Modulational instability in repulsive Bose-Einstein condensates

Modulational instability is an important hydrodynamic phenomenon typically associated with self-focusing (i.e., attractive) interactions in fluids, emerging in a variety of fields including water waves, nonlinear optics, and superfluids. This instability is associated with an exponentially growing wave mode, often associated with rogue waves in shallow water and solitons in optical fiber. Here we describe experimental demonstrations of modulational instability in atomic Bose-Einstein condensates with repulsive interactions. We describe the methodologies for observing these wave phenomena in two-component atomic condensates, followed by recent observations of the Peregrine soliton and barrier induced modulational instability.