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The results of studies of the optical and transport properties of epitaxial Pb0.74Sn0.26Te(In) films with a differently treated surface are presented. A comparative analysis of the results makes it possible to establish that the characteristic features of the photoconductivity spectra are defined by processes in the bulk while the surface states have practically no effect on the spectra. At the same time, the photoresponse kinetics and the rate of relaxation processes are heavily dependent on the state of the surface. The characteristic relaxation times of nonequilibrium charge carriers (about several milliseconds) are such that it is possible to use the methods of standard Fourier-transform spectroscopy to characterize optical transitions in the bulk.

In this paper, we report on photoconductivity induced by high-power laser radiation with a frequency of 2 THz in Hg0.87Cd0.13Te-based epitaxial structures. Experimental results obtained for a set of the samples with variable geometric parameters allow us to determine the photoresponse features of both bulk and nonlocal contributions to the net response. We show that the persistent photoconductivity effect originates from the non-equilibrium processes related to the bulk carrier excitation.

Topological crystalline insulators form a class of semiconductors for which surface electron states with the Dirac dispersion relation are formed on surfaces with a certain crystallographic orientation. Pb1−xSnxTe alloys belong to the topological crystalline phase when the SnTe content x exceeds 0.35, while they are in the trivial phase at x < 0.35. For the surface crystallographic orientation (111), the appearance of topologically nontrivial surface states is expected. We studied the photoelectromagnetic (PEM) effect induced by laser terahertz radiation in Pb1−xSnxTe films in the composition range x = (0.11–0.44), with the (111) surface crystallographic orientation. It was found that in the trivial phase, the amplitude of the PEM effect is determined by the power of the incident radiation, while in the topological phase, the amplitude is proportional to the flux of laser radiation quanta. A possible mechanism responsible for the effect observed presumes damping of the thermalization rate of photoexcited electrons in the topological phase and, consequently, prevailing of electron diffusion, compared with energy relaxation.

We have shown that in heterostructures based on the topological phase Hg 1-x Cd x Te, the properties of two-dimensional topological states formed at the trivial buffer - topological film heterointerface differ from the states at the topological film - vacuum interface, and it is the former that are responsible for the appearance of PT-symmetric photoconductivity. It has been shown that the source of nonequilibrium electrons is the film bulk, while the place of manifestation of the effect is the trivial buffer - topological film heterointerface.

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 studied the photoelectromagnetic (PEM) effect induced by laser terahertz radiation in $\text{Pb}_{1_{-x}}\text{Sn}_{x}\text{Te}$ films in the composition range $x=(0.15-0.45)$ . A transition from the trivial phase to the topological crystalline insulator phase occurs within this composition interval at $x=0.35$ . The films had the crystallographic orientation $ $ , for which the appearance of topologically nontrivial surface states is expected. It was found that in the trivial phase, the amplitude of the PEM effect is determined by the power of the incident radiation, while in the topological phase, the amplitude is proportional to the flux of laser radiation quanta. Possible mechanism leading to the observed effect is discussed.

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.

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.

We present results on non-local terahertz photoconductivity in Hg 1-x Cd x Te thick epitaxial films with the inverted energy spectrum $(\mathrm{x}\lt \sim 0.16)$. We show that the phototransport features observed in magnetic field indicate realization of the nonlocal electron transport regime, which is inherent to 2D topological insulators. We discuss the results in terms of a qualitative model that takes into account coexistence of the bulk transport and boundary conductive channels.

In this work, we demonstrate that the terahertz photoconductivity in Hg 1-x Cd x Te epitaxial films (x < 0.15) is featured by superimposed positive and negative components. The dramatic drop of the positive photoresponse as the sample thickness decreases indicates that the bulk-related processes crucially contribute to the non-equilibrium transport. At the same time, the features observed in magnetic field provide us with arguments for an edge transport channel formation. We discuss the sign-alternating photoconductivity kinetics in terms of a model that takes into account both the bulk electron excitation and the surface/interface state contribution.

We show that the terahertz photoconductivity in Hg 1-x Cd x Te-based films with the inversed band structure corresponding to the topological phase is asymmetric in magnetic field which may be considered as a T-symmetry breaking. Beside that, the photoconductivity is asymmetric for two mirror-symmetric pairs of potential probes which may be treated as a P- symmetry breaking. At the same time, the photoconductivity remains intact upon simultaneous swapping of both magnetic field and potential probe couple revealing thus the PT-symmetry.

In this paper, we show that the positive terahertz photoconductivity in Hg l-x Cd x Te solid solutions being in a topological phase ( $(\mathrm{x} ) strongly depends on the combination of both the external magnetic field direction and the potential probe position. The asymmetry in the photoresponse in the magnetic field may be due to the spin-related features of the carriers excited.

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.