V. I. Sokolenko

Critical current, pinning, and the resistive state of superconducting single-crystal niobium with various types of defect structure

The critical current, pinning, and resistive state of single-crystal niobium of texture orientation are studied in various structural states obtained by rolling by 42% at 20 K and subsequent polishing of the surface layers. It is found that the heterogeneous structures typical of the strained sample have a lower current-carrying capacity even after thinning to ∼10% on account of the enhancement of the thermomagnetic instability in the parts with fragmented structure in the subsurface layers. For the case of a homogeneous defect structure of the mid-region of a sample with a uniformly distributed dislocation density of 1.3×1011 cm−2 in the resistive state a correlation between the component of the normal current and the critical current density is found, in accordance with the concepts of flux creep due to the scatter of the local values of Jc.

Change of the superconducting, transport, and microscopic properties of transition metals upon introduction of interstitial impurities and deformation-induced defects

For group-V transition metals (Nb, Ta) containing different concentrations of interstitial impurities (O, C, N, H) and deformation-induced defects, a numerical calculation of various effective microscopic characteristics averaged over the Fermi surface and of the band parameters in the framework of the Friedel model is carried out using the experimentally determined values of the superconducting transition temperature Tc and the temperature dependence of the resistivity in the interval Tc<T≲300 K. The causes of the polar character discerned in the influence of interstitial impurities and dislocations on the investigated physical characteristics are discussed.

Resistivity and microscopic characteristics of platinum in different structural states

The experimental temperature dependence of the resistivity of platinum in different structural states is processed using the two-band Mott–Wilson model of a transition metal. It is found that impurities, deformation defects, and quenching defects have essentially different influences on the Debye temperature, the intensity of electron–electron Coulomb scattering, and the intensity of intra- and interband electron–phonon scattering. A number of effective microscopic characteristics and band parameters are calculated in the Friedel model. The mechanisms by which the different structural factors influence the investigated complex of physical characteristics of platinum are analyzed.

Pinning and critical currents of heterogeneous superconductors in different structural states

The critical currents of a composite Nb3Sn-based superconductor are found to increase as a result of low intensity ultrasonic interactions. In terms of a mechanism for interactions of vortices with grain boundaries based on electron scattering at the boundaries, a reduction in the compressive forces in an Nb3Sn layer during ultrasonic interactions leads to an increase in the elementary pinning force because of a reduction in the coherence length and increases in the superconducting transition temperature and the electron specific heat. It is shown that for single-crystal niobium with a high density of uniformly distributed dislocations in fields close to Hc2, the field dependence of the volume pinning force corresponds to a system of effective point centers that satisfy a rarefaction criterion. The first order interaction of a vortex with these kinds of pinning centers greatly exceeds the characteristic for interactions of the vortex with single helical and edge dislocations.

Effect of Magnetic-Lattice Transformations on Low-Temperature Plasticity and Fracture of Chromium Single Crystals

Analysis of the literature indicates an interconnection of the lattice and magnetic subsystems of chromium, which suggests the further study of changes in low-temperature plasticity and fracture of chromium resulting from magneto-lattice transformations during thermal cycling in the range of 373↔77 K. For the initial microalloyed Eu single crystals of Cr (the content of impurities Ci ≈ 10–3 wt.%) with orientations [110] and [112] along the compression axes, a monotonous growth of the yield point was observed in the temperature range from room temperature to T = Tx ≈ 170 K (Tx is the temperature of cold-brittleness or the temperature of ductile-brittle transition). When the temperature was lowered in the range of Tx–4.2 K, the samples became brittle, with a decrease in the fracture stress, which is due to the growth of local stresses at the concentrators. As a result of thermal cycling accompanied by multiple phase transformations from the paramagnetic to the antiferromagnetic AF1 and AF2 states and vice versa, a decrease in the Tx value of chromium was observed of ≈ 20 K. The effect is related to the occurrence of relaxation processes near stress concentrators due to the displacement of the walls of domain structures and also cyclic striction deformations that compensate for the volume change caused by overheating and supercooling of the sample due to a rapid change in temperature.Analysis of the literature indicates an interconnection of the lattice and magnetic subsystems of chromium, which suggests the further study of changes in low-temperature plasticity and fracture of chromium resulting from magneto-lattice transformations during thermal cycling in the range of 373↔77 K. For the initial microalloyed Eu single crystals of Cr (the content of impurities Ci ≈ 10–3 wt.%) with orientations [110] and [112] along the compression axes, a monotonous growth of the yield point was observed in the temperature range from room temperature to T = Tx ≈ 170 K (Tx is the temperature of cold-brittleness or the temperature of ductile-brittle transition). When the temperature was lowered in the range of Tx–4.2 K, the samples became brittle, with a decrease in the fracture stress, which is due to the growth of local stresses at the concentrators. As a result of thermal cycling accompanied by multiple phase transformations from the paramagnetic to the antiferromagnetic AF1 and AF2 states and vice v...

Effect of coherent twin boundaries on the electron-phonon interaction in single crystals of tin

Electron-phonon interaction near the twinning plane in a single crystal of tin in normal state is studied experimentally using the method of the point contact spectroscopy. Considerable softening of the electron-phonon interaction function is observed. The order of magnitude of the increase of superconducting transtion temperature near the twinning plane is estimated.