Phase coherence and superfluid-insulator transition in a disordered Bose-Einstein condensate

We have studied the effects of a disordered optical potential on the transport and phase coherence of a Bose-Einstein condensate (BEC) of Li-7 atoms. At moderate disorder strengths (V-D), we observe inhibited transport and damping of dipole excitations, while in time-of-flight images, random but reproducible interference patterns are observed. In situ images reveal that the appearance of interference is correlated with density modulation, without complete fragmentation. At higher V-D, the interference contrast diminishes as the BEC fragments into multiple pieces with little phase coherence.