Response function of strongly interacting Fermi gas in a virial expansion

The dynamic response functions of strongly interacting fermion gas in homogeneous space are investigated in a virial expansion to second order. The density response function exhibits transition from atomic to molecular response, as the interaction strength increases and the system undergoes BCS-BEC crossover. The qualitative features of density and spin response agree with measurements from the Bragg spectroscopy experiments. The virial response is exact at low density and high temperature, therefore providing a benchmark for many-body response.