Stationary and Nonstationary Fluid Flow of a Bose-Einstein Condensate Through a Penetrable Barrier

We experimentally study the fluid flow induced by a broad, penetrable barrier moving through an elongated dilute gaseous Bose-Einstein condensate. The barrier is created by a laser beam swept through the condensate, and the resulting dipole potential can be either attractive or repulsive. We examine both cases and find regimes of stable and unstable fluid flow: At slow speeds of the barrier, the fluid flow is steady due to the superfluidity of the condensate. At intermediate speeds, we observe an unsteady regime in which the condensate gets filled with dark solitons. At faster speeds, soliton formation completely ceases, and a remarkable absence of excitation in the condensate is seen again.