Giant magneto-optical Kerr effect and universal Faraday effect in thin-film topological insulators

Phys Rev Lett. 2010 Jul 30;105(5):057401. doi: 10.1103/PhysRevLett.105.057401. Epub 2010 Jul 26.


Topological insulators can exhibit strong magneto-electric effects when their time-reversal symmetry is broken. In this Letter we consider the magneto-optical Kerr and Faraday effects of a topological insulator thin film weakly exchange coupled to a ferromagnet. We find that its Faraday rotation has a universal value at low frequencies θF=tan(-1)α, where α is the vacuum fine structure constant, and that it has a giant Kerr rotation θK=π/2. These properties follow from a delicate interplay between thin-film cavity confinement and the surface Hall conductivity of a topological insulator's helical quasiparticles.