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In this paper, we show that electron states formed in topological insulators at the interfaces topological phase–trivial phase and topological phase–vacuum may possess different properties. This is demonstrated on an example of heterostructures based on thick topological Hg1−xCdxTe films, in which the PT-symmetric terahertz photoconductivity is observed. It is shown that the effect originates from features of the interface topological film–trivial buffer/cap layer. The PT-symmetric terahertz photoconductivity is not provided by electron states formed at the interface topological film–vacuum.

We show that the terahertz (THz) photoconductivity in the topological phase of Hg1–xCdxTe-based structures exhibits the apparent PT- (parity-time) symmetry whereas the P-symmetry and the T-symmetry, separately, are not conserved. Moreover, it is demonstrated that the P- and T-symmetry breaking may not be related to any type of the sample anisotropy. This result contradicts the apparent symmetry arguments and means that there exists an external factor that interacts with the sample electronic system and breaks the symmetry. We show that deviations from the ideal experimental geometry may not be such a factor.

Terahertz photoconductivity in heterostructures based on n-type Hg1−xCdxTe epitaxial films both in the topological phase (x < 0.16, inverted band structure, zero band gap) and the trivial state (x > 0.16, normal band structure) has been studied. We show that both the positive photoresponse in films with x < 0.16 and the negative photoconductivity in samples with x > 0.16 have no low-energy threshold. The observed non-threshold positive photoconductivity is discussed in terms of a qualitative model that takes into account a 3D potential well and 2D topological Dirac states coexisting in a smooth topological heterojunction.

For the Hg1−x Cd x Te-based structures, it is shown that the transition from the direct to invertеd spectrum is accompanied by the sign change for the signals related to the terahertz photoconductivity and to the magnetophotogalvanic effect. Within the range of chemical compositions corresponding to the inverted spectrum, the photoconductivity kinetics exhibits specific features, which can result from the surface topological states.

In this paper, we show that the terahertz photoconductivity and the magnetophotogalvanic effect in Hg 1−x Cd x Te films change the sign across the topological transition from the inverted to the trivial electron energy spectrum. Electron energy spectrum modification influenced by magnetic field and topological surface states formation are taken into account to discuss the observed photoelectric phenomena features.