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تسجيل مستخدم جديدEngineering topological surface-states: HgS, HgSe and HgTe
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Using density functional electronic structure calculations, we establish the consequences of surface termination and modification on protected surface-states of metacinnabar (beta-HgS). Whereas we find that the Dirac cone is isotropic and well-separated from the valence band for the (110) surface, it is highly anisotropic at the pure (001) surface. We demonstrate that the anisotropy is modified by surface passivation because the topological surface-states include contributions from dangling bonds. Such dangling bonds exist on all pure surfaces within the whole class HgX with X = S, Se, or Te and directly affect the properties of the Dirac cone. Surface modifications also alter the spatial location (depth and decay length) of the topologically protected edge-states which renders them essential for the interpretation of photoemission data.
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We report ab initio calculations of the electronic band structure and the phonon dispersion relations of the zincblende-type mercury chalcogenides (beta-HgS, HgSe, and HgTe). The latter have been used to evaluate the temperature dependence of the spe
cific heat which has been compared with experimental data. The electronic band structure of these materials has been confirmed to have an inverted direct gap of the alpha-tin type, which makes HgSe and HgTe semimetallic. For beta-HgS, however, our calculations predict a negative spin-orbit splitting which restores semiconducting properties to the material in spite of the inverted gap. We have calculated the spin-orbit induced linear terms in k which appear at the Gamma_8 valence bands. We have also investigated the pressure dependence of the crystal structure and the phonons.
We report transport studies on a three dimensional, 70 nm thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate. The strain induces a band gap in the otherwise semi-metallic HgTe, which thus becomes a three dimensional topologic
al insulator. Contributions from residual bulk carriers to the transport properties of the gapped HgTe layer are negligible at mK temperatures. As a result, the sample exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states.
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 va
riety 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 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, it is essential to increase the HgTe thickness further. In this work, by combining transport measurements together with capacitance spectroscopy, we perform an analysis of a 200-nm partially relaxed HgTe film. The Drude fit of the classical magnetotransport reveals the ambipolar electron-hole transport with a high electron mobility. A detailed analysis of Shubnikov-de Haas oscillations in both conductivity and capacitance allows us to distinguish three groups of electrons, identified as electrons on top and bottom surfaces and bulk electrons. The indirect bulk energy gap value is found to be close to zero. It is established that the significant gap decrease does not affect the surface states, which are found to be well resolved and spin nondegenerate. The presented techniques allow investigations of other three-dimensional TIs, regardless of the presence of bulk conductivity.
Preceded by the discovery of topological insulators, Dirac and Weyl semimetals have become a pivotal direction of research in contemporary condensed matter physics. While easily accessible from a theoretical viewpoint, these topological semimetals po
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