Superfluid Fermi gases in the BEC-BCS crossover realize a model system for strong correlations. In this talk I will present our recent experimental results on 3D and 2D Fermi gases. In 3D Fermi gases, we map out the critical velocity along the crossover by perturbing the gas with a small obstacle. We also measure the speed of sound vs and compare vc with vs. We find good agreement with numerical simulations in the BEC regime, validating the approach. In the strongly correlated regime, our measurements of vc provide a testing ground for theoretical approaches.
In lower dimensional gases quantum fluctuations play a more important role than in three dimensions. 2D systems are particularly hard to describe, since the exact analytical solutions available in 1D do not exist. Moreover, logarithmic divergences appear in most quantities. We measure the speed of sound in 2D Fermi gases, which depends on the precise scattering physics and the equation of state. Our measurements are sensitive enough to rule out the applicability of mean field theories including phase and amplitude fluctuations. They agree well with quantum Monte Carlo simulations. Finally, I will present an experiment in which we probe the coherence properties of 2D gases across the Berezinskii-Kosterliss Thouless transition.