Witten effect in a crystalline topological insulator

TL;DR

This paper explores the Witten effect in topological insulators, demonstrating fractional charge binding to monopoles and proposing an experimental setup to test this fundamental physics prediction.

Contribution

It provides the first numerical demonstration of the Witten effect in a topological insulator model and suggests a method to experimentally observe the phenomenon.

Findings

Fractional electric charge binds to magnetic monopoles in topological insulators.

Numerical evidence supports the existence of the Witten effect in these materials.

Proposed experimental scheme to generate artificial monopoles for testing the effect.

Abstract

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.