Terahertz spectroscopy of topological insulator HgTe

Topological insulators are novel materials with unusual and nontrivial properties like protected conducting states at the surface. Unusual electrodynamic properties of topological insulators are expected like giant Kerr rotation or universal step in the Faraday effect. Within the proposed project the theoretical predictions will be proven experimentally with the help of unique magneto-optical technique at terahertz and infrared frequency ranges. Samples with transparent electrode will allow to study electrodynamic response of topological states in the vicinity of Dirac point with the gate-controlled carriers density. In addition, these experiments will lead to a development of electrically tuned polarization rotator in the terahertz range.

Topological insulators are a new class of materials which are insulating inside their interior but conducting on the surfaces. The most promising property of this surface conduction is a high degree of spin polarization, that is, the direction of the electron`s spin being locked to the direction of travel. Materials with such properties could provide the ground for the novel spin- based elements in the field of spintronics. One of the predictions for spin polarized surfaces was that the light travelling through such surface will change the direction of oscillation only by a certain quantized amount. More precisely, the angle by which the direction of oscillation is changed can only be a multiple of the fine structure constant a = 1/137, a constant which is not specific to any material, but is fundamental to our universe. The most important result of this project, the experimental observation of this quantized rotation caused by the topological surfaces, was observed in mercury telluride thin films, the particular material studied within the project. The fact that the measured rotation is not material dependent can be used for metrological measurements of fundamental constants. Generally, the property of a material being conducting, semiconducting or insulating depends on the properties of electrons inside the material, which can be described by a so-called band structure of the material. Besides the main goal, a special method to study the band structure of materials was also developed within the present project and successfully applied to mercury telluride.