Witten effect in a crystalline topological insulator

It has been noted a long time ago that a term of the form theta(e(2)/2 pi h)B.E may be added to the standard Maxwell Lagrangian without modifying the familiar laws of electricity and magnetism. theta is known to particle physicists as the axion field and whether or not it has a nonzero expectation value in vacuum remains a fundamental open question of the standard model. A key manifestation of the axion term is the Witten effect: a unit magnetic monopole placed inside a medium with theta not equal 0 is predicted to bind a (generally fractional) electric charge -e(theta/2 pi+n) with n integer. Here we conduct a test of the Witten effect based on the recently established fact that the axion term with theta = pi emerges naturally in the description of the electromagnetic response of a class of crystalline solids called topological insulators-materials distinguished by strong spin-orbit coupling and nontrivial band structures. Using a simple physical 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.