Non-equilibrium strong-coupling theory for a driven-dissipative ultracold Fermi gas in the BCS-BEC crossover region

TL;DR

This paper develops a non-equilibrium strong-coupling theory for ultracold Fermi gases in the BCS-BEC crossover, revealing anomalous pairing fluctuations and re-entrant superfluid phase transitions driven by chemical potential bias.

Contribution

It introduces a novel non-equilibrium strong-coupling framework combining T-matrix approximation with Keldysh Green's functions for ultracold Fermi gases.

Findings

Chemical potential bias induces FFLO-type pairing fluctuations without spin imbalance.

Re-entrant superfluid phase transition behavior observed in the BCS-unitary regime.

Enhanced pseudogap phenomena due to non-equilibrium pairing fluctuations.

Abstract

We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.