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.

Persistent photoconductivity (PPC) spectra of HgTe/CdHgTe heterostructures with double quantum wells with different cap layers have been studied in the radiation excitation range 0.62–3.1 eV. We have shown that the material of the cap layer defines key features of the PPC spectra—local extrema—and their origin. An unusual oscillatory behavior of the PPC spectra is demonstrated. Such a behavior is shown to be independent of both cap and barrier layers.

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.

In this work, we conduct research of spectral and power characteristics of quantum cascade lasers (QCLs) based on a GaAs/Al0.15Ga0.85As active region emitting at 2.3 (A), 3.2 (B) and 4.1 (C) THz. The QCL devices had a double-metal Au waveguide and operated in pulsed mode with 1.5–9 us pulses at 20 Hz repetition rate. Using the integral output power curves measured with different pulse durations, we consider the potential mechanisms of QCL temperature degradation using Arrhenius plots. Moreover, we present the spectra of the lasers measured at fixed operating points for devices A, С and with current scanning for device B in a wide temperature range from 5 to 120 K. We hope that our results will prove useful for research concerning QCL maximum operating temperatures.

In this work, we conduct research of spectral and power characteristics of quantum cascade lasers (QCLs) based on a GaAs/Al0.15Ga0.85As active region emitting at 2.3 (A), 3.2 (B) and 4.1 (C) THz. The QCL devices had a double-metal Au waveguide and operated in pulsed mode with 1.5–9 μs pulses at 20 Hz repetition rate. Using the integral output power curves measured with different pulse durations, we consider the potential mechanisms of QCL temperature degradation using Arrhenius plots. Moreover, we present the spectra of the lasers measured at fixed operating points for devices A, C and with current scanning for device B in a wide temperature range from 5 to 120 K. We hope that our results will prove useful for research concerning QCL maximum operating temperatures.

We conduct research of quantum cascade lasers (QCLs) emitting at 2.3, 3.2 and 4.1 THz. Measuring the output power, we consider the potential mechanisms of QCL temperature degradation. Additionally, we present the spectra of the lasers measured in a wide temperature range 5 -120 K.

We report a detailed study of the bipolar persistent photoconductivity in an HgTe/CdHgTe double quantum well (DQW), which can be a perspective for studying topological states in these structures. Photoconductivity spectra measurements in the range of 1.1–3.1 eV as well as transport measurements under different illumination conditions were performed at T = 4.2 K. Based on the results, the processes occurring in the structure under illumination and leading to a change in the carrier concentration in the DQW have been established. They include interband generation in the CdTe cap layer and in the CdHgTe barrier layer and electron transitions from the spin-split band in the CdHgTe barrier layer to the conduction band in the CdTe cap layer. The presence of the CdTe cap layer and the appropriate cadmium fraction in the CdHgTe barrier layers have been shown to be the main factors determining the key features of the spectra. Finally, we suggest an effective method of controlling the conductivity type of HgTe/CdHgTe structures using light with different wavelengths.

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.

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.

Анализ вольт-амперных характеристик позволил определить механизмы проводимости, соответствующие различным состояниям каналов протекания при резистивных переключениях в пленках порфиразинов. Варьирование температуры, структуры диэлектрической матрицы и типа основных носителей заряда позволило оценить применимость модели проводящих каналов для описания транспорта и определить механизмы проводимости для каждого состояния системы. Ключевые слова: пленки порфиразинов, транспорт носителей заряда, резистивное переключение.

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.

We studied the photoelectromagnetic (PEM) effect induced by laser terahertz radiation in Pb1-xSnxTe 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.