Topological insulators are materials which are insulating in the bulk and which reveal conducting surface states. The electrodynamics of topological insulators is described by modified Maxwell’s equations, which contain additional terms that couple an electric field to a magnetization, such that the coupling coefficient is quantized. The new term leads to universal values of Faraday rotation angle equal to the fine structure constant when a linearly polarized terahertz radiation passes through a topological insulator. This regime is obtained in high magnetic field and in the quantum regime.

Further talks in the series of our Szilard Colloquium: http://physics.bme.hu/kollokvium?language=hu