Exploring the ferromagnetic behaviour of a repulsive Fermi gas via spin dynamics

TL;DR

This study investigates whether short-range repulsive interactions alone can induce ferromagnetism in ultracold Fermi gases, revealing signatures of a metastable ferromagnetic phase through spin dynamics and susceptibility measurements.

Contribution

It demonstrates the emergence of a metastable ferromagnetic state in a repulsive Fermi gas, linking dynamical spin responses to ferromagnetic instability without complex band effects.

Findings

Increased spin susceptibility near critical repulsion strength

Observation of domain immiscibility indicating metastability

Signatures of a Stoner-like ferromagnetic phase

Abstract

Ferromagnetism is a manifestation of strong repulsive interactions between itinerant fermions in condensed matter. Whether short-ranged repulsion alone is sufficient to stabilize ferromagnetic correlations in the absence of other effects, like peculiar band dispersions or orbital couplings, is however unclear. Here, we investigate ferromagnetism in the minimal framework of an ultracold Fermi gas with short-range repulsive interactions tuned via a Feshbach resonance. While fermion pairing characterises the ground state, our experiments provide signatures suggestive of a metastable Stoner-like ferromagnetic phase supported by strong repulsion in excited scattering states. We probe the collective spin response of a two-spin mixture engineered in a magnetic domain-wall-like configuration, and reveal a substantial increase of spin susceptibility while approaching a critical repulsion strength. Beyond this value, we observe the emergence of a time-window of domain immiscibility, indicating the metastability of the initial ferromagnetic state. Our findings establish an important connection between dynamical and equilibrium properties of strongly-correlated Fermi gases, pointing to the existence of a ferromagnetic instability.