Nonlinear Acoustics in Ultracold Fermi Fluids

Near- and far-from-equilibrium dynamics are among the most interesting phenomena in quantum many-body systems and pose major challenges to theoretical descriptions. Here we use a degenerate Fermi gas in a box trap as a platform to investigate nonlinear sound waves in ultracold Fermi fluids. The uniform density and sharp boundary condition of the setup allow for clean excitation and observation of waves. We benchmark our measurements in the linear response regime by measuring the density-density response in ideal and weakly interacting Fermi gases, which in the T=0 limit, is the celebrated Lindhard function. In the nonlinear response regime, we encounter several surprises, including the emergence of a local minimum in the response versus drive frequency of a weakly interacting gas, and the unexpectedly high nonlinear sensitivity of the unitary gas.