R. V. Parfen’ev

Tellurium recrystallization under microgravity conditions and the resulting properties of samples

Three experiments on the tellurium recrystallization by a modified Bridgman method were performed under microgravity conditions on board the Mir orbital space laboratory using a ChSK-1 Kristallizator furnace. The physical properties of samples were studied, including the final crystal structure, the distribution of impurities and defects, and the charge carrier concentration and mobility. The results were compared to the analogous parameters of crystals remelted using the same method under the normal gravity conditions. It is established that the samples recrystallized in a close volume under the on-board microgravity conditions “break off” from the container walls and touch the walls only in a few points. This circumstance gives rise to special effects, such as the growth of crystals with a free surface and deep supercooling. Study of the distribution of electrically active impurities over the length of ingots shows evidence of the presence of thermocapillary convective flows in the melt under the microgravity conditions. The flows tend to increase upon separation of the melt from the container walls. The contributions due to impurities and electrically active structural defects to the charge carrier distribution are taken into account. The single-crystal sample obtained upon the partial recrystallization of tellurium in a close container volume under the on-board microgravity conditions exhibits the electrical characteristics comparable to those of a crystal grown by the Czochralski technique under the normal gravity conditions.

Some results of the growth of semiconductor crystals in microgravity conditions (to the 50th anniversary of Yuri Gagarin’s flight into space)

The history of the growth of semiconductor crystals aboard space vehicles and their subsequent investigation has been described shortly. It has been shown using Ge(Ga), GaSb(Si), and GaSb(Te) crystals as an example that the formation of segregation growth striations can be avoided during their recrystallization by the vertical Bridgman method in conditions of physical simulation of microgravity on the Earth, mainly due to the essential weakening of the thermal gravitation convection. By their structure and impurity distribution, they approach the crystals grown in space. The investigation of recrystallization of Te has made it possible to determine the role of the detachment effect characteristic of the microgravity conditions and the features of the microstructure of the samples that crystallize with a free surface. The analysis of the results obtained from experiments in space allows us to better understand the processes occurring during the crystallization of the melts and to improve the crystal growth in terrestrial conditions.

Superconductivity of (Sn1−zPbz)1−xInxTe alloys

The temperature dependences of the electrical resistivity of (Sn1−zPbz)1−x InxTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (Tc and Hc2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature Tc and of the second critical magnetic field Hc2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn1−zPbz)1−x InxTe increases, Tc and Hc2 decrease as the Fermi level EF (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when EF leaves the LΣ saddle point in the valence-band energy spectrum.

Effect of hydrostatic compression on the indium-impurity-induced superconducting transition in Pb0.3Sn0.7Te

This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the critical temperature Tc and the second critical magnetic field Hc2, in the (Pb0.3Sn0.7)0.95In0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes a superconducting transition at Tc ∼ 2.3 K, i.e., close to the maximum value Tc ∼ 2.9 K found for the (PbzSn1 − z)0.95In0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence Tc(P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution and indicate a shift in the position of the Fermi level EF with increasing pressure within the impurity band of the In quasi-local states. In this case, EF passes through a maximum in the density of impurity states at P = 3–5 kbar.

Low-temperature conductivity and the Hall effect in (PbzSn1 − z)0.84In0.16Te semiconducting solid solutions

The temperature dependences of the conductivity and the Hall effect in heavily doped polycrystalline samples of the (PbzSn1 − z)0.84In0.16Te solid solutions with lead content varied within the 0 ≤ z ≤ 0.9 interval have been studied. For x ≤ 0.65, the material undergoes a superconducting transition at a critical temperature Tc ≤ 4.2 K in a magnetic field Hc2(0 K) ∼ 50 kOe. As the lead concentration is increased to z ≤ 0.9, a clearly pronounced trend to transfer of the material to the dielectric state is observed at helium temperatures. The observed behavior is related to the variation in the band structure of the solid solutions with variations in the material composition, doping level, and position of the indium impurity band. The dependences of the resistivity, Hall effect, and superconducting characteristics of (PbzSn1 − z)0.84In0.16Te on the temperature and the composition of the solid solutions is observed to be related to the variation in its band structure as tin atoms are replaced with lead in the metallic sublattice of the compound.

Superconductor-insulator transition in (PbzSn1−z)0.84In0.16Te

The superconductor-insulator transition that occurs at liquid helium temperatures in the (PbzSn1−z)0.84In0.16Te semiconductor system with varying lead concentration z = 0.5–0.9 is experimentally investigated. The transition is attributed to the change in the energy characteristics of In impurity centers due to the variation in the amount of lead. The insulator state appears with the transition from the mixed band-impurity conduction, which is characterized by resonant scattering of carriers into the quasilocal indium impurity states, to the hopping conduction between indium impurity states. The sample with z = 0.8 is found to exhibit a variable range hopping conduction described by Mott’s law. Factors that lead to the hopping conduction via impurity states are considered.

Dependence of the superconducting transition parameters on the solid-solution composition and Te excess in Sn1−zPbzTe:In

A low-temperature (0.4–4.2 K) measurement of the temperature dependences of the resistivity of two series of samples, SnTe1+y and Sn0.8Pb0.2Te1+y solid solution, doped with 5 at.% In, is reported. The parameters of the superconducting transition, namely, the critical temperatures Tc and the second critical magnetic field Hc2, and their dependences on tellurium excess (0⩽y⩽0.06) have been determined. The observed variation of the critical parameters with increasing tellurium excess in the samples is associated with a change in the filling by holes of the indium-impurity resonance states.

Size effects in electrical and magnetic properties of quasi-one-dimensional tin wires in asbestos

Bulk composites have been prepared based on one-dimensional fibers of natural chrisothil-asbestos with various internal diameters (d = 6–2.5 nm) filled with tin. The electrical and magnetic properties of quasi-one-dimensional Sn wires have been studied at low temperatures. The electrical properties have been measured at T = 300 K at a pressure P = 10 kbar. It has been found that the superconducting (SC) characteristics of the nanocomposites (critical temperature Tc and critical magnetic field Hc) increase as the Sn filament diameter decreases. The temperature spreading of the resistive SC transition also increases as the Sn filament diameter decreases, which is explained by the SC order parameter fluctuations. The size effects (the increase in critical temperature Tc and transition width ΔTc) in Sn nanofilaments are well described by the independent Aslamazov–Larkin and Langer–Ambegaokara fluctuation theories, which makes it possible to find the dependence of Tc of the diffuse SC transition on the nanowire diameter. Using the temperature and magnetic-field dependences of the magnetic moment M(T, H), it has been found that the superconductor–normal metal phase diagram of the Sn–asbestos nanocomposite has a wider region of the SC state in T and H as compared to the data for bulk Sn. The magnetic properties of chrisotil-asbestos fibers unfilled with Sn have been studied. It has been found that the Curie law is fulfilled and that the superparamagnetism is absent in such samples. The obtained results indicate the absence of magnetically ordered impurities (magnetite) in the chrisotil-asbestos matrix, which allowed one to not consider the problem of the interaction of the magnetic subsystem of the asbestos matrix and the superconducting subsystem of Sn nanowires.

Low-temperature electrical conductivity and the superconductor-insulator transition induced by indium impurity states in (Pb0.5Sn0.5)1 − xInxTe solid solutions

A study is reported of the low-temperature electrophysical (including superconducting) characteristics of the (Pb0.5Sn0.5)1 − xInxTe semiconducting solid solutions with an indium content variable within x = 0.05–0.20. A decrease in the impurity content x in the material has been found to bring about a decrease in the superconducting transition temperature Tc and the onset of an “insulating” state of the material. These effects manifest themselves in an increase in the low-temperature (T = 4.2 K) resistivity of (Pb0.5Sn0.5)0.95In0.05Te by more than three orders of magnitude as compared to that of (Pb0.5Sn0.5)0.8In0.2Te. A decrease in the In content in the solid solution also gives rise to a radical change in the shape of the temperature dependence of the electrical resistivity from a metallic behavior in the material with x = 0.20 (decrease in the electrical resistivity with decreasing temperature in the range 300–4.2 K) to a semiconducting behavior in a sample with x = 0.05 (exponential increase in the resistivity at T < 25 K). This transition to the insulating state with decreasing content of the impurity should be assigned to the displacement of the impurity band of quasi-local indium states toward the top of the light-hole valence band of the material and its emergence into the band gap of the solid solution.

Transverse Nernst-Ettingshausen effect, resonant scattering, and superconductivity in SnTe: In

AbstractThe temperature dependences of the coefficient of the transverse Nernst-Ettingshausen effect in SnTe: In samples with different indium contents (1–16 at %) in the temperature range 100–300 K and the electrical resistivity at temperatures of 1.2–4.2 K and in magnetic fields of up to 10 kOe are investigated. The data obtained indicate the presence of resonant hole scattering into the band of quasi-local In impurity states in Sn1−xInxTe samples with In content x ≥ 0.05 and a superconducting transition with a critical temperature Tc ∼ 1.5–2.2 K. The SnTe: In samples with the degree of filling of impurity states by electrons, which is close to 1/2, and the Fermi level ɛF pinned in the vicinity of the minimum energy dependence of the relaxation time τ(ɛ) are characterized by inhomogeneities of a new type, i.e., inhomogeneities of the scattering parameter r = ϖlnτ/∂lnɛ|