Showing papers by "Nikolay N. Mikhailov published in 2018"
TL;DR: By measuring the temperature dependence of B_{c}, this work directly extracts the critical temperature T_{c} at which the bulk band gap vanishes and the topological phase transition occurs, and above this critical temperature, the opening of a trivial gap is clearly observed.
Abstract: We report a direct observation of temperature-induced topological phase transition between trivial and topological insulator in HgTe quantum well. By using a gated Hall bar device, we measure and represent Landau levels in fan charts at different temperatures and we follow the temperature evolution of a peculiar pair of "zero-mode" Landau levels, which split from the edge of electron-like and hole-like subbands. Their crossing at critical magnetic field Bc is a characteristic of inverted band structure in the quantum well. By measuring the temperature dependence of Bc, we directly extract the critical temperature Tc at which the bulk band-gap vanishes and the topological phase transition occurs. Above this critical temperature, the opening of a trivial gap is clearly observed.
45 citations
TL;DR: 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.
Abstract: Terahertz photoconductivity in heterostructures based on n-type Hg1-x Cd x Te epitaxial films both in the topological phase (x 0.16, normal band structure) has been studied. We show that both the positive photoresponse in films 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.
19 citations
TL;DR: In this paper, high quality HgTe/(Cd,Hg)Te quantum wells (QWs) with a width close to the critical one $d_c$ corresponding to the topological phase transition and graphene like band structure were reported.
Abstract: We report the studies of high-quality HgTe/(Cd,Hg)Te quantum wells (QWs) with a width close to the critical one $d_c$, corresponding to the topological phase transition and graphene like band structure in view of their applications for Quantum Hall Effect (QHE) resistance standards. We show that in the case of inverted band ordering, the coexistence of conducting topological helical edge states together with QHE chiral states degrades the precision of the resistance quantization. By experimental and theoretical studies we demonstrate how one may reach very favorable conditions for the QHE resistance standards: low magnetic fields allowing to use permanent magnets ( B $\leq$ 1.4T) and simultaneously realtively high teperatures (liquid helium, T $\geq$ 1.3K). This way we show that HgTe QW based QHE resistance standards may replace their graphene and GaAs counterparts and pave the way towards large scale fabrication and applications of QHE metrology devices.
14 citations
TL;DR: In this article, the activation energy values obtained from the temperature dependences of the longitudinal resistance in the plateau regions of the quantum Hall effect with the filling factors ν = 1, 2, 3 make it possible to estimate the effective mass and the g factor of electrons in the system under study.
Abstract: The magnetic-field (0 T < B < 9 T) dependence of the longitudinal and Hall resistances at fixed temperatures (2 K < T < 50 K) for the HgCdTe/HgTe/HgCdTe system with a HgTe quantum well 20.3 nm in width are measured. The activation analysis of the magnetoresistance curves is used as a tool for identifying the mobility gaps between neighboring Landau levels. The activation-energy values obtained from the temperature dependences of the longitudinal resistance in the plateau regions of the quantum Hall effect with the filling factors ν = 1, 2, 3 make it possible to estimate the effective mass and the g factor of electrons in the system under study. Indications concerning the possibility of large values of the g factor (≅ 80) are obtained.
6 citations
TL;DR: In this article, the authors report on the stimulated emission obtained in the wavelength range of 20.3-17.4 μm on the interband transitions at T = 8-50 K in HgCdTe quantum wells placed in a dielectric waveguide formed from wide gap CdHgTe solid solution.
Abstract: We report on the stimulated emission obtained in the wavelength range of 20.3–17.4 μm on the interband transitions at T = 8–50 K in HgCdTe quantum wells placed in a dielectric waveguide formed from wide-gap CdHgTe solid solution. Heterostructures with HgCdTe quantum wells are interesting for designing long-wavelength lasers operating in the wavelength range of 25–60 μm, which is not covered by currently available quantum cascade lasers. It is shown that the maximum temperature of stimulated emission is determined by the position of lateral maxima in the dispersion dependences in the first valence subband of the quantum well. Methods for suppressing nonradiative recombination in the structures with HgCdTe quantum wells are discussed.
6 citations
TL;DR: In this paper, the defect structure of arsenic-implanted (with the energy of 190 keV) films grown by molecular-beam epitaxy are carried out using secondary-ion mass spectroscopy, transmission electron microscopy, optical reflection in the visible region of the spectrum, and electrical measurements.
Abstract: Complex studies of the defect structure of arsenic-implanted (with the energy of 190 keV) Cd
x
Hg
1–x
Te (x = 0.22) films grown by molecular-beam epitaxy are carried out. The investigations were performed using secondary-ion mass spectroscopy, transmission electron microscopy, optical reflection in the visible region of the spectrum, and electrical measurements. Radiation donor defects were studied in n+–p- and n+–n-structures obtained by implantation and formed on the basis of p-type and n-type materials, respectively, without activation annealing. It is shown that in the layer of the distribution of implanted ions, a layer of large extended defects with low density is formed in the near-surface region followed by a layer of smaller extended defects with larger density. A different character of accumulation of electrically active donor defects in the films with and without a protective graded-gap surface layer has been revealed. It is demonstrated that p+–n- structures are formed on the basis of n-type material upon activation of arsenic in the process of postimplantation thermal annealing with 100% activation of impurity and complete annihilation of radiation donor defects.
6 citations
TL;DR: In this article, the effects of the residual photoconductivity in HgTe/CdHgTe (013) double quantum-well heterostructures were studied at T = 4.2 K.
Abstract: The effects of the residual photoconductivity in HgTe/CdHgTe (013) double quantum-well heterostructures are studied at T = 4.2 K. It is shown that the residual photoconductivity in this system has a bipolar character, i.e., both positive and negative persistent photoconductivity is present depending on the illumination wavelength.
6 citations
TL;DR: The photoluminescence of bulky films and heterostructures with HgCdTe quantum wells was studied in the far IR range upon interband optical excitation as discussed by the authors.
Abstract: The photoluminescence of bulky films and heterostructures with HgCdTe quantum wells is studied in the far IR range upon interband optical excitation. Photoluminescence lines are found whose position is independent of temperature, and intensity nonmonotonically changes with increasing temperature. These lines are shown to be related to the radiative recombination of holes by singly ionized mercury vacancies. A drastic increase in the equilibrium recombination of such centers involved in the formation of the photoluminescence signal with the temperature change from 40 to 70 K leads to the nonmonotonic temperature dependence of the photoluminescence intensity.
5 citations
TL;DR: In this article, the effect of cyclotron resonance on the photoconductivity of a two-dimensional electron-hole system was investigated, and it was shown that a decrease in the electron density by an order of magnitude does not lead to a significant reduction in the photoconductor signal.
Abstract: Terahertz cyclotron-resonance photoconductivity in a two-dimensional electron–hole system under conditions where the cyclotron resonance occurs owing to the absorption of radiation by electrons whose density is one to three orders of magnitude lower than the hole density is experimentally investigated for the first time. Information on the behavior of the main parameters (i.e., amplitude and broadening) characterizing resonance photoconductivity as a function of wavelength, temperature, and electron density is obtained. On this basis, it is concluded that resonance photoconductivity in the system under study results from cyclotron resonance caused by transitions between the partially filled zeroth Landau level and the first Landau level of electrons, and resonance broadening is caused by scattering on a short-range screened impurity potential. It is found that a decrease in the electron density by an order of magnitude does not lead to a significant reduction of the photoconductivity signal; moreover, at a wavelength of 432 μm, the signal even grows slightly. This fact can be associated with the effective enhancement of the field of the incident radiation in the system under study.
5 citations
TL;DR: The thermopower in a two-dimensional semimetal existing in HgTe quantum wells 18-21 nm thick has been studied experimentally and theoretically for the first time as mentioned in this paper.
Abstract: The thermopower in a two-dimensional semimetal existing in HgTe quantum wells 18–21 nm thick has been studied experimentally and theoretically for the first time It has been found theoretically and experimentally that the thermopower has two components—diffusion and phonon-drag—and that the second component is several times larger than the first It has been concluded that the electron–hole scattering plays an important role in both mechanisms of the thermopower
5 citations
21 Dec 2018
TL;DR: In this article, the authors present a theoretical model which accounts for both the edge and bulk contributions to the electrical conductivity and thermoelectric effect in a 2D topological insulator, including the effects of edge to bulk leakage.
Abstract: The thermoelectric response of HgTe quantum wells in the state of two-dimensional topological insulator (2D TI) has been studied experimentally. Ambipolar thermopower, typical for an electron-hole system, has been observed across the charge neutrality point, where the carrier type changes from electrons to holes according to the resistance measurements. The hole-type thermopower is much stronger than the electron-type one. The thermopower linearly increases with temperature. We present a theoretical model which accounts for both the edge and bulk contributions to the electrical conductivity and thermoelectric effect in a 2D TI, including the effects of edge to bulk leakage. The model, contrary to previous theoretical studies, demonstrates that the 2D TI is not expected to show anomalies of thermopower near the band conductivity threshold, which is consistent with our experimental results. Based on the experimental data and theoretical analysis, we conclude that the observed thermopower is mostly of the bulk origin, while the resistance is determined by both the edge and bulk transport.
TL;DR: The electron cyclotron resonance spectra in classical and quantizing magnetic fields in asymmetric heterostructures with HgCdTe/CdHgTe quantum wells with selective barrier doping are investigated in this article.
Abstract: The electron cyclotron resonance spectra in classical and quantizing magnetic fields in asymmetric heterostructures with HgCdTe/CdHgTe quantum wells with selective barrier doping are investigated. Self-consistent calculations of the energy spectra at B = 0 and Landau levels in the framework of the 8-band Kane model using the Hartree approximation are made. The strong (~10%) splitting of the cyclotron resonance line observed in weak fields is attributed to the Rashba effect in samples with inverted and normal band structures. The evolution of absorption lines upon a variation in the magnetic field is investigated up to 34 T, when the magnetic quantization already dominates over Rashba splitting.
TL;DR: In this paper, the performance of MIS structures based on n(p)-Hg1−xCdxTe (at x from 0.22 to 0.40) with SiO2/Si3N4, Al2O3, and CdTe/Al 2O3 insulators is studied experimentally at 77 K.
Abstract: Admittance of MIS structures based on n(p)- Hg1–xCdxTe (at x from 0.22 to 0.40) with SiO2/Si3N4, Al2O3, and CdTe/Al2O3 insulators is studied experimentally at 77 K. Growth of an intermediate CdTe layer during epitaxy results in the almost complete disappearance of the hysteresis of electrophysical characteristics of MIS structures based on graded-gap n-HgCdTe for a small range of the voltage variation. For a wide range of the voltage variation, the hysteresis of the capacitance-voltage characteristics appears for MIS structures based on n-HgCdTe with the CdTe/Al2O3 insulator. However, the hysteresis mechanism differs from that in case of a single-layer Al2O3 insulator. For MIS structures based on p-HgCdTe, introduction of an additional CdTe layer does not lead to a significant decrease of the hysteresis phenomena, which may be due to the degradation of the interface properties when mercury leaves the film as a result of low-temperature annealing changing the conductivity type of the semiconductor.
TL;DR: In this article, the Fourier transform was used to determine the sign of resonance circular polarization of transitions between Landau levels of carriers, which allows identifying observed intraband and interband transitions in the far and middle infrared ranges.
Abstract: Spectra of magnetoabsorption and Faraday rotation in HgTe/CdHgTe heterostructures with single and double quantum wells in high magnetic fields up to 11 T have been studied by the Fourier-transform spectroscopy method. The study of Faraday rotation spectra makes it possible to determine the sign of resonance circular polarization of transitions between Landau levels of carriers, which allows identifying observed intraband and interband transitions in the far and middle infrared ranges.
TL;DR: In this paper, the authors report results of systematic measurements of charge transport properties of the 20.5-nm-wide HgTe-based quantum well in perpendicular magnetic field, performed under hydrostatic pressures up to 15.1 kbar.
Abstract: We report results of systematic measurements of charge transport properties of the 20.5-nm-wide HgTe-based quantum well in perpendicular magnetic field, performed under hydrostatic pressures up to 15.1 kbar. At ambient pressure, transport is well described by the two-band semiclassical model. In contrast, at elevated pressure, we observed nonmonotonic pressure dependence of resistivity at the ``charge neutrality point.'' For pressures lower than $\ensuremath{\approx}9\phantom{\rule{0.28em}{0ex}}\mathrm{kbar}$, resistivity grows with pressure, in accord with expectations from the band structure calculations and the model incorporating effects of disorder on transport in two-dimensional (2D) semimetals with indirect band overlap. For higher pressures, the resistivity saturates and starts decreasing upon further increase of pressure. Above $\ensuremath{\approx}14\phantom{\rule{0.28em}{0ex}}\mathrm{kbar}$ the resistance value and the magnetoresistance character sharply change, which may indicate significant change of electronic structure due to new electronic phase formation or some structural transitions. The data also reveal strong influence of disorder on transport in 2D electron-hole system with a small band overlap.
TL;DR: In this article, the analysis of transmission and reflection spectra of HgTe films of different thicknesses in the far-infrared range recorded in a wide temperature range was performed in order to detect the above interband transitions.
Abstract: Strained 80-nm-thick HgTe films belong to a new class of materials referred to as three-dimensional topological insulators (i.e., they have a bulk band gap and spin-nondegenerate surface states). Though there are a number of studies devoted to analysis of the properties of surface states using both transport and magnetooptical techniques in the THz range, the information about direct optical transitions between bulk and surface bands in these systems has not been reported. This study is devoted to the analysis of transmission and reflection spectra of HgTe films of different thicknesses in the far-infrared range recorded in a wide temperature range in order to detect the above interband transitions. A peculiarity at 15 meV, which is sensitive to a change in the temperature, is observed in spectra of both types. Detailed analysis of the data obtained revealed that this feature is related to absorption by HgTe optical phonons, while the interband optical transitions are suppressed.
TL;DR: In this article, the authors present a theoretical model which accounts for both the edge and bulk contributions to the electrical conductivity and thermoelectric effect in a 2D topological insulator, including the effects of edge to bulk leakage.
Abstract: The thermoelectric response of HgTe quantum wells in the state of two-dimensional topological insulator (2D TI) has been studied experimentally. Ambipolar thermopower, typical for an electron-hole system, has been observed across the charge neutrality point, where the carrier type changes from electrons to holes according to the resistance measurements. The hole-type thermopower is much stronger than the electron-type one. The thermopower linearly increases with temperature. We present a theoretical model which accounts for both the edge and bulk contributions to the electrical conductivity and thermoelectric effect in a 2D TI, including the effects of edge to bulk leakage. The model, contrary to previous theoretical studies, demonstrates that the 2D TI is not expected to show anomalies of thermopower near the band conductivity threshold, which is consistent with our experimental results. Based on the experimental data and theoretical analysis, we conclude that the observed thermopower is mostly of the bulk origin, while the resistance is determined by both the edge and bulk transport.
TL;DR: In this paper, it is shown that the ratios of the Zeeman splitting to the orbital one are close to each other for both types of structures, with a normal and inverted spectrum and they are close enough to the values calculated within kP method.
Abstract: The Zeeman splitting of the conduction band in the HgTe quantum wells both with normal and inverted spectrum has been studied experimentally in a wide electron density range. The simultaneous analysis of the Shubnikov–de Haas oscillations in low magnetic fields at different tilt angles and of the shape of the oscillations in moderate magnetic fields gives a possibility to find the ratio of the Zeeman splitting to the orbital one and anisotropy of g-factor. It is shown that the ratios of the Zeeman splitting to the orbital one are close to each other for both types of structures, with a normal and inverted spectrum and they are close enough to the values calculated within kP method. In contrast, the values of g-factor anisotropy in the structures with normal and inverted spectra are strongly different and for both cases differ significantly from the calculated ones. We assume that such disagreement with calculations is a result of the interface inversion asymmetry in the HgTe quantum well, which is not taken into account in the kP calculations.
TL;DR: In this paper, the impact of the presence of the near-surface graded-gap layers with an increased content of CdTe on the admittance of MIS structures based on MBE-grown n-Hg1-xCdxTe (x = 0.22-0.23) with the Al2O3 insulating coating has been experimentally studied.
Abstract: The impact of the presence of the near-surface graded-gap layers with an increased content of CdTe on the admittance of MIS structures based on MBE-grown n-Hg1–xCdxTe (x = 0.22–0.23) with the Al2O3 insulating coating has been experimentally studied. It has been shown that the structures with a gradedgap layer are characterized by a deeper and wider capacitance dip in the low-frequency capacitance–voltage (CV) characteristic and by higher values of the differential resistance of the space-charge region than the structures without such a layer. It has been found that the main features of the hysteresis of capacitance dependences typical of the graded-gap structures with SiO2/Si3N4 are also characteristic of the MIS structures with the Al2O3 insulator. The factors that cause an increase in the CV characteristic hysteresis upon formation of the graded-gap layer in structures with SiO2/Si3N4 or Al2O3 are still debatable, although it may be assumed that oxygen plays a certain role in formation of this hysteresis.
TL;DR: In this article, the authors investigated the potential of HgTe/HgCdTe quantum wells for long-wavelength interband lasers (λ = 15-30 μm) and showed that the carrier heating under intense optical pumping is the main factor limiting the SE wavelength and intensity.
Abstract: We investigate the prospects of HgTe/HgCdTe quantum wells for long-wavelength interband lasers (λ = 15–30 μm). The properties of stimulated emission (SE) and magnetoabsorbtion data of QWs structures with wide-gap HgCdTe dielectric waveguide provide an insight on dominating non-radiative carrier recombination mechanism. It is shown that the carrier heating under intense optical pumping is the main factor limiting the SE wavelength and intensity, since the Auger recombination is greatly enhanced when carriers populate high energy states in the valence band.
TL;DR: Gavrilenko, V.V.Mikhailov, S.A.Dubinov, N.N.Teppe, C.I.Sirtori Institute for Physics of Microstructures, Nizhny Novgorod, Russia, gavr@ipmras.ru as mentioned in this paper
Abstract: V.I.Gavrilenko, V.V.Rumyantsev, A.A.Dubinov, S.V.Morozov, N.N.Mikhailov, S.A.Dvoretsky, F.Teppe, C.Sirtori Institute for Physics of Microstructures, Nizhny Novgorod, Russia, gavr@ipmras.ru A.V.Rzhanov Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia Charles Coulomb Laboratory, University of Montpellier & CNRS, Montpellier, France Laboratory of Materials and Quantum Phenomena, University Paris-Diderot, Paris, France
TL;DR: In this paper, a toy electrostatic model involving the surface states localized at quantum well interfaces was proposed to explain the spin splitting of the first conduction subband H1 in 18-22nm HgTe quantum wells.
Abstract: We report on beating appearance in Shubnikov-de Haas oscillations in conduction band of 18-22nm HgTe quantum wells under applied top-gate voltage. Analysis of the beatings reveals two electron concentrations at the Fermi level arising due to Rashba-like spin splitting of the first conduction subband H1. The difference dN_s in two concentrations as a function of the gate voltage is qualitatively explained by a proposed toy electrostatic model involving the surface states localized at quantum well interfaces. Experimental values of dN_s are also in a good quantitative agreement with self-consistent calculations of Poisson and Schrodinger equations with eight-band kp Hamiltonian. Our results clearly demonstrate that the large spin splitting of the first conduction subband is caused by surface nature of $H1$ states hybridized with the heavy-hole band.
TL;DR: In this article, the mobility and quantum time of Dirac electrons in HgTe quantum wells with near-critical thickness corresponding to the transition from the direct to inverted spectrum are experimentally and theoretically investigated.
Abstract: The mobility and quantum time of Dirac electrons in HgTe quantum wells with near-critical thickness corresponding to the transition from the direct to inverted spectrum are experimentally and theoretically investigated. The nonmonotonic dependence of the mobility on the electron concentration is experimentally established. The theory of the scattering of Dirac electrons by impurities and irregularities of the well boundaries leading to well thickness fluctuations is constructed. The comparison of this theory with an experiment shows their good agreement and explains the observed nonmonotonic behavior by a decrease in the ratio between the de Broglie wavelength of Dirac electrons and the characteristic size of irregularities with increasing electron concentration. It is established that the transport time is larger than the quantum time by almost an order of magnitude in the case of the dominance of roughness scattering. The transition from macroscopic to mesoscopic samples is studied and an abrupt decrease in both the mobility and quantum time is observed. This behavior is attributed to the size effect on the free path length.