The peculiarities of electronic transport of the WTe2 single crystal, in particular, the minimum on the temperature dependence of the resistivity in a magnetic field and quadratic field dependence of the Hall resistivity, were studied. The values of the mean free path were estimated. It is suggested that the observed quadratic field dependence of the Hall resistivity may be associated with the strong electron-surface scattering mechanism, which was previously observed in compensated metals with a closed Fermi surface, under conditions of inhomogeneous distribution of electric current over the conductor cross section.

PtSn4 and WTe2 single crystals were grown, and the resistivity was studied in the temperature range from 4.2 to 80 K in various magnetic fields up to 10 T in detail. It is suggested that the observed quadratic temperature dependence of the electrical resistivity at low temperatures in zero field can be due to, in addition to electron-electron scattering, the “electron-phonon-surface” interference scattering mechanism. The transition from high effective magnetic fields to weak ones, which is observed in compensated conductors with a closed Fermi surface, was proposed as a possible explanation for the minimum on the temperature dependence of the resistivity of PtSn4 and WTe2 in a magnetic field. The values of the mean free path of current carriers were estimated in these materials.

A stepwise behavior of the Hall magnetoresistance ρ_yx(B) with an almost vertical increase around zero and a further trend close to h/e^2 has been revealed in a double quantum well consisting of two 8.5-nm-thick HgTe layers separated by a 3-nm barrier. The band structure of the double quantum well is characterized by a sharp maximum in the center of the Brillouin zone, which is close in energy to the lateral maximum. The observed sharp increase in ρ_yx(B) near zero is consistent with a vanishingly low concentration of light holes at this maximum, but the further almost horizontal behavior is inconsistent with the classical description of ρ_yx(B) for a high concentration of low-mobility holes at the lateral maximum and implies quantum effects. A high sensitivity of the observed behavior of ρ_yx(B) to external fields (electric field perpendicular to the layers and a parallel magnetic field) has been detected in agreement with change in fine balance of the positions of the central and lateral maxima. It has been shown that this property occurs because the double quantum well has a specific dipole moment distinguishing it from the single quantum well with similar features of the band structure; consequently, the response of the single quantum well should be much weaker.

The effects observed in the Shubnikov–de Haas oscillation regime in the HgCdTe/HgTe/HgCdTe heterostructure with a wide (20.3 nm) HgTe quantum well with an inverted band structure are discussed. In a topologically trivial 2D system, a π-shift of magnetooscillation phase is found. A thorough experimental study and theoretical analysis of the data is presented to understand the physical causes of this anomalous phase shift. The effective electron mass m c / m 0 = (0.022 ± 0.002) obtained from the region of doubly degenerate magnetoresistance peaks is approximately half the theoretical estimates. In the region of stronger magnetic fields, for non-degenerate magnetoresistance peaks, we have m c / m 0 = (0.034 ± 0.003), which is in good agreement with both theoretical predictions and experimental results obtained from the analysis of activation conductivity in the quantum Hall effect regime. The reasons for this discrepancy are discussed.

Electrical resistivity, magnetoresistivity, and the Hall effect have been studied in a single crystal of topological semimetal WTe 2 in the temperature range from 12 to 200 K under magnetic fields up to 9 T. A quadratic temperature dependence of the electrical resistivity in the absence of field and conductivity in a magnetic field is found at low temperatures, which appears to be associated with contributions from various scattering mechanisms. Single-band and two-band models were used to analyze data on the Hall effect and magnetoresistivity. These results indicate electron–hole compensation with a slight predominance of electron charge carriers.

Представлены результаты исследований специфики энергетического спектра гетероструктур на основе HgTe, характеризуемого инверсным расположением зон, образованных s-орбиталями (состояния типа E), и зон, сформированной p-орбиталями (состояния типа H), при толщинах слоев HgTe, близких к критической величине в 6.3 нм, и более.

A single crystal of a topological Weyl semimetal WTe2 was grown and its electrical resistivity and galvanomagnetic properties (magnetoresistivity and the Hall effect) were investigated in detail in the temperature range from 1.8 K to 300 K and in magnetic fields of up to 9 T.

The effects observed in the Shubnikov - de Haas oscillation regime in the HgCdTe/HgTe/HgCdTe heterostructure with a wide (20.3 nm) HgTe quantum well with an inverted band structure are discussed. In a topologically trivial 2D system, Pi(Greek letter) - shift of magnetooscillation phase is found. A thorough experimental study and theoretical analysis of the data is presented to understand the physical causes of this anomalous phase shift. The value of the effective electron mass m_c/m_0 = (0.022+-0.002), obtained from the region of doubly degenerate peaks of magnetoresistance, is approximately half the theoretical estimates. In the region of stronger magnetic fields for nondegenerate peaks of magnetoresistance, we have m_c/m_0 = (0.034+-0.003), which is in good agreement with both theoretical predictions and experimental results obtained from the analysis of activation conductivity in the quantum Hall effect regime. The reasons for this discrepancy are discussed.

WTe2 and MoTe2 single crystals were grown, some of them were quenched, and the following properties were studied: electroresistivity in the temperature range from 1.8 to 300 K, magnetoresistivity at temperatures from 1.8 to 300 K in magnetic fields of up to 9 T. On the one hand, quenching leads to dramatic changes in the behaviour and value of the electroresistivity of MoTe2; the type of the electroresistivity changes from “semiconductor” to “metallic”, and the electroresistivity values of MoTe2 before and after quenching differ by 8 orders of magnitude (!) at low temperatures. On the other hand, quenching is shown not to lead to significant changes in the behaviour and value of the electroresistivity of WTe2. A relatively small increase in the electroresistivity of quenched WTe2 at low temperatures can be associated with the scattering of current carriers by structural defects. The magnetoresistivity of MoTe2 increases from 7 to 16% in a field of 9 T at a temperature of 12 K as a result of quenching. The magnetoresistivity of WTe2 is shown to reach ∼1700% in a field of 9 T at 2 K. The behaviour of the magnetoresistivity of non-quenched samples is typical for compensated conductors with a closed Fermi surface.

The longitudinal ρxx(B, T) and Hall ρxy(B, T) resistances are experimentally investigated in n-InGaAs/GaAs nanostructures with a single and double quantum wells in the magnetic field range B = 0–2.5 T and temperatures T = 1.8–20 K. It is shown that the origin of the temperature-independent point located at ωcτ≅1 on the ρxx(B, T) curves is due to the combined action of the classical cyclotron motion and the quantum interference effects of weak localization and electron-electron interaction. The results obtained indicate that the transition from the dielectric phase to the phase of the quantum Hall effect is a crossover from weak localization (quantum interference effects in a weak magnetic field) to strong localization in quantizing magnetic fields in the quantum Hall effect regime.

We report on the observation of an unconventional structure of the quantum Hall effect (QHE) in a p-type HgTe/CdxHg1-xTe double quantum well (DQW) consisting of two HgTe layers of critical thickness. The observed QHE is a reentrant function of magnetic field between two i = 2 states (plateaus at rho(yx) = h/ie(2)) separated by an intermediate i = 1 state in the shape of a flat-top peak placed on the remarkably long i = 2 plateau. This anomalous i = 1 peak separates two different regimes: (i) a traditional QHE at relatively low fields corresponding to a small density of mobile holes p(s) and (ii) a high-field QHE with a 2-1 plateau-plateau transition corresponding to a much larger p(s). The high-field part is strongly sensitive to external influences such as gate voltages, in contrast to the low-field part, which is much less responsive. We explain the observed behavior by analyzing the calculated trajectories of the Fermi level E-F(B) between hole-like and electron-like Landau levels (LLs). At low fields, E-F is captured by the lateral maximum (LM) of the valence subband, and only holes in the center of the Brillouin zone contribute to QHE, while holes in the LM are inactive. In contrast, at fields above the reentry, E-F rises significantly higher than LM, approaching the zero-mode LLs, and all holes come into play in QHE. At intermediate fields, the reentrance is caused by a combination of two factors in the specific energy spectrum of this DQW: (i) the superposition of an electron-like LL on hole-like LLs and (ii) the stabilizing influence of the LM reservoir on E-F(B).

The electro- and magnetoresistivity of MoTe 2 single crystals before and after quenching were measured at temperatures from 1.8 to 300 K and in magnetic fields of up to 9 T. It was demonstrated that quenching can lead to strong changes in values of the electro-and magneresistivity studied as well as in their temperature and field dependences. The peculiarities of these electronic transport characteristics changes were studied in detail.

We provide a systematic measurement of the longitudinal ρxx and Hall ρxy resistivities of quantum Hall transition in a two-dimensional electron system In0.9Ga0.1As/In0.81Al0.19As with strong spin-orbit coupling. For half-integer filling factors the linear temperature dependence of the effective quantum Hall effect plateau-to-plateau transition width ΔB(T) is observed in contrast to scaling behavior for systems with short-range disorder. The recent prediction that the width of transition region remains finite when extrapolated to zero temperature, resulting from Landau level mixing, is more preferable. The shift of the transition point in magnetic field with the temperature is found to originate from the mixing between Landau levels due to the inelastic scattering.

The temperature dependences of the Hall coefficient and magnetoresistivity of a p-type HgTe/CdHgTe double quantum well with HgTe layers of critical thickness in the temperature range T = 35–300 K under magnetic fields up to 9 T are investigated. The position of the earlier observed reentrant quantum Hall transition from plateau i = 1 to plateau i = 2 is found to be close to the transition field from light to heavy holes with an increase in the magnetic field in the classical Hall effect. It is found that thermally activated light electrons contribute to the Hall effect along with light and heavy holes at T ≥ 35 K. The activation energy of electrons is estimated from the temperature dependence of the electron concentration as 28 meV, which exceeds the calculated value from the lateral maximum of the valence subband to the edge of the lowest conduction subband, probably because of heterostructure asymmetry.

We report on the longitudinal and Hall resistivities of a HgTe quantum well with inverted energy spectrum (dQW = 20.3 nm) measured in the quantum Hall (QH) regime at magnetic fields up to 9 T and temperatures 2-50 K. The temperature dependence of the QH plateau-plateau transition (PPT) widths and of variable range hopping (VRH) conduction on the Hall plateaus are analyzed. The data are presented in a genuine scale form both for PPT regions and for VRH regime. Estimations for the degree of the carrier localization length divergence reveal a decisive role of the long-range random potential (the potential of remote ionized impurities) in the localization - delocalization processes in the QH regime for the system under study.

We report an experimental study of quantum conductivity corrections for two-dimensional electron gas in a GaAs/InGaAs/GaAs single and double quantum wells in a wide temperature range (1.8-100) K. We perform a comparison of our experimental data for the longitudinal conductivity at zero magnetic field to the theory of interaction-induced corrections to th transport coefficients. In the temperature range from 10 K up to (45-60) K, wich covers the ballistic interaction regimes for our samples, a rather good agreement between the theory and our experimental results has been found.

AbstractThe temperature dependences of the Hall coefficient and magnetoresistivity of a p -type HgTe/CdHgTe double quantum well with HgTe layers of critical thickness in the temperature range T = 35–300 K under magnetic fields up to 9 T are investigated. The position of the earlier observed reentrant quantum Hall transition from plateau i = 1 to plateau i = 2 is found to be close to the transition field from light to heavy holes with an increase in the magnetic field in the classical Hall effect. It is found that thermally activated light electrons contribute to the Hall effect along with light and heavy holes at T ≥ 35 K. The activation energy of electrons is estimated from the temperature dependence of the electron concentration as 28 meV, which exceeds the calculated value from the lateral maximum of the valence subband to the edge of the lowest conduction subband, probably because of heterostructure asymmetry.

AbstractThe longitudinal ρ_ xx and Hall ρ_ xy resistances were measured in the region of the quantum phase transitions for the quantum Hall effect regime with magnetic fields up to 12 T at temperatures of T = 0.4–30 K in two-dimensional electron systems n -In_0.9Ga_0.1As/In_0.81Al_0.19As. The nonuniversal scaling behavior of the temperature dependence of the width of the resistance ρ_ xx peaks related to the effect of the large-scale random potential and of Landau-level mixing with opposite spin directions was found.

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.

The name of the third author should read E. V. Ilchenko.