R. A. Lunin

Quantum oscillations of Hall resistance, magnetoresistance in a magnetic field up to 54 T and the energy spectrum of Sn doped layered semiconductors p-(Bi1−xSbx)2Te3

The Hall effect and the Shubnikov–de Haas (SdH) effect have been investigated in magnetic fields up to 54 T in p-(Bi1−xSbx)2Te3 (0 ≤ x ≤ 1.0) Sn doped single crystals. Doping of (Bi1−xSbx)2Te3 with tin has shown that Sn exhibits acceptor properties in all crystals. We discuss the valence band structure of (Bi1−xSbx)2Te3 with the upper valence band (light hole band (LHB)), the lower valence band (heavy hole band (HHB)) and Sn-induced impurity band (IB). The Hall resistivity ρH as a function of magnetic field shows quantization in the form of plateaus. The calculated Landau levels of the LHB with the best-fit parameters are in agreement with the experiment. The oscillation of ρH is due to the presence of the carrier reservoir. The impurity resonant band with a high density of states or the HHB with a higher hole effective mass serve as the reservoir.

Transport and optical properties of tin-delta-doped GaAs structures

The transport and optical properties of tin δ layers in GaAs are investigated as functions of the Sn concentration. The Shubnikov-de Haas and Hall effects are measured in the temperature range 0.4–12 K in magnetic fields up to 38 T. The band diagrams and quantum mobilities of electrons in the quantum-well subbands are calculated. Features associated with electronic transitions from quantum-well levels are found in the photoluminescence spectra of the structures. Oscillations of the resistance are observed in a magnetic field parallel to the δ layer and are attributed to features in the density of states at the Fermi level.

Electron transport and optical properties of shallow GaAs/InGaAs/GaAs quantum wells with a thin central AlAs barrier

Shallow GaAs/InGaAs/GaAs quantum well structures with and without a three-monolayer thick AlAs central barrier have been investigated for different well widths and Si doping levels. The transport parameters are determined by resistivity measurements in the temperature range 4–300 K and magnetotransport in magnetic fields up to 12 T. The (subband) carrier concentrations and mobilities are extracted from the Hall data and Shubnikov–de Haas oscillations. We find that the transport parameters are strongly affected by the insertion of the AlAs central barrier. Photoluminescence spectra, measured at 77 K, show an increase of the transition energies upon insertion of the barrier. The transport and optical data are analysed with the help of self-consistent calculations of the subband structure and envelope wavefunctions. Insertion of the AlAs central barrier changes the spatial distribution of the electron wavefunctions and leads to the formation of hybrid states, i.e., states which extend over the InGaAs and the delta-doped layer quantum wells.

Experimental determination of the electron effective masses and mobilities in each dimensionally-quantized subband in an InxGa1 − xAs quantum well with InAs inserts

HEMT structures with In0.53Ga0.47As quantum well are synthesized using molecular-beam epitaxy on InP substrates. The structures are double-side Si δ-doped so that two dimensionally-quantized subbands are occupied. The effect of the central InAs nanoinsert in the quantum well on the electron effective masses m* and mobilities in each subband is studied. For experimental determination of m*, the quantum μq and transport μt mobilities of the two-dimensional electron gas in each dimensionally-quantized subband, the Shubnikov-de Haas effect is measured at two temperatures of 4.2 and 8.4 K. The electron effective masses are determined by the temperature dependence of the oscillation amplitudes, separating the oscillations of each dimensionally-quantized subband. The Fourier spectra of oscillations are used to determine the electron mobilities μq and μt in each dimensionally-quantized subband. It is shown that m* decreases as the InAs-nanoinsert thickness d in the In0.53Ga0.47As quantum well and electron mobilities increase. The maximum electron mobility is observed at the insert thickness d = 3.4 nm.

Structure and properties of solid solutions in the Mg-Al-B system

Compounds with the general formula Mg1−xAlxB2 were obtained by two-step ceramic synthesis. All compounds were characterized by X-ray diffraction, NMR spectroscopy, and by four point probe resistivity measurements in various magnetic fields method. The diborides unit cell parameters were determined as a function of the Al mole fraction. With the vaues of x up to 0.40 (where x is the composition of the stock prepared for sintering), the unit cell parameters of Mg1−xAlxB2 are similar to those of pure MgB2 and the superconducting transition temperature was lowered. For stock compositions of 0:25 ≤ x ≤ 0:60, the products contain a superstructure, also superconducting phase, which becomes the only product at x = 0:50, and at x > 0:60 this phase is replaced by AlB2-based solid solutions.

Quenching of persistent photoconductivity and decrease of electron concentration by high electric fields in GaAs delta-doped by Sn on vicinal substrate structures

Abstract This paper reports the measurements of high electric field transport and the persistent photoconductivity in delta-doped by Sn on vicinal and singular substrate GaAs structures. Transport properties of the hot electron gas were measured in the temperature range 4.2 K

New insights into the plateau-insulator transition in the quantum Hall regime

Abstract We have measured the quantum critical behavior of the plateau–insulator (PI) transition in a low-mobility InGaAs/GaAs quantum well. The longitudinal resistivity measured for two different values of the electron density follows an exponential law, from which we extract critical exponents κ=0.54 and 0.58, in good agreement with the value (κ=0.57) previously obtained for an InGaAs/InP heterostructure. This provides evidence for a non-Fermi liquid critical exponent. By reversing the direction of the magnetic field we find that the averaged Hall resistance remains quantized at the plateau value h/e2 through the PI transition. From the deviations of the Hall resistance from the quantized value, we obtain the corrections to scaling.

Study of the paramagnetic centers in heterofullerides MnM′3-nC60 (M = K, Rb, Cs; M′ = Be, Mg, Ca, Ba; n = 1, 2)

New heterofullerides Cs2MC60, CsM2C60, Rb2MC60, K2MC60, and KM2C60 (M = Be, Mg, Ca, Ba) have been synthesized; the temperature dependences of the magnetic susceptibilities of these compounds in the temperature range from 4.2 to 297 K have been measured. Among these heterofullerides, K2MgC60, KMg2C60, K2CaC60, K2BeC60, and Rb2BeC60 pass to the superconducting state at temperatures Tc = 13–24.3 K. The paramagnetic electronic states of the compounds have been studied by EPR at temperatures of 105–300 K, which shows the existence of two types of paramagnetic centers, related to oxygen defects and conduction electrons.

Effect of selenium on the galvanomagnetic properties of the diluted magnetic semiconductor Hg1−xMnxTe1−ySey

The galvanomagnetic properties of single crystals of the diluted magnetic semiconductor Hg1−xMnxTe1−ySey (0.01<y<0.1) with x=0.05 and 0.14 have been investigated in the temperature range 4.2–300 K. Peculiarities of the temperature dependence of the Hall coefficient RH and its complicated behavior in a magnetic field are quantitatively explained by the existence of three groups of current carriers: electrons and two types of holes with different mobilities for which the temperature dependence of concentration and mobility was obtained. A transition from p-type to n-type conductivity was observed with increase of Se content, alongside with a simultaneous change of sign of the magnetoresistance from negative to positive.

Influence of buffer-layer construction and substrate orientation on the electron mobilities in metamorphic In0.70Al0.30As/In0.76Ga0.24As/In0.70Al0.30As structures on GaAs substrates

The influence of construction of the buffer layer and misorientation of the substrate on the electrical properties of In0.70Al0.30As/In0.76Ga0.24As/In0.70Al0.30As quantum wells on a GaAs substrate is studied. The temperature dependences (in the temperature range of 4.2 K < T < 300 K) and field dependences (in magnetic fields as high as 6 T) of the sample resistances are measured. Anisotropy of the resistances in different crystallographic directions is detected; this anisotropy depends on the substrate orientation and construction of the metamorphic buffer layer. In addition, the Hall effect and the Shubnikov–de Haas effect are studied. The Shubnikov–de Haas effect is used to determine the mobilities of electrons separately in several occupied dimensionally quantized subbands in different crystallographic directions. The calculated anisotropy of mobilities is in agreement with experimental data on the anisotropy of the resistances.