Unconventional superfluid in a two-dimensional Fermi gas with anisotropic spin-orbit coupling and Zeeman fields

TL;DR

This paper explores the complex phase diagram of a 2D ultracold Fermi gas with anisotropic spin-orbit coupling and Zeeman fields, revealing exotic superfluid and FFLO states with unique stability and features.

Contribution

It provides the first detailed analysis of the stability and characteristics of FFLO states in a 2D Fermi gas with anisotropic SOC and Zeeman fields, highlighting new phenomena.

Findings

Identification of stable FFLO states under SOC and Zeeman fields

Discovery of a fully gapped FFLO state

Effects of Zeeman fields on pairing stability and phase competition

Abstract

We study the phase diagram of a two-dimensional ultracold Fermi gas with the synthetic spin-orbit coupling (SOC) that has recently been realized at NIST. Due to the coexistence of anisotropic SOC and effective Zeeman fields in the NIST scheme, the system shows rich structure of phase separation involving exotic gapless superfluid and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing states with different center-of-mass momentum. In particular, we characterize the stability region of FFLO states and demonstrate their unique features under SOC. We then show that the effective transverse Zeeman field in the NIST scheme can qualitatively change the landscape of the thermodynamic potential which leads to intriguing effects such as the disappearance of pairing instability, the competition between different FFLO states, and the stabilization of a fully gapped FFLO state. These interesting features may be probed for example by measuring the in-situ density profiles or by the momentum-resolved radio-frequency spectroscopy.