We study the spin Hall effect in two-dimensional topological insulators with “Mexican hat” dispersion and a ring-shaped Fermi surface which are formed due to the band inversion. Electron transitions between different isoenergetic contours and the quantum metric of band states play an important role in the transport properties of such materials, since they largely determine the spatial distribution of the electron charges screening the impurity potential and the scattering probability (Shchamkhalova and Sablikov, 2025). Here we study a spin-dependent skew scattering, which is enabled by the second-order scattering processes, and show that the extrinsic spin-Hall current (SHC) can significantly exceed the intrinsic SHC arising from the Berry curvature. Furthermore, due to Mexican-hat dispersion, the SHC exhibits a very unusual dependence on the Fermi energy (EF). The extrinsic SHC reaches a maximum at some EF, then decreases with increasing EF and can even change a sign. This complicated behavior reflects an interplay of energy dependencies of such important factors as probabilities of inter- and intra-contour transitions, as well as different electron velocities in two contours.

Scattering by charged impurities is known to mainly determine transport properties of electrons in modern quantum materials, but it remains poorly studied for materials with Mexican hat dispersion. Due to such nontrivial features as a singular density of states and a ring-shaped Fermi surface, electron-electron interaction and electron transitions between different isoenergetic contours are of key importance in this materials. We show that these factors significantly affect both the spatial profile of the screened potential of Coulomb centers and the dependence of mobility on temperature and electron density. The screened potential is calculated within the random phase approximation. The transport properties are determined without using the usual relaxation time approximation, since the distribution function in energy space is a vector defined by a system of two equations.

Dynamic conductance and time-of-flight current instability in a quantum wire connected to electron reservoirs under DC bias voltage are studied in the absence of a gate screening the Coulomb interaction of electrons. Due to a strong electron-electron interaction, dramatic rearrangements of the charge density distribution and the potential landscape in the wire occur at a sufficiently high DC bias voltage. The applied voltage is screened mainly near the cathode contact, and an almost flat potential profile is established in the most of the wire. Thus, the size of the region of a population inversion of electronic states greatly increases, and the band of wave vectors that form unstable modes of electronic waves significantly reduces. As a result, the conditions for the occurrence of the time-of-flight instability are greatly facilitated and the negative dynamic conductivity increases.

Bound electron pairs formed due to the peculiarities of the band dispersion of electrons in crystals attract much interest because they can carry charge and spin even in the absence of band conductivity. However, such an important parameter of bound pairs as the effective mass is still poorly understood. We carry out this study for materials described by the Bernevig-Hughes-Zhang model in the electron-hole symmetric case and find a clear relationship between the effective mass and the energy of a bound pair at rest. The dependence of mass on energy has a specific form for each of different types of pairs, but a common feature is the change of the mass sign when energy passes through the middle of the band gap. The sign is negative when the energy is in the lower half of the gap, and positive in the upper half.

Bound electron pairs (BEPs) arising due to peculiarities of the band structure of topologically non‐trivial materials are of interest as charge and spin carriers with energies in the bandgap. Moreover, being composite bosons, they can also possess completely unusual collective properties. In this regard, highest importance is the problem of their decay time. The processes of radiative decay of BEPs are studied, considering the electron–electron interaction in the final state, into which the BEPs decay. It is found that the decay time of singlet BEPs lies in the nanosecond range and significantly exceeds other characteristic electron relaxation times. The radiative decay of triplet BEPs is impossible.

Bound electron pairs (BEPs) with energy in the bandgap are interesting because they can participate in charge and spin transport in modern topologically nontrivial materials. The problem of their stability is addressed and the radiative decay of the BEPs formed due to the negative reduced effective mass in 2D topological insulators is studied. The decay time is found to be rather large on the scale of the characteristic relaxation times of the electron system and significantly dependent on the topological properties and dispersion of the band states. In the topological phase, the decay time is much longer than in the trivial one, and is estimated as ≈1 ns for the HgTe/CdHgTe heterostructures. However, the longest decay time is in the topological phase with nearly flat dispersion in the band extrema.

We present a first study of films of the quaternary Bi2-xSbxTe3-ySey solid solutions on (0001) sapphire substrates grown by atmospheric pressure MOVPE. Trimethylbismuth, trimethylantimony, diisopropy-lselenide and diethyltelluride were used as precursors. To passivate the free bonds of the substrate and to improve the epitaxy, a thin (15 nm) ZnTe buffer layer was first grown. EDX analysis of the films grown at a temperature of 445 degrees C and about 10-fold excess of chalcogen in the vapor phase indicates on their compliance with V2VI3 stoichiometry. AFM and SEM investigations showed that at the initial stage of deposition the Stranski-Krastanov growth mode is dominant. Complete coalescence of nanoislands occurs at a thickness about 60 nm and further film formation is in the 2D layer-by-layer growth mode. A high mole fraction of antimony in the vapor phase leads to bad crystalline quality of the films and even to their discontinuity. Transport properties of the Bi2-xSbxTe3-ySey films were evaluated using Van der Pauw Hall effect measurements in the range of temperatures of 10-300 K. Some films are always n- or p-type; in other samples the change of conductivity from p-to n-type was observed when the temperature decreases. (C) 2017 Elsevier B.V. All rights reserved.

The films of Sb-Te system have been deposited by MOVPE on (0001) Al2O3 substrates with thin ZnTe buffer layers at different temperatures and Te/Sb ratios in the vapor phase. X-ray diffractometry, SEM microscopy, Raman and EDX spectroscopy were used to study as-grown films. The surface morphology and stoichiometry of Sb-Te films strongly depend on Te/Sb ratio in vapor phase. We have deposited the phases of homologous series nSb2·mSb2Te3 with following stoichiometries: Sb2Te3, Sb4Te5, Sb8Te9, Sb10Te9, Sb4Te3, Sb2Te, Sb8Te3, Sb10Te3, Sb16Te3, Sb18Te3 and Sb. Transport properties of Sb2Te3, Sb4Te5, Sb8Te9, Sb4Te3, Sb2Te were evaluated using Van der Pauw technique at 300K.

We show that the charged tip of the probe microscope, which is widely used in studying the electron transport in low-dimensional systems, induces a spin current. The effect is caused by the spin–orbit interaction arising due to an electric field produced by the charged tip. The tip acts as a spin-flip scatterer giving rise to the spin polarization of the net current and the occurrence of a spin density in the system.

Thin solid layers that are formed upon heating of the gaseous trimethylbismuth–isopropylselenide–hydrogen system on the (0001) Al2O3 and singular and vicinal (100) GaAs surfaces are studied. The conditions for deposition of metal Bi and phases of Bi4Se3, BiSe, and topological insulator Bi2Se3 using the MOCVD method are determined. Pure metastable phase BiSe is obtained for the first time. Bi2Se3 films with a thickness of no less than 200 nm, a relatively low volume concentration of 3 ×1018 cm–3, and a high mobility of carriers at 300 K (1000 cm2 V–1 s–1) are fabricated.

We studied the metalorganic vapor phase epitaxy (MOVPE) of (B1-xSbx)(2)Se-3 solid solution films with a different Sb content on (001) Al2O3 substrates with thin ZnSe buffer layer in the range of temperatures 250-480 degrees C. As-grown films were studied by atom force and scanning electron microscopy (AFM and SEM), Raman spectroscopy and X-ray diffractometry (XRD) techniques. To determine the elemental composition of the grown films, we used an energy dispersive spectrometer (EDS). The dependencies of the crystal structure of films on the growth temperature and Sb content (0 <= x <= 1) were explored. At different growth temperatures we obtained the following bismuth compounds: the films grown at the temperature of 370 degrees C or lower consist of the pure Bi phase, whereas we got the Bi4Se3 phase at 380 degrees C, the phase BiSe at 430 degrees C and Bi2Se3 at the temperature of 460 degrees C or above. We found out that at the temperature of 480 degrees C the single-phase films of (B1-xSbx)(2)Se-3 with rhombohedral and orthorhombic lattices are realized when x is less than 0.25 and greater than 0.935, respectively. For 0.25 < x < 0.935 the grown films are composites of rhombohedral and orthorhombic phases. At the temperature of 440 degrees C we obtained films consisting of three rhombohedral phases (B1-xSbx)(4)Se-3, (Bi1-xSbx) Se and Bi. The room temperature transport properties of rhombohedral samples were characterized using the Van der Pauw technique. (C) 2015 Elsevier B.V. All rights reserved.

We have deposited films of Bi-Te system by atmospheric pressure MOVPE on (0001) Al2O3 substrates with thin ZnTe or thick GaN buffer layers at different temperatures and Te/Bi ratio in the vapor phase. As-grown films were studied by X-ray diffractometry, SEM microscopy and Raman spectroscopy. To determine the elemental composition of the films, an energy dispersive spectrometer was used. Single-phase films of Bi2Te3, Bi4Te5, BiTe, Bi10Te9, Bi4Te3, Bi3Te2 have been grown and growth parameter ranges for obtaining different phases were defined. It was found that under the same growth condition different phases of the Bi-Te system realize depending on the film's thickness. Thus, when growing of Bi2Te3 films by MOCVD method the careful control of the phase composition is required.

We report on a metal organic vapor epitaxy (MOVPE) of Bi2Te3−xSex films over the entire range of compositions (0≤x≤3) for the first time. The films were grown on Al2O3(0001) substrates at 465°C using trimethylbismuth (Bi2Me3), diethyltellurium (Et2Te) and diisopropylselenium (iPro2Se) as metalorganic sources. To realize the 2D growth mode and to grow films with flat surfaces and high crystalline quality, a thin ZnTe buffer layer was used. As-grown films were studied using optical and AFM microscopy techniques and X-ray diffraction. It was found that under steady growth conditions the composition of Bi2Te3−xSex films strongly depends on the film thickness. But a high rate of interdiffusion of chalcogens at the growth temperature rapidly leads to a homogeneous composition of the film in the growth direction. Dependence of the intensity of X-ray reflection (0012) on the composition of Bi2Te3−xSex films x has extremes near x=1 (Bi2Te2Se) and x=2 (Bi2Se2Te). The AFM micrographs and profiles show large (above 2μm) triangle-shaped atomically flat terraces with step height of a quintuple layer (0.90nm) of the tetradymite-type compounds. The electronic properties of the grown films have been characterized via four probe magnetotransport measurements.

We study the formation of spontaneous spin polarization in inhomogeneous electron systems with pair interaction localized in a small region that is not separated by a barrier from surrounding gas of non-interacting electrons. Such a system is interesting as a minimal model of a quantum point contact, in which the electron-electron interaction is strong in a small constriction coupled to electron reservoirs without barriers. Based on the analysis of the grand potential within the self-consistent field approximation, we find that the formation of the polarized state strongly differs from the Bloch or Stoner transition in homogeneous interacting systems. The main difference is that a metastable state appears in the critical point in addition to the globally stable state, so that when the interaction parameter exceeds a critical value, two states coexist. One state has spin polarization and the other is unpolarized. Another feature is that the spin polarization increases continuously with the interaction parameter and has a square-root singularity in the critical point. We study the critical conditions and the grand potentials of the polarized and unpolarized states for one-dimensional and two-dimensional models in the case of extremely small size of the interaction region.

We show that quasi-bound electron states are formed in a quantum wire as a result of electron backscattering in the transition regions between the wire and the electron reservoirs, to which the wire is coupled. The backscattering mechanism is caused by electron density oscillations arising even in smooth transitions due to the reflection of electrons not transmitting through the wire. The quasi-bound states reveal themselves in resonances of the electron transmission probability through the wire. The calculations were carried out within the Hartree-Fock approximation using quasi-classic wavefunctions.

ZnCdSe/ZnSSe MQW structures for an electron beam pumped VCSEL with resonant periodic gain were grown by MOVPE at 425-470 degrees C. Strong contamination of the structure by Ga from a GaAs substrate was found and its effect on the growth rate and photoluminescence characteristics was studied. A protective thin ZnSSe layer deposited at lower temperature (350 degrees C) or thin layers of ZnS and ZnS/ZnSSe SL grown at temperature 425-470 degrees C prevent Ga penetration and allowed improving the quality and periodicity of the structure. Based on the grown MQW structure, green VCSEL was fabricated. Lasing at 542 nm with 3 W output power was achieved at RT and 40 keV. The threshold was as low as 8 A/cm(2). (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We review the mechanisms resulting in absolute negative conductivity in two-dimensional electron systems subjected to a magnetic field which can be relevant to the so-called zero-resistance and zero-conductance states stimulated by microwaves observed in recent experiments.

We investigate a potential landscape of a ballistic quantum wire (QW) adiabatically connected to electron reservoirs with an applied voltage V. Three qualitatively different types of potential distribution are found to appear depending on a bias. Two characteristic voltages V1 and V2 dependent on positive background charge density and geometry of the structure determine the bias intervals, where these regimes realize. At low bias, VV2 the potential in the central part of the QW becomes nonuniform since the anode reservoir cannot supply enough electrons to screen the electric field.