Strong-Coupling Effects on Quantum Transport in an Ultracold Fermi Gas
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Strong‑Coupling Effects on Quantum Transport in an Ultracold Fermi Gas Koichiro Furutani1 · Yoji Ohashi1 Received: 9 July 2019 / Accepted: 20 May 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract We theoretically investigate quantum conductance across a point contact between two strongly-interacting ultracold Fermi gases. We evaluate the tunneling current, taking into account pairing fluctuations in the Fermi gases within the framework of a combined strong-coupling T-matrix approximation with the Schwinger-Keldysh non-equilibrium formalism. We show that quasi-particle conductance is gradually suppressed, as one passes through the BCS–BEC crossover region from the weakcoupling side. On the other hand, with increasing the strength of a pairing interaction, the fluctuation-pair conductance is found to remarkably increase in the crossover region, which is followed by the suppression in the strong-coupling regime. Keywords Quantum transport · Pairing fluctuations · BCS–BEC crossover
1 Introduction The recent progress in cold Fermi gas physics has contributed to the development of quantum many-body physics. For example, since the realization of the BCS (Bardeen–Cooper–Schrieffer)–BEC (Bose–Einstein condensation) crossover phenomenon in 40 K and 6 Li Fermi gases [1–4], strong-coupling effects on various thermodynamic quantities have extensively been studied in this system, both experimentally [5, 6] and theoretically [7–10]. In addition, a two-terminal setup has recently been realized in a 6 Li Fermi gas [11, 12], so that we can now go beyond the previous equilibrium studies to the non-equilibrium transport physics, by maximally using the high tunability of this atomic gas system. In the conventional electronic mesoscopic system, various interesting phenomena associated with the strong confinement have been discussed, e.g., quantization of conductance [13–15]. In the above-mentioned two-terminal setup experiment, using a * Koichiro Furutani 6tho.re‑m‑[email protected] 1
Department of Physics, Keio University, 3‑14‑1, Hiyoshi, Kohoku‑ku, Yokohama 223‑8522, Japan
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Vol.:(0123456789)
Journal of Low Temperature Physics
tunable pairing interaction associated with a Feshbach resonance [16], Esslinger and co-workers have found remarkable deviation of the conductance from the ordinary Landauer expression in the BCS–BEC crossover region. As a possible explanation for this anomaly, the importance of pairing fluctuations in the two reservoirs connecting to each other through the junction has been pointed out [17, 18]. Indeed, including this effect within the framework of the Gaussian fluctuation theory, Ref. [17] has succeeded in explaining the above-mentioned experimental result near the superfluid phase transition temperature Tc . However, the understanding of the transport property of an ultracold Fermi gas in the whole interaction and temperature region still remains as an exciting challenge. In this paper, we theoretically deal with a quantum point contact between two ultracold Fermi ga
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