Terahertz magneto-optics of Dirac fermions

Topological insulators are insulators in the bulk but they reveal topologically protected conducting states on the surface. The electrodynamics of topological insulators is presently one of the most intensive fields of research in solid state physics. Recently, the predicted universal Faraday rotation in topological insulators has been observed experimentally by several groups. These experiments uncovered a number of problems in the physics of topological insulators, including the impossibility of observing the predicted half quantum of the Faraday rotation. The question of whether this is just an experimental problem, or a fundamental physical limitation, remains puzzling. In addition, even the terahertz quantum Hall effect in conventional two-dimensional quantum wells remains largely unexplored. Therefore, in several cases, it remains unclear whether the observed new effects are characteristic for topological insulators, or whether they are manifestations of the conventional quantum Hall effect. In this project we plan to undertake several experimental steps towards the understanding of the magneto-optical dynamics of topological insulators in the quantum regime. In order to achieve these goals, several comparative experiments, in conventional and topological quantum wells, will be carried out covering a broad range of physical parameters. Additionally, we will target an improvement in the accuracy of the quantum Hall standard in the dynamic regime.

Topological insulators are promising materials for spintronics, a field of research that tries to utilize the spin of an electron for information processing, in addition to its charge, as in the conventional electronics. Spintronic devices can achieve much higher computational power compared to the conventional computers. Mercury telluride (HgTe) thin films studied in the present project are the cleanest examples of topological insulators. The property of being the topological insulator is determined by the band structure of the material. The main goal of the project is the experimental determination of band structures of HgTe films with different thickness. The systematic study of the present work has established both the parameter boundaries and intrinsic laws determining these boundaries between different states of HgTe thin films. Transitions between Dirac semimetal, indirect semimetal and 3D topological insulator were identified in experiment and described by the theory. Apart from the main part of the project, a better understanding of light-matter interaction was also achieved, recognizing the plasmonic properties of 2D semiconductors at high frequencies, contrary to the dissipative conductivity at low frequencies. These findings are of high relevance already for the upcoming high-frequency 5G networks.