Critical Velocity in the BEC-BCS Crossover

We map out the critical velocity in the crossover from Bose-Einstein condensation to Bardeen-Cooper-Schrieffer superfluidity with ultracold Li-6 gases. A small attractive potential is dragged along lines of constant column density. The rate of the induced heating increases steeply above a critical velocity upsilon(c). In the same samples, we measure the speed of sound upsilon(s) by exciting density waves and compare the results to the measured values of upsilon(c.) We perform numerical simulations in the Bose-Einstein condensation regime and find very good agreement, validating the approach. In the strongly correlated regime our measurements of upsilon(c) provide a testing ground for theoretical approaches.