Population imbalanced lattice fermions near the BCS-BEC crossover: II. The FFLO regime and its thermal signatures

TL;DR

This study investigates the thermal stability and signatures of the FFLO state in a population-imbalanced 2D attractive Hubbard model near the BCS-BEC crossover, revealing a low critical temperature and complex thermal behavior.

Contribution

It provides the first detailed analysis of thermal effects on the FFLO state in this regime, highlighting a very low T_c and the persistence of finite momentum pairing signatures at high temperatures.

Findings

Very low T_c for FFLO transition, much below mean field estimate

Weak finite momentum pairing signatures persist at high temperatures

Transition from pseudogapped to gapless density of states with increasing temperature

Abstract

We study the effect of high population imbalance in the two dimensional attractive Hubbard model, in the coupling regime corresponding to BCS-BEC crossover, and focus on thermal effects on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Using a method that retains all the classical thermal fluctuations on the FFLO state we estimate a very low $T_c$ and infer a strongly first order normal state to FFLO transition. The $T_c$ is an order of magnitude below the mean field estimate. We track the fermionic momentum distribution, the density of states, and the pairing structure factor deep into the normal state. The pairing structure factor retains weak signature of finite momentum pairing to a high temperature despite the low $T_c$ itself, while the spin resolved density of states changes from the `pseudogapped' FFLO character to gapless and pseudogapped again with increasing temperature. These results map out the rather narrow temperature window, and diverse physical indicators, relevant to FFLO states on a cold atom optical lattice, and complements our work on the lower field `breached pair' state.