Screening and Non-Equilibrium Kohn Anomalies in Floquet-Driven Two-Dimensional Electron Systems

Periodic driving of quantum systems offers a new platform for controlling and engineering materials’ properties. We present a theory for the time-averaged polarization function of a monochromatically irradiated two-dimensional electron gas. Our theory shows that non-equilibrium Floquet excitations lead to the appearance of new non-analytic features in the polarization function, i.e., non-equilibrium Kohn anomalies. We obtain approximate analytic formulas for the polarization function and Kohn anomalies by approximating the Green’s functions in the Keldysh-Floquet formulation of the polarization function in terms of successive powers of light-matter coupling. We find that irradiation introduces considerable corrections to the n=0 equilibrium-like Kohn anomaly. Additionally, our results show that the Kohn anomalies associated with electrons traversing Floquet sidebands with |n|>0 are proportional to even powers of the light-matter coupling and that all the Kohn anomalies share the same non-analytic behavior. We discuss the consequences of these new Kohn anomalies on the screened Coulomb potential, Friedel oscillations, and the RKKY interaction.