Topological Surface States in Thick Partially Relaxed HgTe Film.

Surface states of topological insulators (TIs) have been playing the central role in the majority of outstanding investigations in low-dimensional electron systems for more than 10 years. TIs based on high quality strained HgTe films demonstrate a variety of subtle physical effects. The strain leads to a bulk band gap, but limits a maximum HgTe strained film thickness, and, therefore, the majority of experiments were performed on the films with a thickness of less than 100 nm. Since a spatial separation of topological states is crucial for the study of a single surface response, the HgTe thickness is essential to be increased further. In this work, by combining transport measurements together with capacitance spectroscopy, we performed the analysis of a 200 nm partially relaxed HgTe film. The Drude fit of the classical magnetotransport revealed the ambipolar electron-hole transport with a high electron mobility. The detailed analysis of Shubnikov-de Haas oscillations, both in conductivity and capacitance, allowed us to distinguish three groups of electrons, identified as electrons on top and bottom surfaces and bulk electrons. The indirect bulk energy gap value was found to be close to zero. It was established that the significant gap decrease does not affect the surface states which are found to be well-resolved and spin non-degenerate. The presented techniques allow the investigations of other 3D TIs, regardless of the presence of bulk conductivity.