Formation of nonlinear X -waves in condensed matter systems

X -waves are an example of a localized wave packet solution of the homogeneous wave equation and can potentially arise in any area of physics exhibiting wave phenomena, such as acoustics, electromagnetism, and quantum mechanics. They have been predicted in condensed matter systems such as atomic Bose-Einstein condensates in optical lattices and were recently observed in exciton-polariton condensates. Here we show that polariton X -waves result from an interference between two separating wave packets that arise from the combination of a locally hyperbolic dispersion relation and nonlinear interactions. We show that similar X -wave structures could also be observed in expanding spin-orbit coupled Bose-Einstein condensates.