Anomalous Edge States and the Bulk-Edge Correspondence for Periodically Driven Two-Dimensional Systems

When a topological insulator is ``driven'' by an applied electromagnetic field, its behavior cannot be predicted based on how it acts when the driving is absent. Theorists show that new conducting edge states can appear and that a new topological invariant, instead of the well-known Chern number, is needed to classify conducting edge states in driven topological insulators.