arXiv subjects

N. N. Mikhailov

Publications and source records attributed to N. N. Mikhailov.

At least 37 records · Page 2Linked to original sources

arXiv2023

Energy spectrum of valence band in HgTe quantum wells on the way from a two to the three dimensional topological insulator

The magnetic field, temperature dependence and the Hall effect have been measured in order to determine the energy spectrum of the valence band in HgTe quantum wells with the width (20-200)nm. The comparison of hole densities determined from the period Shubnikov-de Haas oscillations and the Hall effect shows that states at the top of valence band are double degenerate in teh entry quantum wells width the width range. The cyclotron mass determined from temperature dependence of SdH oscillations increases monotonically from (0.2-0.3) mass of the free electron, with increasing hole density from 2e11 to 6e11 cm^-2. The determined dependence has been compared to theoretical one calculate within the four band kp model. The experimental dependence was found to be strongly inconsistent with this predictions. It has been shown that the inclusion of additional factors (electric field, strain) does not remove the contradiction between experiment and theory. Consequently it is doubtful that the mentioned kp calculations adequately describe the valence band for any width of quantum well.

cond-mat.mes-hall↗

arXiv2023

Gate tunable terahertz cyclotron emission from two-dimensional Dirac fermions

Two-dimensional Dirac fermions in HgTe quantum wells close to the topological phase transition can generate significant cyclotron emission that is magnetic field tunable in the Terahertz (THz) frequency range. Due to their relativistic-like dynamics, their cyclotron mass is strongly dependent on their electron concentration in the quantum well, providing a second tunability lever and paving the way for a gate-tunable, permanent-magnet Landau laser. In this work, we demonstrate the proof-of-concept of such a back-gate tunable THz cyclotron emitter at fixed magnetic field. The emission frequency detected at 1.5 Tesla is centered on 2.2 THz and can already be electrically tuned over 250 GHz. With an optimized gate and a realistic permanent magnet of 1.0 Tesla, we estimate that the cyclotron emission could be continuously and rapidly tunable by the gate bias between 1 and 3 THz, that is to say on the less covered part of the THz gap.

cond-mat.mes-hall↗

arXiv2023

Transport properties of a 1000-nm HgTe film: the interplay of surface and bulk carriers

We report on systematic study of transport properties of a 1000-nm HgTe film. Unlike to thinner and strained HgTe films, which are known as high-quality three-dimensional (3D) topological insulators, the film under study is much thicker than the limit of pseudomorphic growth of HgTe on a CdTe substrate. Therefore, it is expected to be fully relaxed and has the band structure of bulk HgTe, i.e., a zero gap semiconductor. Nevertheless, since the bands inversion the two-dimensional (2D) topological surface states are still expected to exist. To check this claim we studied classical and quantum transport response of the system. We demonstrate that by tuning the top-gate voltage one can change the electron-dominating transport to the hole one. The highest electron mobility is found to be more than $300 \times 10^3$ cm$^2$/Vs. The system exhibits Shubnikov-de Haas (SdH) oscillations with a complicated pattern and shows up to 5 independent frequencies in corresponding Fourier spectra. They are attributed to the topological surface states, Volkov-Pankratov states and spin-degenerate bulk states in the accumulation layer near the gate. The observed peculiarities of the quantum transport are the strong SdH oscillations of the Hall resistance, and the suppressed oscillatory response of the topological surface states.

cond-mat.mes-hall↗

arXiv2023

Terahertz cyclotron emission from two-dimensional Dirac fermions

Since the emergence of graphene, we have seen several proposals for the realization of Landau lasers tunable over the terahertz frequency range. The hope was that the non-equidistance of the Landau levels from Dirac fermions would suppress the harmful non-radiative Auger recombination. Unfortunately, even with this non-equidistance an unfavorable non-radiative process persists in Landau-quantized graphene, and so far no cyclotron emission from Dirac fermions has been reported. One way to eliminate this last non-radiative process is to sufficiently modify the dispersion of the Landau levels by opening a small gap in the linear band structure. A proven example of such gapped graphene-like materials are HgTe quantum wells close to the topological phase transition. In this work, we experimentally demonstrate Landau emission from Dirac fermions in such HgTe quantum wells, where the emission is tunable by both the magnetic field and the carrier concentration. Consequently, these results represent an advance in the realization of terahertz Landau lasers tunable by magnetic field and gate-voltage.

cond-mat.mes-hall↗

arXiv2022

Quantum oscillations of transport coefficients and capacitance: an unexpected manifestation of the spin-Hall effect

The results of systematic experimental studies of quantum oscillations of resistivity, Hall coefficient and capacitance in GaAs and In$_x$Ga$_{1-x}$As quantum wells (QWs) with a simple electron spectrum and HgTe QWs with a complicated non-parabolic spectrum and strong spin-orbit interaction are reported. A striking result is the ratio of the amplitudes of the resistance and Hall coefficient oscillations. In GaAs QW with a simple spectrum characterized by negligibly small Zeeman and spin-orbit splitting, the ratio of amplitudes is close to that predicted theoretically. In HgTe QWs, this ratio is very different and behaves differently in QWs with normal and inverted electron spectra. In HgTe WQs with a normal spectrum, it tends to a theoretical value with an increase of the filling factor ($N$), while for HgTe QWs with an inverted spectrum, it differs significantly from the theoretical one for all available $N$. It is assumed that such a difference in the ratio of amplitudes in GaAs and HgTe QWs is due not to the peculiarities of the energy spectrum of HgTe, but to the peculiarities of electron scattering due to spin-orbit interaction with the potential of the scatterers. This assumption is justified by analysis of experimental results obtained for a heterostructure with a In$_{0.2}$Ga$_{0.8}$As QW, which spectrum is very close to the GaAs QW spectrum, but characterizes by much stronger spin-orbit splitting value. It has been found that the positions of the resistance and capacitance oscillations, the difference between the phases of the resistance and Hall coefficient oscillations and its $N$ dependence are close to those observed in GaAs QW. At the same time the ratio of the amplitude of the resistance oscillations to the Hall coefficient oscillations and its $N$ dependence differs very strongly and they are close to that observed in HgTe quantum wells.

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arXiv2022

Scattering anisotropy in HgTe (013) quantum well

We report on a detailed experimental study of the electron transport anisotropy in HgTe (013) quantum well of 22 nm width in the directions $[100]$ and $[03\bar{1}]$ as the function of the electron density $n$. The anisotropy is absent at minimal electron density near the charge neutrality point. The anisotropy increases with the increase of n and reaches about 10% when the Fermi level is within the first subband H1. There is a sharp increase of the anisotropy (up to 60%) when the Fermi level reaches the second subband E2. We conclude that the first effect is due to the small intra-subband anisotropic interface roughness scattering, and the second one is due to the strongly anisotropic inter-subband roughness scattering, but the microscopical reason of such a strong change in the anisotropy remains unknown.

cond-mat.mtrl-sci↗

arXiv2022

Quantum transport of Dirac fermions in HgTe gapless quantum wells

We study transport properties of HgTe quantum wells with critical well thickness, where the band gap is closed, and the low energy spectrum is described by a single Dirac cone. In this work, we examined both macroscopic and micron-sized (mesocopic) samples. In micron-sized samples, we observe a magnetic field induced, quantized resistance ($\sim h/2e^{2}$) at Landau filling factor $ν=0$, corresponding to the formation of helical edge states centered at the charge neutrality point (CNP). In macroscopic samples, the resistance near zero Landau level (LL) reveals strong oscillations, which we attribute to scattering between the edge $ν=0$ state and bulk $ν\neq 0$ hole LL. We provide a model taking an empirical approach to construct a LL diagram based on a reservoir scenario, formed by the heavy holes.

cond-mat.mes-hall↗

arXiv2022

THz ratchet effect in HgTe interdigitated structures

The emergence of ratchet effects in two-dimensional materials is strongly correlated with the introduction of asymmetry into the system. In general, dual-grating-gate structures forming lateral asymmetric superlattices provide a suitable platform for studying this phenomenon. Here, we report on the observation of ratchet effects in HgTe-based dual-grating-gate structures hosting different band structure properties. Applying polarized terahertz laser radiation we detected linear and polarization independent ratchets, as well as an radiation-helicity driven circular ratchet effect. Studying the ratchet effect in devices made of quantum wells (QWs) of different thickness we observed that the magnitude of the signal substantially increases with decreasing QW width with a maximum value for devices made of QWs of critical thickness hosting Dirac fermions. Furthermore, sweeping the gate voltage amplitude we observed sign-alternating oscillations for gate voltages corresponding to p-type conductivity. The amplitude of the oscillations is more than two orders of magnitude larger than the signal for n-type conducting QWs. The oscillations and the signal enhancement are shown to be caused by the complex valence band structure of HgTe-based QWs. These peculiar features of the ratchet currents make these materials an ideal platform for the development of THz applications.

cond-mat.mes-hall↗

arXiv2021

Transformation of an energy spectrum and wave functions in the crossover from two- to three-dimensional topological insulator in HgTe quantum wells: long and thorny way

A magnetotransport and quantum capacitance of the two-dimensional electron gas in HgTe/Cd$_x$Hg$_{1-x}$Te quantum wells of a width ($20.2-46.0$)~nm are experimentally investigated. It is shown that the first energy subband of spatial quantization is split due to the spin-orbit interaction and the split branches are single-spin, therewith the splitting strength increases with the increase of the quantum well width. The electron effective masses in the branches are close to each other within the actual density range. Magneto-intersubband oscillations (MISO) observed in the structures under study exhibit the growing amplitude with the increasing electron density that contradicts to the expected decrease of wave function overlap for the rectangular quantum well. To interpret the data obtained, we have used a self-consistent approach to calculate the electron energy spectrum and the wave function within framework of the \emph{kP}-model. It has been in particular shown that the MISO amplitude increase results from the increasing overlap of the wave functions due to their shift from the gate electrode with the gate voltage increase known as phenomenon of the negative electron polarizability. The results obtained from the transport experiments are supported by quantum capacitance measurements.

cond-mat.mes-hall↗

arXiv2021

Thermoelectric transport in a three-dimensional HgTe topological insulator

The thermoelectric response of 80-nm-thick strained HgTe films of a three-dimensional topological insulator (3D TI) has been studied experimentally. An ambipolar thermopower is observed where the Fermi energy moves from conducting to the valence bulk band. The comparison between theory and experiment shows that the thermopower is mostly due to the phonon drag contribution. In the region where the 2D Dirac electrons coexist with bulk hole states, the Seebeck coefficient is modified due to 2D electron - 3D hole scattering.

cond-mat.mes-hall↗

arXiv2021

Engineering topological phases in triple HgTe/CdTe quantum wells

Quantum wells formed by layers of HgTe between Hg$_{1-x}$Cd$_x$Te barriers lead to two-dimensional (2D) topological insulators, as predicted by the BHZ model. Here, we theoretically and experimentally investigate the characteristics of triple HgTe quantum wells. We describe such heterostructure with a three dimensional $8\times 8$ Kane model, and use its eigenstates to derive an effective 2D Hamiltonian for the system. From these we obtain a phase diagram as a function of the well and barrier widths and we identify the different topological phases composed by zero, one, two, and three sets of edge states hybridized along the quantum wells. The phase transitions are characterized by a change of the spin Chern numbers and their corresponding band inversions. Complementary, transport measurements are experimentally investigated on a sample close to the transition line between the phases with one and two sets of edges states. Accordingly, for this sample we predict a gapless spectrum with low energy bulk conduction bands given by one parabolic and one Dirac band, and with edge states immersed in the bulk valance bands. Consequently, we show that under these conditions, local and non-local transport measurements are inconclusive to characterize a sole edge state conductivity due to bulk conductivity. On the other hand, Shubnikov-de Haas (SdH) oscillations show an excellent agreement with our theory. Particularly, we show that the measured SdH oscillation frequencies agrees with our model and show clear signatures of the coexistence of a parabolic and Dirac bands.

cond-mat.mes-hall↗

arXiv2021

Transport through the network of topological channels in HgTe based quantum well

Topological insulators represent a new quantum state of matter which is characterized by edge or surface states and an insulating band gap in the bulk. In a two dimensional (2D) system based on the HgTe quantum well (QW) of critical width random deviations of the well width from its average value result in local crossovers from zero gap 2D Dirac fermion system to either the 2D topological insulator or the ordinary insulator, forming a complicated in-plane network of helical channels along the zero-gap lines. We have studied experimentally the transport properties of the critical width HgTe quantum wells near the Dirac point, where the conductance is determined by a percolation along the zero-gap lines. The experimental results confirm the presence of percolating conducting channels of a finite width. Our work establishes the critical width HgTe QW as a promising platform for the study of the interplay between topology and localization.

cond-mat.mes-hall↗

arXiv2021

HgTe quantum wells for QHE metrology under soft cryomagnetic conditions: permanent magnets and liquid ${^4He}$ temperatures

HgTe quantum wells with a thickness of ${\sim}$7 nm may have a graphene-like band structure and have been recently proposed to be potential candidates for quantum Hall effect (QHE) resistance standards under the condition of operation in the fields above certain critical field $B_c$, above which the topological phase (with parasitic edge conduction) disappears. We present experimental studies of the magnetoresistance of different of HgTe quantum wells as a function temperature and magnetic field, determining the critical magnetic field $B_c$. We demonstrate that for QWs of specific width $B_c$ becomes low enough to grant observation of remarkably wide QHE plateaus at the filling factor ${v=-1}$ (holes) in relaxed cryomagnetic conditions: while using commercial 0.82 T Neodymium permanent magnets and temperature of a few Kelvin provided by ${^4He}$ liquid system only. Band structure calculations allow us to explain qualitatively observed phenomena due to the interplay between light holes and heavy holes energy sub-bands (side maxima of the valence band). Our work clearly shows that the peculiar band structure properties of HgTe QWs with massless Dirac fermions make them an ideal platform for developing metrological devices with relaxed cryomagnetic conditions.

cond-mat.mes-hall↗

arXiv2021

$4π$-periodic supercurrent tuned by an axial magnetic flux in topological insulator nanowires

Topological insulator (TI) nanowires in proximity with conventional superconductors have been proposed as a tunable platform to realize topological superconductivity and Majorana zero modes (MZM). The tuning is done using an axial magnetic flux $Φ$ which allows transforming the system from trivial at $Φ=0$ to topologically nontrivial when half a magnetic flux quantum $Φ_0/2$ threads the wire's cross-section. Here we explore the expected topological transition in TI-wire-based Josephson junctions as a function of magnetic flux by probing the $4π$-periodic fraction of the supercurrent, which is considered as an indicator of topological superconductivity. Our data suggest that this $4π$-periodic supercurrent is at lower magnetic field largely of trivial origin, but that at magnetic fields above $\simΦ_{0}/4$ topological $4π$-periodic supercurrents take over.

cond-mat.mes-hall↗

arXiv2021

Magnetohydrodynamics and electro-electron interaction of massless Dirac fermions

The magnetotransport properties of massless Dirac fermions in a gapless HgTe quantum well are investigated. In samples with narrow channels, a large negative magnetoresistance with a Lorentzian profile is observed, which is interpreted as a manifestation of electron viscosity due to electron-electron interaction. Comparison of experiment with theory yields the shear stress relaxation time of the Dirac fermions caused by electron-electron scattering.

cond-mat.mes-hall↗

arXiv2021

Localization of the Helical Edge States in the Absense of External Magnetic Field

Theoretically, the helical edge states of two-dimensional topological insulators are protected from coherent backscattering due to nonmagnetic disorder provided electron interactions are not too strong. Experimentally, the edges typically do not demonstrate the systematic and robust quantization, at the same time little is known about the sub-Kelvin temperature behavior. Here, we report the surprising localization of the edge states in an 8 nm HgTe quantum well in zero magnetic field at millikelvin temperatures. Additionally, the magnetoresistance data at 0.5 K for the edges few micrometers long suggests the field-dependent localization length $l_B\propto B^{-α}$, with $α$ ranging approximately from $1.6$ to $2.8$ at fields $B\lesssim0.1\,\text{T}$ and $α\approx1.1$ at higher fields up to $0.5\,\text{T}$. In the frame of disordered interacting edge, these values of $α$ correspond to the Luttinger liquid parameters $K\approx 0.9-1.1$ and $K\approx 0.6$, respectively. We discuss possible scenarios which could result in the zero magnetic field localization.

cond-mat.mes-hall↗

arXiv2021

Multiple crossing of Landau levels of two-dimensional fermions in double HgTe quantum wells

The double quantum well systems consisting of two HgTe layers separated by a tunnel-transparent barrier are expected to manifest a variety of phase states including two-dimensional gapless semimetal and two-dimensional topological insulator. The presence of several subbands in such systems leads to a rich filling factor diagram in the quantum Hall regime. We have performed magnetotransport measurements of the HgTe-based double quantum wells in both gapless and gapped state and observed numerous crossings between the Landau levels belonging to different subbands. We analyze the Landau level crossing patterns and compare them to the results of theoretical calculations.

cond-mat.mes-hall↗

arXiv2020

Topological insulators based on HgTe

The most interesting experimental results obtained in studies of 2D and 3D topological insulators (TIs) based on HgTe quantum wells and films are reviewed. In the case of 2D TIs, these include the observation of nonlocal ballistic and diffusion transport, the magnetic breakdown of 2D TIs, and an anomalous temperature dependence of edge-channel resistance. In 3D TIs, a record-setting high mobility of surface two-dimensional Dirac fermions (DFs) has been attained. This enabled determining all the main TI parameters (the bulk gap and the density of DFs on both of its surfaces) and provided information on the phase of the Shubnikov - de Haas oscillations of DFs, which indicates the rigid topological coupling between the fermion spin and momentum. Prospects for further research are discussed in the conclusion.

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