D. V. Shamshur

Superconductivity and magnetism in In-doped lead-tin tellurides

Abstract Our studies of solid solutions PbzSn1−zTe doped with In have concentrated on checking for a superconducting transition in the temperature variation of both the resistivity and the magnetization and of an influence on that behaviour of an applied magnetic field. The magnetization of the samples was measured in a SQUID magnetometer using a small field (15 Oe) cooling method. We obtained the dependences of the superconducting critical parameters as a function of the In concentration and the content of isovalent lead. The data for superconductivity correlate with normal state properties and with an influence of the matrix composition on the energy band position of the In states. We observed an enhancement of superconductivity when the Fermi level pinned by the In impurity band enters the heavy hole Σ-band.

Low-temperature transport properties of multigraphene films grown on the SiC surface by sublimation

Multigraphene films grown by sublimation on the surface of a semi-insulating 6H-SiC substrate have been studied. It is shown that pregrowth annealing of the substrate in a quasiclosed growth cell improves the structural quality of a multigraphene film. Ohmic contacts to the film have been fabricated, and the Hall effect has been studied at low temperatures. It is found that a 2D electron gas exists in the films. It is concluded that the conductivity of the film is determined by defects existing within the graphene layer or at the interface between the graphene film and a SiC substrate.

Anomalous electron transport in doped uncompensated p-GaAs/AlGaAs quantum wells: evidence of virtual Anderson transition

For highly doped uncompensated p-type layers located within the central part of GaAs/AlGaAs quantum wells, we observed the activated low-temperature behavior of conductivity. The low values of the activation energy, ?4 = (1?3)?meV, cannot apparently be ascribed to standard mechanisms. We attribute this behavior to the existence of a narrow band of extended states near the maximum of the density of states in the impurity band. The Hubbard repulsion prevents metallic transport of holes over these states. However, the minority carriers?electrons?supplied by background defects and situated at low temperatures within the tail of the impurity band can be activated to the above mentioned band of extended states. We refer to this behavior as the virtual Anderson transition since the conductance is maintained by the extended states formed within the impurity band though the conductivity is not metallic. The low-temperature () conductance is strongly non-Ohmic: the I?V curves are S-shaped that leads to a breakdown behavior. We explain the observed low threshold fields () by the fact that we are dealing with the impact ionization of the electrons from the states below the chemical potential to the band of extended impurity states situated close to the chemical potential, the ionization energy being small.

Electrical conductivity and superconductivity of ordered indium-opal nanocomposites

The electrical conductivity is measured experimentally and the parameters of the superconducting transition are determined in a regular spatial network of multiply connected submicron-sized indium grains embedded in voids of an ordered opal dielectric matrix. The In-opal nanocomposite was prepared by pressure injection of the molten metal into voids of opal samples. Arrays of In grains of different sizes were produced by properly varying the characteristic geometric sizes of the opal voids, which offered the possibility of observing quantitative and qualitative changes in the temperature dependence of electrical resistance and studying the size effects on the critical temperature and critical magnetic field in the In-opal nanocomposites. It was found that, as the coherence length becomes comparable to the size of the superconducting grains, the parameters of the superconducting transition in the nanocomposite increase sharply.

Low-temperature hopping conduction over the upper Hubbard band in p-GaAs/AlGaAs multilayered structures

The low-temperature 2D variable range hopping conduction over the states of the upper Hubbard band is investigated in detail for the first time in multilayered Be-doped p-type GaAs/AlGaAs structures with quantum wells of 15-nm width. This situation was realized by doping the layer in the well and a barrier layer close to the well for the upper Hubbard band (A+ centers) in the equilibrium state filled with holes. The conduction was of the Mott hopping type in the entire temperature range (4−0.4 K). The positive and negative magnetoresistance branches as well as of non-Ohmic hopping conduction at low temperature are analyzed. The density of states and the localization radius, the scattering amplitude, and the number of scatterers in the upper Hubbard band are estimated. It is found that the interference pattern of phenomena associated with hopping conduction over the A+ band is qualitatively similar to the corresponding pattern for an ordinary impurity band, but the tunnel scattering is relatively weak.

Transition from strong to weak localization in the split-off impurity band in two-dimensional p-GaAs/AlGaAs structures

A crossover from strongly localized behavior to weak localization (SL-WL) was observed in two-dimensional modulation-doped GaAs/Al0.3Ga0.7 As structures as the impurity concentration increased. In this case, it was observed that the low-temperature dependence of the conductivity changed its character (from exponential to logarithmic) and the magnetoresistance changed its sign (from linear negative to root positive). For 2D structures, it is shown that this transition proceeds in the impurity band separated from the valence band by the mobility gap, whereas the effective mass in the impurity band is larger than in the valence band.

Metal-insulator transition in n-3C-SiC epitaxial films

The paper reports a study of galvanomagnetic properties of n-3C-SiC/n-6H-SiC heterostructures at liquid-helium temperatures. 3C-SiC epitaxial layers were grown by sublimation epitaxy in a vacuum on the (0001)C face of 6H-SiC substrates produced by the Lely method and 4H-SiC substrates grown by modified Lely method. The x-ray topography demonstrated the high quality structure of the epitaxial layers and the absence of any transition regions between 3C-SiC epitaxial layer and substrate. The low-temperature conductivity and magnetoresistance of the films have been studied as functions of their doping level and structural quality. It was found that the metal-insulator transition occurs in the n-3C-SiC layer at concentrations Nd−Na≤3×1017 cm−3.

Effect of pressure on the parameters of the superconducting transition in In-doped PbzSn1−zTe semiconducting solid solutions

The effect of hydrostatic pressures up to 10 kbar and lead content on the superconducting and electrical characteristics of indium-doped (5 at. %) PbzSn1−zTe semiconducting solid solutions is studied. Experimental correlations are found for these pressure dependences. When the pressure is raised to P = 6.8 kbar in this material with a lead content z = 0.45 the superconductivity threshold increases to Tc = 1.7 K, which is typical of the compound with z = 0.05 at atmospheric pressure. Increasing the hydrostatic pressure P > 3 kbar in (Pb0.05Sn0.95)0.95In0.05Te causes Tc to fall below 1 K. These features, along with the bell-shaped Tc(P) curve for z = 0.3, are explained by a shift in the energy EIn of the quasilocal states and the Fermi level which is stabilized by them, from the L-band into the Σ-valence band with a high density of states (most noticeable for z = 0.45). A concentration threshold for emergence of the Fermi level from the Σ-valence band with increasing lead content was found using Hall effect...

Electrical characteristics of multigraphene films grown on high-resistivity silicon carbide substrates

Multigraphene films grown by sublimation of the surface of semi-insulating 6H-SiC substrates in a vacuum have been studied. The films exhibit a semiconductor-type conductivity. A conclusion is made that this type of conduction is supposedly determined by defects present between separate graphene crystals constituting the carbon layers under study.

Slow relaxation of magnetoresistance in AlGaAs–GaAs quantum well structures quenched in a magnetic field

We observed a slow relaxation of the magnetoresistance in response to an applied magnetic field in selectively doped p-GaAs-AlGaAs structures with a partially filled upper Hubbard band. We have paid special attention to excluding the effects related to temperature fluctuations. Although these effects are important, we have found that the general features of slow relaxation persist. This behavior is interpreted as related to the properties of the Coulomb glass formed by charged centers with account taken of spin correlations, which are sensitive to an external magnetic field. Variation of the magnetic field changes the numbers of the impurity complexes of different types. As a result, it affects the shape and depth of the polaron gap formed at the states belonging to the percolation cluster responsible for the conductance. The suggested model explains both the qualitative behavior and the order of magnitude of the slowly relaxing magnetoresistance.