On the Stability of the Repulsive Fermi Gas with Contact Interactions

We report the creation and the study of the stability of a repulsive quasi-homogeneous spin-$1/2$ Fermi gas with contact interactions. For the range of scattering lengths $a$ explored, the dominant mechanism of decay is a universal three-body recombination towards a Feshbach bound state. We observe that the recombination coefficient $K_3\propto ε_\text{kin} a^6$, where the first factor, the average kinetic energy per particle $ε_\text{kin}$, arises from a three-body threshold law, and the second one from the universality of recombination. Both scaling laws are consequences of Pauli blocking effects in three-body collisions involving two identical fermions. As a result of the interplay between Fermi statistics and the momentum dependence of the recombination process, the system exhibits non-trivial temperature dynamics during recombination, alternatively heating or cooling depending on its initial quantum degeneracy. The measurement of $K_3$ provides an upper bound for the interaction strength achievable in equilibrium for a uniform repulsive Fermi gas.