Electrodynamics of MnBi 2 Te 4 intrinsic magnetic topological insulators

We report on the electrodynamics of MnBi 2 Te 4 thin films, an intrinsic magnetic topological material. We study its optical conductivity from terahertz (THz) to ultraviolet (UV) frequencies as a function of the film thickness, highlighting the presence of surface topological states superimposed on the bulk electrodynamics response. For the thinnest film, where the charge transport is dominated by Dirac surface states, we investigate the effect of the phase transition from the high-temperature topological protected state to the low-temperature magnetic (time-reversal broken) state by measuring the optical conductivity across the Néel temperature. At low temperatures, the breaking of the time reversal symmetry affects the optical conductivity, indicating that a magnetic-induced gap opens below T N .