Stoner ferromagnetism of a strongly interacting Fermi gas in the quasirepulsive regime

TL;DR

This paper develops a nonperturbative theoretical approach to study the Stoner ferromagnetic transition in a strongly interacting Fermi gas near a Feshbach resonance, providing phase diagrams and insights into experimental observability.

Contribution

It introduces a nonperturbative method for analyzing the quasirepulsive upper branch and maps the finite-temperature phase diagram for Stoner instability.

Findings

Results align with quantum Monte-Carlo simulations at zero temperature.

At resonance, the transition temperature is near the Fermi temperature.

The molecule formation rate is negligible near the transition, aiding experimental detection.

Abstract

Recent advances in rapidly quenched ultracold atomic Fermi gases near a Feshbach resonance have brought about a number of interesting problems, in the context of observing the long-sought Stoner ferromagnetic phase transition. The possibility of experimentally obtaining a "quasirepulsive" regime in the upper branch of the energy spectrum due to the rapid quench is currently being debated, and the Stoner transition has mainly been investigated theoretically by using perturbation theory or at high polarization, due to the limited theoretical approaches in the strongly repulsive regime. In this work, we present a nonperturbative theoretical approach to the quasirepulsive upper branch of a Fermi gas near a broad Feshbach resonance, and we determine the finite-temperature phase diagram for the Stoner instability. Our results agree well with the known quantum Monte-Carlo simulations at zero temperature, and we recover the known virial expansion prediction at high temperature for arbitrary interaction strengths. At resonance, we find that the Stoner transition temperature becomes of the order of the Fermi temperature, around which the molecule formation rate becomes vanishingly small. This suggests a feasible way to observe Stoner ferromagnetism in the nondegenerate temperature regime.