Exotic pairing states in a Fermi gas with three-dimensional spin-orbit coupling

TL;DR

This paper explores exotic superfluid pairing states in a 3D spin-orbit coupled Fermi gas, revealing complex phase transitions and novel Fulde-Ferrell-Larkin-Ovchinnikov states with unique properties.

Contribution

It introduces the stability and characteristics of FFLO states induced by spin-orbit coupling and Zeeman fields in three-dimensional Fermi gases, highlighting their unconventional features.

Findings

First-order phase transitions between different superfluid phases

Existence of nodal and fully gapped FFLO states with unique momentum-space features

FFLO states induced by spin-orbit coupling and Fermi surface deformation

Abstract

We investigate properties of exotic pairing states in a three-dimensional Fermi gas with three-dimensional spin-orbit coupling and an effective Zeeman field. The interplay of spin-orbit coupling, effective Zeeman field and pairing can lead to first-order phase transitions between different phases, and to interesting nodal superfluid states with gapless surfaces in the momentum space. We then demonstrate that pairing states with zero center-of-mass momentum are unstable against finite center-of-mass momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, with the center-of-mass momentum of the pairs opposite to the direction of the effective Zeeman field. Unlike conventional FFLO states, these FFLO states are induced by the coexistence of spin-orbit coupling and Fermi surface deformation, and have intriguing features like first-order transitions between different FFLO states, nodal FFLO states with gapless surfaces in momentum space, and exotic fully gapped FFLO states. With the recent theoretical proposals for realizing three-dimensional spin-orbit coupling in ultracold atom gases, our work is helpful for the future experimental studies, and provides valuable information for the general understanding of pairing physics in spin-orbit coupled fermionic systems.