Engineering Topological Surface States: HgS, HgSe, and HgTe

Abstract

Using density functional electronic structure calculations, we establish the consequences of surface termination and modification on protected surface-states of metacinnabar (β-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.PACS numbers: 72.80.Sk An attractive perspective for spintronics was opened up by the discovery 1-3 and manufacturing 4-6 of topological insulators (TIs). Inside, in its bulk, a TI is insulating but on its boundary -the material's edges, surfaces, or interfaces -a TI is conducting. The first TI in which the topological character of its edge-states was established is HgTe, a mercury chalcogenide material of the HgX family (where X=S, Se, Te) with a zinc-blende crystal structure 4 . The understanding of the precise properties of the topological edge-states in the HgX family of TIs -and an understanding of how these can be altered and engineered -is of fundamental importance in the field. It is of course also of direct relevance to the development of future spintronics applications, which aims to exploit the spin-momentum locking of TI boundary electrons 7,8 allowing for instance the generation of edge-spin-currents by circularly polarized light 9-11 . The precise location and electronic structure of the topological surface-states, in particular their dispersion, play furthermore an essential role in determining their spin-transport characteristics. It is for instance well-known that the momentum-spin locking usually renders disordered TIs poor spin conductors: When the electronic spin is tied to its momentum each scattering event randomizes the spin 12,13 . It was recently established that the resulting limit on spin relaxation time τ s for edge-state electrons strongly depends on their dispersion and momentum space anisotropy 14 .Here we show that for the HgX class of mercury-based II-VI semiconductors in the zinc-blende structure there is a strong influence of existing dangling bonds on the properties of topological surface-states. Thus, by surface passivation, it is possible to achieve a strong modification of the spatial location of the topological states, their rate of decay into the bulk, the resulting Dirac cone dispersion and, in particular, the Dirac cone anisotropy. Such alterations are possible because the presence of topological edge-states is absolutely fixed by the topological invariants of the bulk electronic structure, but the shape of their dispersion, their locat...

First principles investigations of HgX (X=S, Se and Te)

“…HgS in the structure of zinc-blende (ZB) is used to produce nanocrystals or thin films [2]. Topological insulators, optoelectronic and spintronic applications, photovoltaic/photoconductive devices IR detectors and emitters, ohmic contact and removing mercury from exhaust gases, are some examples of their technological applications of mercury chalcogenides [3][4][5][6][7].…”

Manifestly gauge-independent formulations of theZ2invariants

“…For example, in a study on strained bulk HgTe material, 23 a noncentrosymmetric system, even with a tight-binding model, was more convenient to complete slab calculations and to look directly at the surface states rather than computing the bulk Z 2 strong invariant.A similar approach was followed in a recent first-principles study on the noncentrosymmetric metacinnabar compound. 24 Thus evaluating the Z 2 invariant is already difficult at the level of tight-binding modeling, but the difficulty becomes overwhelming when attempting first-principles electronic structure calculations. This aspect was recently discussed in Ref.…”

Spin texture and mirror Chern number in Hg-based chalcogenides

“…Because of the cubic symmetry, the light hole and heavy hole bands are degenerate at the Γ point, yielding a zero energy gap in HgTe (semi-metal phase). In contrast, HgS shows a "negative" SOC splitting [15,16], in which Γ 8 bands are below Γ 7 band. Thus, the valence and conduction bands are formed by Γ 8 and Γ 7 bands, respectively, with a non-zero energy gap.…”

“…1. Their band structures exhibit a band inversion at the Γ point, which induces the TI phase in both compounds [12][13][14][15]. The band inversion occurs between Hg-s and Te-p (S-p) bands for HgTe (HgS) near the Fermi energy.…”