Dynamic Spin Response of a Strongly Interacting Fermi Gas

We present an experimental investigation of the dynamic spin response of a strongly interacting Fermi gas using Bragg spectroscopy. By varying the detuning of the Bragg lasers, we show that it is possible to measure the response in the spin and density channels separately. At low Bragg energies, the spin response is suppressed due to pairing, whereas the density response is enhanced. These experiments yield the first independent measurements of the spin-parallel and spin-antiparallel dynamic and static structure factors, which provide insight into the different features of density and spin response functions. At high momentum the spin-antiparallel dynamic structure factor displays a universal high frequency tail, proportional to ω^-5/2, where ℏω is the probe energy.