Higher-order Floquet topological phases with corner and bulk bound states

Periodically driven systems can be quite different from those in equilibrium. For example, vortices stirred in a fluid can trap nearby objects, such as bubbles. The authors report their theoretical discovery of new phases of a two-dimensional quantum system under periodic drive, in which nonequilibrium states are stabilized at the corners as well as within stirred vortices in the bulk. These are higher-order topological phases, exhibiting the intimate connection between corner and bulk vortex structures generated by spatiotemporal modulations.