Surface-quantized anomalous Hall current and the magnetoelectric effect in magnetically disordered topological insulators.

We study theoretically the role of quenched magnetic disorder at the surface of topological insulators by numerical simulation and scaling analysis based on the massive Dirac fermion model. This addresses the problem of Anderson localization on chiral anomaly. It is found that all the surface states are localized, while the transverse conductivity is quantized to be ±e2/2h as long as the Fermi energy is within the bulk gap. This greatly facilitates the realization of the topological magnetoelectric effect proposed by Qi et al. [Phys. Rev. B 78, 195424 (2008)] with the surface magnetization direction being controlled by the simultaneous application of magnetic and electric fields.