  It has been noted a long time ago that a term of the form theta (e^2/2\pi h)
B dot 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 non-zero theta is predicted to bind a
(generally fractional) electric charge -e(theta/2 pi+n) with n integer. Here we
conduct a first 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 new class of crystalline solids called
topological insulators - materials distinguished by strong spin-orbit coupling
and non-trivial band structure. 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 the first experimental test of Witten's
prediction.
