We address the general problem of magnetic-field-induced corner states in quantum spin Hall insulators (QSHIs). Our analytical findings reveal that when applied to the QSHIs in zinc-blende semiconductor quantum wells (QWs), the presence of corner states extends beyond the anticipated range of meeting edges, surpassing the limitations imposed by crystal symmetry. We clearly demonstrate that, in the most general scenario, magnetic field-induced corner states in QSHIs are not topological. However, we find that the presence of crystal symmetry can stabilize these states only under specific orientations of the in-plane magnetic field and meeting edges. Therefore, contrary to previous assumptions, our research unveils that QSHIs in the presence of a magnetic field cannot be accurately considered as higher-order topological insulators. Furthermore, the lack of an inversion center in zinc-blende semiconductor QWs enables the emergence of corner states through the influence of a perpendicular magnetic field.

Quantum spin Hall transport in InAs/GaInSb-based two-dimensional topological insulators can be limited by parasitic bulk and edge contributions. We demonstrate that these limitations are effectively mitigated through electrostatic control in dual-gated InAs/GaInSb/InAs trilayer quantum wells grown on AlSb quasi-substrates. In macroscopic Hall bars exceeding the phase coherence length, a multi-probe analysis reveals an insulating bulk and a constant edge resistance over a wide electric-field range. In microscopic devices with edge lengths below the phase coherence lengths, the edge resistance remains robust and quantized accross a broad field range, revealing the intrinsic resilience of helical edge channels to electric-field perturbations. Only beyond a threshold value, parasitic edge contributions emerge. These results establish dual gating as a reliable strategy to suppress parasitic conduction while stabilizing helical edge transport, providing a versatile and reproducible platform for tunable topological transport in III-V quantum spin Hall systems.

By using the self-consistent Born approximation, we investigate topological phase transitions in double HgTe quantum wells (QWs) induced by the short-range impurities. Following the evolution of the density of states and the spectral function, we demonstrate multiple closings and openings of the band gap with the increase of the disorder strength due to the mutual inversions between the first and second electronlike and holelike subbands. We show that starting from a band insulator in the clean limit, under the influence of disorder, the double HgTe QW undergoes a transition, first into a semimetal state similar to "bilayer graphene" and then into a high-order topological insulator state with a double band inversion. We find out that all disorder-induced transitions can be fully characterized by introducing a non-Hermitian quasiparticle Hamiltonian encoding the band structure renormalization and quasiparticle decay.

Overcoming the limitations of the von Neumann architecture requires new computational paradigms capable of solving complex problems efficiently. Quantum and neuromorphic computing rely on unconventional materials and device functionalities, yet achieving resilience to imperfections and reliable operation remains a major challenge. This has motivated growing interest in topological materials that provide robust and low-power operation while preserving coherence. However, integrating coherent topological transport with non-volatile memory functionality in a single reconfigurable device has remained challenging. In this work, we demonstrate a topological field-effect memristor based on inverted InAs/GaInSb/InAs trilayer quantum wells operating in the quantum spin Hall regime. The intrinsic floating-gate behavior allows one to reconfigure the transistor functionality into memristive functionality with broad electric-field tunability. Unlike other memristor implementations, one resistance state is governed entirely by dissipationless, coherent transport through helical edge channels, while the other arises from incoherent bulk conduction. By combining electrically tunable coherent and incoherent transport with memory functionality, our device realizes a prototypical topological electronic element that integrates coherent transport and adaptive memristive behavior, paving the way for hybrid quantum-neuromorphic architectures.

The quantum spin Hall effect (QSHE), a hallmark of topological insulators, enables dissipationless, spin-polarized edge transport and has been predicted in various two-dimensional materials. However, challenges such as limited scalability, low-temperature operation, and the lack of robust electronic transport have hindered practical implementations. Here, we demonstrate the QSHE in an InAs/GaInSb/InAs trilayer quantum well structure operating at elevated temperatures. This platform meets key criteria for device integration, including scalability, reproducibility, and tunability via electric field. When the Fermi level is positioned within the energy gap, we observe quantized resistance values independent of device length and in both local and nonlocal measurement configurations, confirming the QSHE. Helical edge transport remains stable up to T = 60 kelvin, with further potential for higher-temperature operation. Our findings establish the InAs/GaInSb system as a promising candidate for integration into next-generation devices harnessing topological functionalities, advancing the development of topological electronics.

Positive terahertz photoconductivity is observed at room temperature in CdHgTe thin films with different Cd contents. We show that electron gas heating caused by Drude-like absorption results in positive photoconductivity because of the interband activation mechanism specific for undoped narrow-gap semiconductors and semimetals. Applying intense terahertz radiation, we observed that the photoconductivity saturates at high intensities, which was found to be caused by absorption bleaching. Both the magnitude of the photoconductivity and the saturation intensity are shown to exhibit an exponential dependence on the hydrostatic pressure. We show that this is a consequence of the fact that both phenomena are controlled by the ratio of energy and momentum relaxation times.

The quantum Hall effect, which exhibits a number of unusual properties, is studied in a gated 1000-nm-thick HgTe film, nominally a three-dimensional system. A weak zero plateau of Hall resistance, accompanied by a relatively small value of Rxx of the order of h/e^2, is found around the point of charge neutrality. It is shown that the zero plateau is formed by the counter-propagating chiral electron-hole edge channels, the scattering between which is suppressed. So, phenomenologically, the quantum spin Hall effect is reproduced, but with preserved ballisticity on macroscopic scales (larger than 1mm). It is shown that the formation of the QHE occurs in a two-dimensional (2D) accumulation layer near the gate, while the bulk carriers play the role of an electron reservoir. Due to the exchange of carriers between the reservoir and the 2D layer, an anomalous scaling of the QHE is observed not with respect to the CNP, but with respect to the first electron plateau.

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap ( 80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

Materials with linear electronic dispersion often feature high carrier mobilities and unusually strong nonlinear optical interactions. In this work, we investigate the THz nonlinear dynamics of one such material, HgCdTe, with an electronic band dispersion heavily dependent on both temperature and stoichiometry. We show how the band gap, carrier concentration and band shape together determine the nonlinear response of the system. At low temperatures, carrier generation from Zener tunneling dominates the nonlinear response with a reduction in the overall transmission. At room temperature, quasi-ballistic electronic dynamics drive the largest observed nonlinear optical interactions, leading to a transmission increase. Our results demonstrate the sensitivity of these nonlinear optical properties of narrow-gap materials to small changes in the electronic dispersion and carrier concentration.

We report on the coexistence of both normal and topological insulating phases in InAs/GaSb bilayer quantum well induced by the built-in electric field tuned optically and electrically. The emergence of topological and normal insulating phases is assessed based on the evolution of the charge carrier densities, the resistivity dependence of the gap via in-plane magnetic fields and the thermal activation of carriers. For the Hall bar device tuned optically, we observe the fingerprints associated with the presence of only the topological insulating phase. For another Hall bar processed identically but with an additional top gate, the coexistence of normal and topological insulating phases is found by electrical tuning. Our finding paves the way for utilizing a new electro-optical tuning scheme to manipulate InAs/GaSb bilayer quantum wells to obtain trivial-topological insulating interfaces in the bulk rather than at the physical edge of the device.

We present a multiprobe transport analysis that effectively separates bulk and edge currents in large Hall bar devices with standard geometries. Applied to transport measurements on all possible four-probe configurations of six-probe Hall-bar devices made of inverted three-layer InAs/(Ga, In)Sb quantum wells (QWs), our analysis not only reveals the presence of dissipative edge currents in the topological gap, but also allows the temperature dependence of bulk and edge conductivity to be evaluated separately. The temperature dependence of the edge conductivity for Hall-bar channels from 10 to 70 mu m in the range of 1.5 to 45 K is consistent with the theoretical expectation of weakly interacting helical edge electrons with backscattering due to localized magnetic moments of charge impurities. We argue that these charge impurities are naturally associated with intrinsic Ga-antisite defects, which act as double acceptors in InAs/(Ga,In)Sb-based QWs.

We report on the observation and comprehensive study of the terahertz radiation induced magneto-photogalvanic effect (MPGE) in bulk CdHgTe crystals hosting Kane fermions. The MPGE has been detected in Cd$_{x}$Hg$_{1-x}$Te films with Cd contents $x = 0.15$ and $0.22$ subjected to an in-plane magnetic field. At liquid helium temperature we observed multiple resonances in MPGE current upon variation of magnetic field. In the $x = 0.22$ with noninverted band structure, the resonances are caused by cyclotron resonance (CR) and photoionization of an impurity level. In the $x = 0.15$ films with an inverted band structure, they originate from the CR and interband optical transitions. Band structure calculated by the Kane model perfectly describes positions of all resonances. In particularly, the resonant MPGE caused by interband transitions excited by THz radiation is caused by the gapless energy spectrum of Kane fermions realized in materials with certain Cd contents and temperature range. In addition to the resonant MPGE current we detected a nonresonant one due to indirect optical transitions (Drude-like). This contribution has a nonmonotonic magnetic field dependence increasing linearly at low magnetic field $B$, approaching a maximum at moderate field and decreasing at high $B$. While the nonresonant MPGE decreases drastically with increasing temperature, it is well measurable up to room temperature. The developed theory demonstrates that the MPGE current arises due to cubic in momentum spin-dependent terms in the scattering probability. The asymmetry caused by these effects results in a pure spin current which is converted into an electric current due to the Zeeman effect.

We report on the emission of Terahertz radiation from Landau quantized Dirac electrons in HgTe/CdHgTe based quantum wells. We show that this cyclotron emission is continuously tunable with magnetic field from 500 GHz up to 2.5 THz. Moreover, given the relativistic nature of charge carriers, the cyclotron mass and therefore the emitted frequency are also tunable with the electron density. These results pave the way for the long-awaited Landau laser which could additionally be tunable by a gate voltage with a fixed magnetic field.

Graphene is a quantum spin Hall insulator, with a nontrivial topological gap induced by the spin-orbit coupling. Such splitting is weak $(\sim 45 \mu$ eV) in the absence of external magnetic field. However, due to rather long spin-relaxation time, graphene is an attractive candidate for applications in quantum technologies. When it is encapsulated in hexagonal boron nitride, the coupling between graphene and the substrate compensates intrinsic spin-orbit coupling and decreases the nontrivial topological gap, which may lead to phase transition into a trivial band insulator state. In this work, we have measured experimentally the zero-field splittings in monolayer and bilayer graphene by the means of subterahertz photoconductivity-based electron spin resonance technique. The dependance in temperature of such splittings have been also studied in the 2-12K range. We observed a decrease of the spin splittings with increasing temperature. Such behavior might be understood from several physical mechanisms that could induce a temperature dependence of the spin-orbit coupling. These includes the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electronphonon interactions, and the presence of a magnetic order at low temperature.

Гетероструктуры на основе CdHgTe с двойными квантовыми ямами (КЯ) являются сложными объектами, обладающими бóльшим числом топологических фаз, чем в гетероструктурах с одиночными КЯ [1].Последние известны прежде всего тем, что они являются первыми экспериментально обнаруженными двумерными топологическими изоляторами [2].Разнообразие возможных состояний в двойных КЯ связано с большим количеством параметров (составы и толщины КЯ и туннельного барьера и т.п.) и, не в последнюю очередь, с тем, что разделяющий КЯ барьер может являться туннельно-прозрачным для электронов, но не для дырок.Многообразие параметров приводит к необходимости проверки их номинальных значений, закладываемых при росте структур, независимыми методами.Одним из таких методов может являться исследование магнитопоглощения.В слабых магнитных полях оно дает информацию об эффективных массах на уровне Ферми, а в сильных (квантующих) -информацию о переходах между уровнями Ландау.В конечном итоге это позволяет восстанавливать реальный зонный спектр исследуемых объектов, определять фактические параметры структур, а также обнаруживать явления, выходящие за рамки «простых» моделей.В настоящем докладе будут представлены результаты исследований магнитопоглощения в квантующих магнитных полях в гетероструктурах HgTe/CdHgTe с двойными квантовыми ямами.Будут продемонстрированы спектры магнитопоглощения, измеренные в магнитных полях до 30 Тл, выявлены многочисленные спектральные особенности и выполнена их интерпретация в рамках четырехзонной модели Кейна [3].Кроме того, будет продемонстрировано дополнительное расщепление основных линий магнитопоглощения, связанных с переходами с «нулевых» уровней Ландау, которое позволит конкретизировать природу структурной асимметрии в двойных квантовых ямах HgTe/CdHgTe, а также определить вклады, обусловленные встроенным электрическим полем, различием толщин квантовых ям и порядка их расположения в структуре [4].Наконец, в докладе будут представлены результаты исследований магнитопоглощения при различных температурах.Будет продемонстрирован фазовый переход между различными топологическими фазами, а также рассмотрена тонкая структура антикроссинга «нулевых» уровней Ландау в образце с «двойной инверсией».

Graphene is a quantum spin Hall insulator with a 45 mu eV-wide nontrivial topological gap induced by the intrinsic spin-orbit coupling. Even though this zero-field spin splitting is weak, it makes graphene an attractive candidate for applications in quantum technologies, given the resulting long spin-relaxation time. On the other side, the staggered sublattice potential, resulting from the coupling of graphene with its boron nitride substrate, compensates intrinsic spin-orbit coupling and decreases the nontrivial topological gap, which may lead to the phase transition into trivial band insulator state. In this work, we present extensive experimental studies of the zero-field splittings in monolayer and bilayer graphene in a temperature range 2-12 K by means of subterahertz photoconductivity-based electron spin-resonance technique. Surprisingly, we observe a decrease of the spin splittings with increasing temperature. We discuss the origin of this phenomenon by considering possible physical mechanisms likely to induce a temperature dependence of the spin-orbit coupling. These include the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electron-phonon interactions, and the presence of a magnetic order at low temperature. Our experimental observation expands knowledge about the nontrivial topological gap in graphene.

Using the self-consistent Born approximation, we study a topological phase transition appearing in bulk HgCdTe crystals induced uncorrelated disorder due to both randomly distributed impurities and fluctuations in Cd composition. By following the density-of-states evolution, we clearly demonstrate the topological phase transition, which can be understood in terms of the disorder-renormalized mass of Kane fermions. We find that the presence of a heavy-hole band in HgCdTe crystals leads to the topological phase transition at much lower disorder strength than is expected for conventional three-dimensional topological insulators. Our theoretical results can also be applied to other narrow-gap zinc-blende semiconductors such as InAs, InSb, and their ternary alloys InAsSb.

Quantum spin Hall insulators (QSHIs) based on HgTe and three-layer InAs/GaSb quantum wells (QWs) have comparable bulk band gaps of about 10-18 meV. The former, however, features a band gap vanishing with temperature, while the gap in InAs/GaSb QSHIs is rather temperature independent. Here, we report on the realization of a large inverted band gap in strained three-layer InAs/GaInSb QWs. By temperature-dependent magnetotransport measurements of gated Hall bar devices, we extract a gap as high as 45 meV. By combining local and nonlocal measurements, we detect edge conductivity at temperatures up to 40 K, possibly of topological origin, with equilibrium lengths of a few micrometers. Our results pave the way for the manipulation of topological edge states at high temperatures in QW heterostructures.

The density dependences of the splitting values of the dominant magneto-absorption lines in undoped p-type double HgTe/CdHgTe quantum wells are studied. An analysis of the splitting values of the magneto-absorption lines on the basis of self-consistent calculations of the band structure has made it possible to clarify the origin of structure inversion asymmetry in double HgTe/CdHgTe quantum wells and to determine the contributions caused by the built-in electric field, the difference between the thicknesses of the quantum wells, and the order of their arrangement in the structure. The discrepancy between the experimental energy splitting of the zero-mode Landau levels, extracted from the analysis of two types of lines, indicates the influence of many-particle effects on the transition energies between Landau levels.

Using an elegant model involving only Γ6c and Γ8v bands, massless Kane fermions were defined as the particles associated with the peculiar band structure of gapless HgCdTe crystals. Although their dispersion relation resembles that of a pseudo-spin-1 Dirac semimetal, these particles were originally considered to be hybrids of pseudospin-1 and -1/2 fermions. Here we unequivocally find that by considering an additional Γ7c conduction band inherent in HgCdTe crystals, the Kane fermions are ultimately two nested Dirac particles. This observation allows the direct application of Lorentz transformations to describe the relativistic behavior of these particles in crossed electric and magnetic fields. By studying the relativistic collapse of their Landau levels at different orientations between the crossed fields and the main crystallographic axes, we demonstrate that the Kane fermions strikingly decay into two independent Dirac particles with increasing of electric field. Our results provide new insight into semi-relativistic effects in narrow-gap semiconductors in crossed electric and magnetic fields.