Topological insulators constitute a novel class of materials in which the topological details of the bulk band structure induce a robust surface state on the edges of the material. Recent theoretical work established that fundamental symmetry considerations of the time reversal protected surface state allow for an additional topological term in Maxwell's laws of electromagnetism, with a universal quantized coefficient proportional to odd multiples of the fine structure constant. Such a term gives rise to several striking effects, the most direct of which is a distinct magnetoelectric response with a static electric field generating a magnetization in the same direction. We will use time domain Terahertz magneto-optical Faraday and Kerr rotation experiments to explore this effect. For an unambiguous demonstration the electro-chemical potential needs to be located close to the Dirac point situated in the bulk energy gap. This necessitates the development of large area gate structures suitable for Terahertz studies and very clean, i. e. bulk insulating materials. To this end, our work will intitially center on strained HgTe.
Terahertz magneto-optical studies further access optical conductivities and as such complement DC transport studies. Due to the high conductivities of Dirac surface states there is a marked response in the transport signal even if bulk carriers are present to some extent. For the development and optimization of new TI materials in which clean contacts may not be established, contactless and non-invasive magneto-optical characterization will therefore be a valuable instrument and as such an important ingredient of the experimental characterization toolbox of the collaborative research center.
[A07.4] A. M. Shuvaev, G. V Astakhov, A. Pimenov, C. Brüne, H. Buhmann and L. W. Molenkamp, Giant Magneto-Optical Faraday Effect in HgTe Thin Films in the Terahertz Spectral Range, Phys. Rev. Lett. 106, 107404 (2011).