Topological Magneto-Electric Effect Decay

We address the influence of realistic disorder on the effective magnetic monopole that is induced near the surface of an ideal topological insulator (TI) by azimuthal currents which flow in response to a suddenly introduced external electric charge. We show that when the longitudinal conductivity ${\sigma }_{xx}=g(e^2\mathrm{/}h)$ is accounted for, the apparent position of a magnetic monopole initially retreats from the TI surface at speed $v_M=\alpha cg$, where $\alpha$ is the fine structure constant and $c$ is the speed of light. For the particular case of TI surface states described by a massive Dirac model, we further find that the temperature T=0 Hall currents vanish once the surface charge has been redistributed to screen the external potential.