Ferromagnetism in repulsive Fermi gases: upper branch of Feshbach resonance versus hard spheres

Understand

We use quantum Monte Carlo, including backflow corrections, to investigate a two-component Fermi gas on the upper branch of a Feshbach resonance and contrast it with the hard sphere gas.

• We find that, in both cases, the Fermi liquid becomes unstable to ferromagnetism at a $k_F a$ smaller than the mean field result, where $k_F$ is the Fermi wavevector and $a$ the scattering length.
• Even though the total energies $E(k_F a)$ are similar in the two cases, their pair correlations and kinetic energies are completely different, reflecting the underlying potentials.
• We discuss the extent to which our calculations shed light on recent experiments.