Strong-Coupling Theory for a Non-equilibrium Unitary Fermi Gas
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Strong-Coupling Theory for a Non-equilibrium Unitary Fermi Gas T. Kawamura1 · D. Kagamihara1 · R. Hanai2,3 · Y. Ohashi1 Received: 2 July 2019 / Accepted: 11 December 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract We study a non-equilibrium unitary Fermi gas, being coupled with two baths with different values of chemical potential. Extending a T -matrix approximation developed in equilibrium Fermi gases to the non-equilibrium steady state by using the Keldysh Green’s function technique, we examine the superfluid phase transition out of equilibrium. We also clarify non-equilibrium effects on the pseudogap phenomenon by strong pairing fluctuations. Keywords Non-equilibrium strong-coupling state · Unitary Fermi gas · T -matrix approximation · Keldysh Green’s function
1 Introduction Since the realization of superfluid 40 K [1] and 6 Li [2] Fermi gases, cold Fermi gas physics has made significant progress. An advantage of this gas system is the high tunability of various physical parameters. Indeed, a tunable pairing interaction associated with a Feshbach resonance has realized the BCS–BEC crossover. Using this, we can now examine Fermi superfluid properties, from the weak-coupling BCS regime to the strong-coupling BEC limit in a unified manner [3]. The intermediate coupling regime, which is also referred to as the unitary regime in the literature, has particularly attracted much attention [4–6], because system properties are dominated by pairing fluctuations there. So far, strong-coupling properties of an ultracold Fermi gas in the BCS–BEC crossover region have dominantly been discussed in the equilibrium case. However,
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T. Kawamura [email protected]
1
Department of Physics, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
2
Department of Physics, James Franck Institute, University of Chicago, Chicago, IL 60637, USA
3
Department of Physics, Osaka University, Toyonaka 560-0043, Japan
123
Journal of Low Temperature Physics
a non-equilibrium quantum state with tunable dissipation has recently been realized [7–9]. In addition, non-equilibrium phenomena have also recently attracted much attention in the strongly correlated electron system [10]. In response to these, the acceleration of the non-equilibrium study would be expected in cold Fermi gas physics. In particular, a non-equilibrium unitary Fermi gas would provide a unique opportunity, to study competition between strong-coupling effects and non-equilibrium effects. In considering a non-equilibrium unitary Fermi gas, a crucial issue is the pair formation out of equilibrium. This kind of problem has also been discussed in exciton–polariton condensates [11–13], because it is essentially a non-equilibrium phenomenon. (One needs to continue pumping photons and supplying electrons and holes to this system, in order to compensate for the decay of excitons.) However, while the latter system is frequently examined in the mean-field level, in the case of a unitary Fermi gas, one needs to go beyond this approx
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