Compressibility of an ultracold Fermi gas with repulsive interactions

Fermi gases with repulsive interactions are characterized by measuring their compressibility as a function of interaction strength. The compressibility is obtained from in-trap density distributions monitored by phase contrast imaging. For interaction parameters k_F a > 0.25 fast decay of the gas prevents the observation of equilibrium profiles. For smaller interaction parameters, the results are adequately described by first-order perturbation theory. A novel phase contrast imaging method compensates for dispersive distortions of the images.