Generic Phase Diagram of Fermion Superfluids with Population Imbalance

TL;DR

This paper presents a microscopic analysis of imbalanced Fermi superfluids, revealing a universal phase diagram with sign-changing gap functions up to a critical imbalance, consistent with recent experimental observations.

Contribution

It provides a comprehensive phase diagram for imbalanced Fermi superfluids, showing the conditions for FFLO-like states and their stability, which was not fully characterized before.

Findings

Sign-changing gap functions occur up to a critical imbalance P_c.

The BCS state without sign change is only stable at non-zero temperatures.

The phase diagram matches recent experimental results.

Abstract

It is shown by microscopic calculations for trapped imbalanced Fermi superfluids that the gap function has always sign changes, i.e., the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state like, up to a critical imbalance $P_c$, beyond which normal state becomes stable, at temperature T=0. A phase diagram is constructed in $T$ vs $P$, where the BCS state without sign change is stable only at $T\neq 0$. We reproduce the observed bimodality in the density profile to identify its origin and evaluate $P_c$ as functions of $T$ and the coupling strength. These dependencies match with the recent experiments.