Universal Faraday Effect in 3D Topological Insulator

Topological insulators and related materials have become a hot topic
of research recently. Following the theoretical predictions and the
results of spin-resolved ARPES studies, the signatures of nontrivial
topological states were observed in direct transport measurements.
In the present work the direct interaction of the electromagnetic radiation
with the 3D topological thin film is realized in the magneto-optic experiments.
The linearly polarized THz waves of a certain frequency are utilized to test
the cosequenses of novel eletrodynamics intrinsic to
to the nontrivial electronic states on the surfaces of the sample.
The MBE grown HgTe thin films were fitted with the semitransparent
gate electrodes. It has allowed to control the position of the Fermi level
and suppress the interband transitions from the valence band Landau levels.
The universal Faraday effect was observed in the broad range of magnetic
fields and gate voltages. The measured terahertz Faraday rotation equal
to the fine structure constant α = e² / 2ε₀hc (in SI
units) is a direct consequence of axion electrodynamics of 3D topological
insulators.