Spontaneous formation of ring dark solitons and vortex dipole necklaces in a two-dimensional atomic superfluid

Unveiling non-equilibrium dynamics of a nonlinear medium is a current frontier across diverse fields. Two-dimensional (2D) quantum gas in an arbitrarily painted box potential offers a versatile platform for studying microscopic details of nonlinear phenomena, ranging from patterned defect formations at repulsive interactions to fragmentation and collapse occurring under attractive interactions. In this talk, we will present the observation of spontaneous defect formation in a 2D superfluid in a circular box. By quenching either the repulsive interaction strength or the confining box potential, we observe ring dark solitons emerging from the edge and their transverse instability at discrete azimuthal angles, resulting in a patterned formation of vortex dipole necklaces. Through collisions of the vortex dipoles with the box trap, we observe vortex unbinding, vortex pinning to the edge, and emission of rarefaction pulses. Our realization of ordered vortices with alternating charges potentially opens a doorway to study a variety of many-body dynamics, including persistent revivals and clusterization of vortex dipoles, which do not occur in disordered vortex matter.