Correlations and Pair Formation in a Repulsively Interacting Fermi Gas

TL;DR

This study investigates a strongly repulsive Fermi gas near a Feshbach resonance, finding it remains paramagnetic and quickly forms pairs, challenging the expectation of a ferromagnetic phase transition.

Contribution

It provides experimental evidence that a strongly repulsive Fermi gas does not transition to ferromagnetism, contrary to some theoretical predictions.

Findings

Fermi gas remains paramagnetic at large scattering lengths

Rapid decay into bound pairs occurs within ~10ħ/E_F

No evidence of ferromagnetic phase transition observed

Abstract

A degenerate Fermi gas is rapidly quenched into the regime of strong effective repulsion near a Feshbach resonance. The spin fluctuations are monitored using speckle imaging and, contrary to several theoretical predictions, the samples remain in the paramagnetic phase for arbitrarily large scattering length. Over a wide range of interaction strengths a rapid decay into bound pairs is observed over times on the order of 10\hbar/E_F, preventing the study of equilibrium phases of strongly repulsive fermions. Our work suggests that a Fermi gas with strong short-range repulsive interactions does not undergo a ferromagnetic phase transition.