Barrier induced modulational instability in a two-component repulsive Bose-Einstein condensate

Modulational instability is an important hydrodynamic phenomenon associated with self-focusing (i.e., attractive) interactions in fluids, emerging in a variety of fields including water waves, nonlinear optics, and atomic superfluids. Surprisingly, this instability can also arise in the case of purely repulsive interactions if a multicomponent system is employed. Here, we show how a two-component Bose-Einstein condensate which is completely described by repulsive interactions can have an emergent self-focusing interaction due to the immiscibility. We show how a localized barrier within a stationary superfluid can induce modulational instability which then spatially spreads out over time. Through a joint experimental and numerical study, we characterize how this phenomenon manifests for multicomponent mixtures, and discuss the reduction to an effective single component model with attractive interactions.