohashi Y.

Journal of Physics: Conference Series （Journal of Physics: Conference Series） 969 （ 1 ） 012013(1-6) 2018年04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

© Published under licence by IOP Publishing Ltd. We theoretically investigate the specific heat at constant volume C V in the BCS(Bardeen-Cooper-Schrieffer)-BEC(Bose-Einstein-condensation)-crossover regime of an ultracold Fermi gas, below the superfluid phase transition temperature T c. Within the strong-coupling framework developed by Nozières and Schmitt-Rink, we show that the temperature dependence of C V drastically changes as one passes through the crossover region, and is sensitive to strong fluctuations in the Cooper channel near the unitarity limit. We also compare our results to a recent experiment on a 6Li unitary Fermi gas. Since fluctuation effects are a crucial key in the BCS-BEC-crossover phenomenon, our results would be helpful in considering how the fermionic BCS superfluid changes into BEC with increasing the interaction strength, from the viewpoint of specific heat.

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Superfluid Fermi atomic gas as a quantum simulator for the study of the neutron-star equation of state in the low-density region

Van Wyk P., Tajima H., Inotani D., Ohnishi A., Ohashi Y.

Physical Review A （Physical Review A） 97 （ 1 ） 013601(1-13) 2018年01月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 24699926

© 2018 American Physical Society. We propose a theoretical idea to use an ultracold Fermi gas as a quantum simulator for the study of the low-density region of a neutron-star interior. Our idea is different from the standard quantum simulator that heads for perfect replication of another system, such as the Hubbard model discussed in high-Tc cuprates. Instead, we use the similarity between two systems and theoretically make up for the difference between them. That is, (1) we first show that the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR) can quantitatively explain the recent experiment on the equation of state (EoS) in a Li6 superfluid Fermi gas in the BCS (Bardeen-Cooper-Schrieffer) unitary limit far below the superfluid phase-transition temperature Tc. This region is considered to be very similar to the low-density region (crust regime) of a neutron star (where a nearly unitary s-wave neutron superfluid is expected). (2) We then theoretically compensate the difference that, while the effective range reff is negligibly small in a superfluid Li6 Fermi gas, it cannot be ignored (reff=2.7 fm) in a neutron star, by extending the NSR theory to include effects of reff. The calculated EoS when reff=2.7 fm is shown to agree well with the previous neutron-star EoS in the low-density region predicted in nuclear physics. Our idea indicates that an ultracold atomic gas may more flexibly be used as a quantum simulator for the study of other complicated quantum many-body systems, when we use not only the experimental high tunability, but also the recent theoretical development in this field. Since it is difficult to directly observe a neutron-star interior, our idea would provide a useful approach to the exploration for this mysterious astronomical object.

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Pseudogap regime of a two-dimensional uniform fermi gas

Matsumoto M., Hanai R., Inotani D., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 87 （ 1 ） 014301(1-9) 2018年01月

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

© 2018 The Physical Society of Japan. We investigate pseudogap phenomena in a two-dimensional Fermi gas. Including pairing fluctuations within a selfconsistent T-matrix approximation, we determine the pseudogap temperature T∗ below which a dip appears in the density of states ρ(ω) around the Fermi level. Evaluating T∗, we identify the pseudogap region in the phase diagram of this system. We find that, while the observed Berezinskii-Kosterlitz-Thouless (BKT) transition temperature TexpBKT in a 6Li Fermi gas is in the pseudogap regime, the detailed pseudogap structure in ρ(ω) at TexpBKT still differs from a fullygapped one, indicating the importance of amplitude fluctuations in the Cooper channel there. Since the observed TexpBKT in the weak-coupling regime cannot be explained by the recent BKT theory which only includes phase fluctuations, our results may provide a hint about how to improve this BKT theory. Although ρ(ω) has not been measured in this system, we show that the assessment of our results is still possible by using the observable Tan's contact.

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Ground-state thermodynamic quantities of homogeneous spin-1/2 fermions from the BCS region to the unitarity limit

Horikoshi M., Koashi M., Tajima H., Ohashi Y., Kuwata-Gonokami M.

Physical Review X （Physical Review X） 7 （ 4 ） 041004(1-15) 2017年10月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2160-3308

The understanding of physical properties of fermions in the unitary regime, where the s-wave scattering length in the collisional channel of particles is longer than both the interparticle distance and the size of the interaction potential, is a crucial issue for electron systems of high-temperature superconductivity, dilute nucleons in nuclei, and neutron stars. We experimentally determine various thermodynamic quantities of interacting two-component fermions at the zero-temperature limit from the BCS region to the unitarity limit. The obtained results are very accurate in the sense that the systematic error is within 4% in the unitary regime. Using this advantage, we can compare our data with various many-body theories. We find that an extended T-matrix approximation, which is a strong-coupling theory involving fluctuations in the Cooper channel, well reproduces our experimental results. We also find that the superfluid order parameter Δ calculated by solving the ordinary BCS gap equation with the chemical potential of interacting fermions is close to the binding energy of the paired fermions directly observed in a spectroscopic experiment and that obtained using a quantum Monte Carlo method.

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Dynamical instability of a driven-dissipative electron-hole condensate in the BCS-BEC crossover region

Hanai R., Littlewood P., Ohashi Y.

Physical Review B （Physical Review B） 96 （ 12 ） 125206(1-19) 2017年09月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 24699950

© 2017 American Physical Society. We present a stability analysis on a driven-dissipative electron-hole condensate in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. Extending the combined BCS-Leggett theory with the generalized random phase approximation to the nonequilibrium case by employing the Keldysh formalism, we show that the pumping and decay of carriers causes a depairing effect on excitons. This phenomenon gives rise to an attractive interaction between excitons in the BEC regime, as well as a supercurrent that anomalously flows antiparallel to θ(r) [where θ(r) is the phase of the condensate] in the BCS regime, both leading to dynamical instabilities of an exciton BEC. Our results suggest that a substantial region of the exciton-BEC phase in the phase diagram (in terms of the interaction strength and the decay rate) is unstable.

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大橋 洋士

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論文 - 大橋　洋士