Dynamical instability of a spin spiral in an interacting Fermi gas as a probe of the Stoner transition

TL;DR

This paper proposes an experiment using a spin spiral in an ultracold Fermi gas to detect the Stoner transition, revealing dynamical instabilities and domain formation that distinguish paramagnetic and ferromagnetic phases.

Contribution

It introduces a novel dynamical probe based on spin spiral instability to identify the Stoner transition in ultracold Fermi gases, overcoming previous experimental challenges.

Findings

Critical slowing down near the transition is suppressed by scattering effects.

A new unstable mode at large wave vectors emerges close to the transition.

Long-wavelength instabilities are suppressed on the paramagnetic side.

Abstract

We propose an experiment to probe ferromagnetic phenomena in an ultracold Fermi gas, while alleviating the sensitivity to three-body loss and competing many-body instabilities. The system is initialized in a small pitch spin spiral, which becomes unstable in the presence of repulsive interactions. To linear order the exponentially growing collective modes exhibit critical slowing down close to the Stoner transition point. Also, to this order, the dynamics are identical on the paramagnetic and ferromagnetic sides of the transition. However, we show that scattering off the exponentially growing modes qualitatively alters the collective mode structure. The critical slowing down is eliminated and in its place a new unstable branch develops at large wave vectors. Furthermore, long-wavelength instabilities are quenched on the paramagnetic side of the transition. We study the experimental observation of the instabilities, specifically addressing the trapping geometry and how phase-contrast imaging will reveal the emerging domain structure. These probes of the dynamical phenomena could allow experiments to detect the transition point and distinguish between the paramagnetic and ferromagnetic regimes.