Pauli Blocking of Collisions in a Quantum Degenerate Atomic Fermi Gas

We have produced an interacting quantum degenerate Fermi gas of atoms composed of two spin states of magnetically trapped ${}^{40}\mathrm{K}$. The relative Fermi energies are adjusted by controlling the population in each spin state. Thermodynamic measurements reveal a resulting imbalance in the mean energy per particle between the two species, which is a factor of 1.4 at our lowest temperature. This imbalance of energy comes from a suppression of collisions between atoms in the gas due to the Pauli exclusion principle. Through measurements of the thermal relaxation rate we have directly observed this Pauli blocking as a factor of 2 reduction in the effective collision cross section in the quantum degenerate regime.