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 ⁷Li atoms. At moderate disorder strengths (VD), 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 VD, the interference contrast diminishes as the BEC fragments into multiple pieces with little phase coherence.