Nonequilibrium BCS-BEC crossover and unconventional FFLO superfluid in a strongly interacting driven-dissipative Fermi gas

TL;DR

This paper reviews recent advances in nonequilibrium BCS-BEC crossover physics in driven-dissipative Fermi gases, highlighting the emergence of exotic superfluid states and the effects of chemical potential differences.

Contribution

It provides a systematic theoretical analysis of the nonequilibrium BCS-BEC crossover using Schwinger-Keldysh Green's functions, revealing novel superfluid phenomena.

Findings

Two-step particle momentum distribution in weak-coupling regime

Anomalous enhancement of pseudogap under nonequilibrium conditions

Emergence of FFLO-type superfluid instability

Abstract

We present a theoretical review of the recent progress in nonequilibrium BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover physics. As a paradigmatic example, we consider a strongly interacting driven-dissipative two-component Fermi gas where the nonequilibrium steady state is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. As a powerful theoretical tool to deal with this system, we employ the Schwinger-Keldysh Green's function technique. We systematically evaluate the superfluid transition, as well as the single-particle properties, in the nonequilibrium BCS-BEC crossover region, by adjusting the chemical potential difference between the reservoirs and the strength of an s-wave pairing interaction associated with a Feshbach resonance. In the weak-coupling BCS side, the chemical potential difference is shown to imprint a two-step structure on the particle momentum distribution, leading to an anomalous enhancement of pseudogap, as well as the emergence of exotic Fulde-Ferrell-Larkin-Ovchinnikov-type superfluid instability. Since various nonequilibrium situations have recently been realized in ultracold Fermi gases, the theoretical understanding of nonequilibrium BCS-BEC crossover physics would become increasingly important in this research field.