Abstract
Herein, studies of the cyclotron resonance (CR) in thick films with different cadmium contents corresponding to materials with and without band inversion, as well as critical content corresponding to an almost linear energy dispersion are presented. The results demonstrate that the formation of 2D topological surface states requires sharp interfaces between layers with and without band inversion, in which case the corresponding CR is clearly observed for the out‐of‐plane orientation of magnetic field but does not show up for an in‐plane orientation. In contrast, all samples having more conventional technological design with smooth interfaces (i.e., containing regions of with gradually changing Cd content x) show equally pronounced CR in both in‐plane and out‐of‐plane magnetic field revealing that CR is excited in effectively 3D states. Modeling of the surface states for different film designs supports main observations. In all samples, additional broad helicity‐independent resonances are observed, which are attributed to photoionization and magnetic freeze‐out of impurity states.
Highlights
The discovery of the quantum spin Hall effect in HgTe quantum wells [1,2] and of topological insulators [3,4,5] stimulated rapidly has been attracted to bulk CdxHg1ÀxTe films with the Cd content x equal or lower than the critical one, xc, defining the phase transition from inverted to noninverted band order.[31]
We find that the number of Volkov– Pankratov states (VPS) emerging at such a smooth interface is highly sensitive to variation of the band gap, as well as to the energy splitting between light- and heavy-holes caused by deformation and the static electric field
The presented low-temperature THz spectroscopy studies of cyclotron resonances (CRs) in thick films of CdxHg1ÀxTe unambiguously show that 2D surface states are present only in structures with a sharp interface separating the internal layer with band inversion (x < 0.17) and a cap layer without band inversion
Summary
The discovery of the quantum spin Hall effect in HgTe quantum wells [1,2] and of topological insulators [3,4,5] stimulated rapidly has been attracted to bulk CdxHg1ÀxTe films with the Cd content x equal or lower than the critical one, xc, defining the phase transition from inverted to noninverted band order.[31] This interest has been driven by the observation of 3D massless Kane growing interest in fundamental properties and possible applica- fermions in films with x 1⁄4 xc,[32] followed by detailed studies tions of HgTe-based materials. An attractive feature of these using magneto- and terahertz-spectroscopy.[33,34,35] Topological materials is that the band inversion, crucial for the formation insulators (TIs) based on bulk films with Cd contents below.
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