Stability of a Fully Magnetized Ferromagnetic state in Repulsively Interacting Ultracold Fermi Gases

TL;DR

This paper investigates the stability of a fully magnetized ferromagnetic state in ultracold Fermi gases with repulsive interactions, finding it generally unstable except under specific resonance conditions.

Contribution

It introduces a variational wave function with short-range correlations to analyze ferromagnetic stability in different models of ultracold Fermi gases.

Findings

Fully polarized Fermi sea is unstable in Hubbard and continuum models with pure repulsion.

Ferromagnetism can occur in resonance models with large positive scattering length.

Exotic correlations might destabilize the ferromagnetic state further.

Abstract

We construct a variational wave function to study whether a fully polarized Fermi sea is energetically stable against a single spin flip. Our variational wave function contains sufficient short-range correlation at least to the same level as Gutzwiller's projected wave function. For Hubbard lattice model and continuum model with pure repulsive interaction, we show a fully polarized Fermi sea is generally unstable even when the repulsive strength becomes infinite. While for a resonance model, ferromagnetic state is possible if the s-wave scattering length is positive and sufficiently large, and the system is prepared in scattering state orthogonal to molecular bound state. However, we can not rule out the possibility that more exotic correlation can destabilize the ferromagnetic state.