Itinerant Ferromagnetism in an Ultracold Atom Fermi Gas

TL;DR

This paper investigates the emergence of ferromagnetism in ultracold Fermi gases, revealing a first-order transition, limitations of Hartree-Fock theory, and enhanced spin coherence near the transition, with implications for recent experiments.

Contribution

It provides a second-order interaction analysis predicting a first-order ferromagnetic transition and highlights the limitations of Hartree-Fock theory in this context.

Findings

Hartree-Fock underestimates ferromagnetic tendency

Ferromagnetic transition is first order at low temperatures

Spin coherence time increases near the transition

Abstract

We address the possible occurrence of ultracold atom ferromagnetism by evaluating the free energy of a spin polarized Fermi gas to second order in its interaction parameter. We find that Hartree-Fock theory underestimates the tendency toward ferromagnetism, predict that the ferromagnetic transition is first order at low temperatures, and point out that the spin coherence time of gases prepared in a ferromagnetic state is strongly enhanced as the transition is approached. We relate our results to recent experiments.