We show the differences in the necessarily formed electronic states at the boundaries of the topological phase with a vacuum and with a trivial buffer in the regions of heterojunction in topological materials based on epitaxial films Hg1 – xCdxTe. It was demonstrated that the PT-symmetric terahertz photoconductivity observed in these structures is due precisely to the states in the region of the topological film/trivial buffer (or cap-layer) interfaces.

An experimental study is performed of the differences between the electronic states necessarily formed at the boundaries of a topological phase in a vacuum and a trivial buffer in the regions of heterojunction in topological materials based on Hg 1 – x Cd x Te epitaxial films. It is shown that the PT -symmetric terahertz photoconductivity observed in the specified structures is due precisely to states in the region of topological film/trivial buffer (or cap layer) interfaces.

The PT -symmetric photoconductivity has been detected for the first time in microwave-irradiated heterostructures based on thick Hg 1 − x Cd x Te films with the CdTe content x corresponding to the topological phase although the magnetic field symmetry ( T symmetry) and the symmetry in the positions of potential contact pairs ( P symmetry) are not conserved separately. The microwave photoconductivity in similar heterostructures based on the trivial Hg 1 − x Cd x Te phase is both P - and T -symmetric.

Positive photoconductivity stimulated by microwave radiation has been observed in Hg 1 – x Cd x Te thick films being in the topological phase at x < 0.16. The effect amplitude is significant at the liquid helium temperatures, and rapidly decreases with the temperature rising. A mechanism for the positive photoconductivity appearance in the topological phase films is suggested.

We report on observation of strong non-local photoconducitivity induced by terahertz laser pulses in non-zero magnetic field in heterostructures based on Hg1-xCdxTe films being in the topological phase. While the zero-field non-local photoconductivity is negligible, it is strongly enhanced in magnetic fields similar to 0.05 T resulting in appearance of an edge photocurrent that exceeds the respective dark signal by orders of magnitude. This photocurrent is chiral, and the chirality changes every time the magnetic field or the electric bias is reversed. Appearance of the non-local terahertz photoconductivity is attributed to features of the interface between the topological film and the trivial buffer.

We report on observation of strong non-local photoconducitivity induced by terahertz laser pulses in non-zero magnetic field in heterostructures based on $\mathrm {Hg}_{{f{1-}{x}}}{\mathrm {Cd}} _{{x}}$Te films being in the topological phase. While the zero-field non-local photoconductivity is negligible, it is strongly enhanced in magnetic fields $\sim 0.05\mathrm{T}$ resulting in appearance of an edge photocurrent that exceeds the respective dark signal by orders of magnitude. This photocurrent is chiral, and the chirality changes every time the magnetic field or the electric bias is reversed. Appearance of the non-local terahertz photoconductivity is attributed to features of the interface between the topological film and the trivial buffer.

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.

Представлены результаты по нелокальной терагерцовой фотопроводимости в толстых эпитаксиальных пленках Hg1-xCdxTe с инвертированным энергетическим спектром (x <~ 0.16). В работе прямо продемонстрировано существование нелокальной компоненты терагерцового фотоответа в толстых эпитаксиальных пленках Hg1-xCdхTe в магнитном поле. Знак нелокального фотоответа зависит от положения потенциального зонда и направления магнитного поля, что указывает на киральность индуцированного неравновесного транспорта. Наблюдаемые нетривиальные особенности фототранспорта можно интерпретировать как проявление образования концевого хирального краевого проводящего канала в топологической фазе сплавов Hg1-xCdхTe. Мы обсуждаем результаты в терминах качественной модели, которая учитывает сосуществование объемного транспортного и граничного проводящих каналов.

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 Hg 1− x Cd x Te 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 report on observation of strong non-local photoconducitivity induced by terahertz laser pulses in non-zero magnetic field in heterostructures based on Hg1−xCdxTe films being in the topological phase. While the zero-field non-local photoconductivity is negligible, it is strongly enhanced in magnetic fields ~ 0.05 T resulting in appearance of an edge photocurrent that exceeds the respective dark signal by orders of magnitude. This photocurrent is chiral, and the chirality changes every time the magnetic field or the electric bias is reversed. Appearance of the non-local terahertz photoconductivity is attributed to features of the interface between the topological film and the trivial buffer.

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.

Abstract The results of studies of the optical and transport properties of epitaxial Pb_0.74Sn_0.26Te(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.

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.

We present results on non-local terahertz photoconductivity in Hg 1-x Cd x Te thick epitaxial films with the inverted energy spectrum (x<; ~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.

In this paper, we show that the positive terahertz photoconductivity in Hg1-xCdxTe solid solutions being in a topological phase (x < 0.16) 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.

Terahertz photoconductivity in heterostructures based on n-type Hg 1− x Cd x Te 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.

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.

For the Hg 1− 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.