Witten effect in a crystalline topological insulator

A term of the form θ(e^2/2πh)B.E may be added to the standard Maxwell Lagrangian without modifying the familiar laws of electricity and magnetism. θ is known as the "axion" field. The axion term gives the Witten effect: a unit magnetic monopole placed inside a medium with θ $\neq$ 0 is predicted to bind a (generally fractional) electric charge -e(θ/2π+n) with n integer. It has been recently shown that the axion term with θ=π emerges naturally in the description of the electromagnetic response of topological insulators. In this paper the authors demonstrate the existence of a fractional charge bound to a monopole by an explicit numerical calculation and 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.

Reference: "Witten effect in a crystalline topological insulator", G. Rosenberg and M. Franz, Phys. Rev. B 82, 035105 (2010)