V. N. Neverov

Magnetotransport in two-dimensional n-InGaAs∕GaAs double-quantum-well structures near the transition from the insulator to the quantum Hall effect regime

The temperature and magnetic-field dependence of the components of the conductivity and resistivity tensors are calculated with the quantum corrections due to the weak localization effect and electron-electron interaction taken into account in the diffusion and ballistic regimes. The corrections to the conductivity from the weak localization and electron-electron interaction and also the influence of spin and oscillation effects are taken into account by a renormalization of the transport relaxation time of the electron momentum, with the result that the Drude conductivity becomes temperature dependent. A calculation of the components of the conductivity and resistivity tensors is carried out with the use of the theoretical values of the parameters of the theory of quantum corrections, which are determined solely by the values of the carrier density and mobility of a particular sample. The results of the calculation are compared with experimental results for two-dimensional n-InGaAs∕GaAs structures with d...

Spin effects in magnetoresistance induced in an n-InxGa1−xAs/GaAs double quantum well by a parallel magnetic field

Magnetoresistance in n-InxGa1−xAs/GaAs (x ≈ 0.18) heterostructures with double quantum wells (DQWs) was studied in the magnetic field parallel to the DQW layer. Specific features of the magnetoresistance, related to the passing of the tunnel gap edges across the Fermi level, are revealed and studied. Agreement between the calculated and experimental positions of the observed features is obtained when the spin splitting of the energy spectrum is taken into account. Earlier, similar features were observed in the magnetoresistance of n-GaAs/AlxGa1−xAs DQW heterostructures, but the spin effects did not manifest themselves.

Probing the p-Ge1-xSix/Ge/p-Ge1-xSix quantum well by means of the quantum Hall effect

We have measured the temperature (0.1?T?15?K) and magnetic field (0?B?32?T) dependences of longitudinal and Hall resistivities for the p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 10?dw?38?nm and hole densities ps = (1-5)?1015?m-2. An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level (LL) gaps deduced from the activation magnetotransport etc) to the quantum well (QW) characteristics has been revealed in the cases when the Fermi level reached the second confinement subband. The background density of states (5-10)?1014?m-2?meV-1 deduced from the activation behaviour of the magnetoresistance was too high to be attributed to the LL tails, but may be accounted for within a smooth random potential model. The hole gas in the Ge QW was found to separate into two sublayers for dw>~35?nm and ps?5?1015?m-2. Concomitantly the positive magnetoresistance emerged in the weakest fields, from which different mobilities in the sublayers were deduced. A model is suggested to explain the existence of the plateaux close to the fundamental values in a system of two parallel layers with different mobilities.

The effect of infrared radiation on quantum magnetotransport in n-InGaAs/GaAs with two strongly coupled quantum wells

Longitudinal ρxx(B) and Hall ρxy(B) magnetoresistances are measured as a function of transverse magnetic field in n-InGaAs/GaAs nanostructures with strongly coupled double quantum wells in the temperature range T = 0.05–60 K and magnetic fields B = 0–16 T before and after low-temperature infrared irradiation. The appearance of persistent photoconductivity causes the type of the temperature dependence of resistance to be changed: from insulator-like (dρ/dT 0) at higher temperatures. It is shown that this is related to the temperature dependence of electron density. The strong temperature dependence of electron mobility observed before the illumination practically disappears after infrared irradiation.

Features of quantum effects in two-dimensional GaAs∕n‐InGaAs∕GaAs structures with double quantum wells

In an n‐InxGa1−xAs∕GaAs double quantum well (x≈0.2) the temperature dependence of the longitudinal resistance ρxx(T) of a 2D electron gas with low mobility and with an electron density close to the B=0 metal–insulator transition is of an “insulator” character in the temperature interval T=1.8–70K(kBTτ∕ℏ=0.1–3.8). Anomalous temperature dependence of σxy(B,T) in the region ωcτ=1 leads to a number of features of the transition from the regime of weak localization and electron–electron interaction to the quantum Hall effect regime at low magnetic fields.

Nonmonotonic temperature dependence of the resistivity of p-Ge/Ge1−xSix in the region of the metal–insulator transition

In a two-dimensional (2D) hole system (multilayer p-Ge/Ge1−xSix) heterostructure with conductivity σ≈e2/h at low temperatures (T≈1.5 K) a transition from the insulator phase (dσ/dT>0) to a “metallic” phase (dσ/dT<0) is observed as the temperature is lowered, behavior that is in qualitative agreement with the predictions of the Finkelstein theory. In a magnetic field B perpendicular to the plane of the 2D layer one observes positive magnetoresistance depending only on the ratio B/T. We attribute the positive magnetoresistance effect to the suppression of the triplet channel of Fermi-liquid electron–electron interaction by the magnetic field owing to the strong Zeeman splitting of the hole energy levels.

Parabolic negative magnetoresistance in p-Ge/Ge1-xSix heterostructures

Quantum corrections for the conductivity due to the weak localization (WL) and the disorder-modified electron-electron interaction (EEI) are investigated for the high-mobility multilayer p-Ge/Ge1−xSex heterostructures at T=(0.1–20) K in magnetic field B up to 1.5 T. Negative magnetoresistance with logarithmic dependence on T and linear in B2 is observed for B⩾0.1 T. Such a behavior is attributed to the connection between the classical cyclotron motion and the EEI effect. The Hartree part of the interaction constant is estimated (Fσ=0.44) and the WL and EEI contributions to the total quantum correction Δσ at B=0 are separated (ΔσWL≈0.3Δσ; Δσee≈0.7Δσ).Quantum corrections to the conductivity due to the weak localization (WL) and the disorder-modified electron-electron interaction (EEI) are investigated for the high-mobility multilayer p-Ge/Ge1-xSix heterostructures at T = (0.1 - 20.0)K in magnetic field B up to 1.5T. Negative magnetoresistance with logarithmic dependence on T and linear in B^2 is observed for B>= 0.1T. Such a behavior is attributed to the interplay of the classical cyclotron motion and the EEI effect. The Hartree part of the interaction constant is estimated (F_/sigma = 0.44) and the WL and EEI contributions to the total quantum correction /Delta /sigma at B = 0 are separated (/Delta /sigma_{WL} ~ 0.3/Delta /sigma; /Delta /sigma_{EEI} ~ 0.7/Delta /sigma).

Scaling in the quantum Hall effect regime in n-InGaAs/GaAs nanostructures

The longitudinal ρxx(B) and Hall ρxy(B) magnetoresistances are investigated experimentally in the integer quantum Hall effect (QHE) regime in n-InGaAs/GaAs double quantum well nanostructures in the range of magnetic fields B = (0–16) T and temperatures T = (0.05–70) K before and after IR illumination. The results are evaluated within the scaling hypothesis with regard to electron-electron interaction.

Contributions of the electron–electron interaction and weak localization to the conductance of p‐Ge∕Ge1−xSix heterostructures

We separate the contributions to the conductance from disorder-modified electron–electron interaction and weak localization for p‐Ge∕Ge1−xSix heterostructures with low carrier mobility at magnetic fields 0⩽B⩽2T for fixed temperatures 0.2K⩽T⩽4.2K. The contribution of the Zeeman splitting to the magnetoresistance is taken into account in the electron–electron interaction, making it possible to get reasonable values of the energy relaxation time (dephasing time τφ) and to obtain its theoretically predicted power-law temperature dependence. Values are estimated for the parameters of these effects: Hartree interaction constant F0σ=−0.51, amplitude of the Fermi-liquid interaction λ=0.40, and Lande factor g=12.0.

Magnetic breakdown and quantum magnetotransport with a constant pseudospin under tilted magnetic fields in an n-In x Ga1−x As/GaAs double quantum well

A procedure is proposed for precise scanning of the (B⊥, B‖) plane between the magnetic field projections that are perpendicular and parallel to (quasi-)two-dimensional layers when measuring their longitudinal and Hall magnetoresistances. Investigations of a n-InxGa1−xAs/GaAs double quantum well (x ≈ 0.2) performed using this procedure make it possible to reveal a number of the features of the magnetoresistance, which appear due to a complex energy spectrum of the double quantum well in a parallel field, and to separate them from the structures associated with the magnetic breakdown. The trajectories representing the features of the magnetoresistance in the (B⊥, B‖) plane are described by the semiclassical calculations of the quantization of the energy spectrum of the double quantum well under the action of the perpendicular field component. The structures appearing due to the magnetic breakdown are amplified with increasing the total magnetic field magnitude and, in the samples with low mobility, completely suppress the features caused by the motion of an electron with a constant pseudospin component. The peaks corresponding to the magnetic breakdown are split in a strong parallel field due to the spin splitting of the Landau levels. These splittings correspond to the effective Landé factor |g*| ≈ 3.