Witten effect in a topological insulator

It has been established recently that topological insulators exhibit non-trivial electromagnetic response described by the `axion' term ${\Delta{\cal L}_{\rm axion}=\theta (e2/2\pi h){\bf E} \cdot {\bf B}}$ with $\theta=\pi$. The parameter $\theta$ is known to particle physicists as the axion field and whether or not it has a nonzero vacuum expectation value remains a fundamental open question of the Standard Model. A key manifestation of axion electrodynamics is the Witten effect, according to which a unit magnetic monopole placed inside an axion medium is predicted to bind fractional charge $-e(\theta/2\pi)$. I will present a proposal for a first test of the Witten effect. Using a simple model for a topological insulator we demonstrate the existence of a fractional charge bound to a monopole by an explicit numerical calculation. We also propose a scheme for generating an `artificial' magnetic monopole in a topological insulator film that may be used to facilitate an experimental test of Witten's prediction in a solid state system.