Floquet Quantum Criticality*

It has recently been shown that periodically driven (Floquet) systems can, in the presence of disorder, support well-defined phase structure, including possibilities unique to this non-equilibrium setting. However, the nature of the critical points separating such phases has been largely unexplored. We introduce a real-space renormalization group procedure for Floquet systems and apply it to the periodically driven interacting Ising chain. Using analytical arguments and numerical calculations of the entanglement entropy scaling, we identify the criticality along the Floquet eigenstate phase transitions and at the multi-critical point of the non-interacting model, finding criticality not found in the un-driven case. We then discuss the stability of such critical points to interactions, and comment on applications to other Floquet phase transitions.