Appearance of modulated spatial pattern in a dual-species BEC during condensate growth.

It has been long known that a dual-species BEC with a sufficiently large repulsive intra-species interaction becomes immiscible. In a recent experiment with Rb85 and Rb87 by the Wieman group at JILA, a complicated pattern of multiple interleaved ``bubbles'' of the the two species has been observed upon the end of evaporative cooling. This observation defies expectations that the ground state should only have a single boundary between the two species. We purpose a model wherein this spatial pattern is formed during condensate growth through a modulation instability mechanism. We model the growth of the condensates with a linear gain term added to the Gross Pitaevskii equation, and obtain qualitatively similar patterns to the experiment. We also predict the conditions which would maximize the number of bubbles.