Threshold perturbations in current-carrying superconducting bridges with a finite length near the critical temperature

Near the critical temperature of a superconducting transition, the energy of the threshold perturbation δF_thr that transfers a superconducting bridge to a resistive state at a current below the critical current I_c has been determined. It has been shown that δF_thr increases with a decrease in the length of a bridge for short bridges with lengths L < ξ (where ξ is the coherence length) and is saturated for long bridges with L ? ξ. At certain geometrical parameters of banks and bridge, the function δF_thr(L) at the current I → 0 has a minimum at L ~ (2–3)ξ. These results indicate that the effect of fluctuations on Josephson junctions made in the form of short superconducting bridges is reduced and that the effect of fluctuations on bridges with lengths ~(2–3)ξ is enhanced.     

Heat evolution due to pulsed thermal perturbations in superconducting composites with a nonuniformly distributed current

Dissipative processes in superconducting composites under the action of current and pulsed thermal perturbations are studied. Results obtained are compared with the predictions following from the existing theory of thermal stabilization of superconducting composites. It is shown that, if the current is nonuniformly distributed over the composite's cross section and the thermal perturbations are short, intense additional energy dissipation may arise both before and after the occurrence of thermomagnetic instability. Associated Joule heat evolution may substantially exceed the thermal loss predicted by the adopted theory of thermal stabilization throughout the temperature range of the composite. Additional dissipation in composite superconductors narrows the range of stable superconductivity, especially when the current is applied continuously, even if external thermal perturbations are weak.     

Structure of high-T c superconducting tape of the second generation with high current-carrying capacity

Commercial high-T _c superconducting tape produced by Superpower Inc. (the U.S.A.) is studied by scanning and transmission electron microscopy. The superconductor structure is shown to consist of a superconducting layer of average composition GdBa_1.5Cu_2.5–3.0O _y coated by a silver layer and buffer layers of LaMnO₃, MgO,Y₂O₃, and Al₂O₃ deposited on a metallic tape of nickel alloy (Ni-Cr-Mo-Fe-W-Co-Mn). The superconducting layer is formed by the superconducting oxide GdBa₂Cu₃O _y containing lamellar inclusions Gd₂O₃.     

Size effect in the problem of thermal stabilization of superconducting composites

The conditions for thermal stabilization of composite superconductors are investigated in the model of a continuous medium with allowance for the transverse thermal conductivity of the components under the assumption that the current is uniformly distributed over the cross section of the current lead. The conditions for complete (steady-state) and partial (unsteady) stability are analyzed. Numerical simulations are carried out for current-carrying elements of circular and rectangular cross section with various transverse dimensions and for various cooling intensities, and the results are compared with the known results of the one-dimensional theory. It is shown that the current of steady-state stabilization in the two-dimensional approximation is always less than the corresponding value calculated in the one-dimensional model. The critical energies of disturbances are found, giving an upper bound on the admissible intensities of heat release distributed uniformly over the cross section of a composite superconductor. For cooled leads a nonmonotonic dependence of the critical energies on the radius of the composite is observed. This function has a minimum which is shifted to larger values of the radius as the heat removal is improved. It is found that in an extremely narrow interval of initial parameters the one-dimensional model gives a slight underestimate of the level of admissible disturbances. On the whole the one-dimensional calculations give higher values than the two-dimensional calculations. The size effect alters the stability conditions to a greater degree when the heat transfer coefficient is increased. The underlying cause of this effect is an increase in temperature at the center of the lead and a simultaneous decrease in temperature at the surface. Zh. Tekh. Fiz. 67, 51–56 (January 1997)     

Phase diagram of a wide current-carrying superconducting film

A model of current heating of a wide superconducting film is suggested assuming nonlinearity of the film conductance. Within this model, the parameters of the film can be characterized by a single dimensionless coefficient of thermal balance which includes both thermal and resistive parameters of the system. The stability of the state of the current-carrying film is analyzed. A phase diagram of the film is constructed in terms of the coefficient of thermal balance and the average current density. The propagation velocity of a stationary nonuniform temperature distribution in the film is calculated as a function of current density at various values of the coefficient of thermal balance.     

Influence of the superconducting state on the thermal instability of low-temperature plastic deformation in crystals

The influence of the superconducting transition on the unstable, discontinuous nature of plastic deformation in crystals at low temperatures (<10 K) is discussed theoretically. It is established, within the mechanism of the thermal instability of low-temperature plastic deformation, that the superconducting state promotes the stabilization of deformation in a superconductor because of the positive sign of the temperature sensitivity coefficient of the flow stresses below the superconducting-transition temperature and the reduced level of energy dissipated by moving dislocations in comparison to the normal metal. The temperature-rate regions for stable and unstable deformation of a superconductor and the dependence of the stress-jump amplitudes on temperature and deforming stress are determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1778–1784 (October 1998)     

Stability of the normal phase in a bounded current-carrying superconducting film

The two-phase equilibrium states of a current-carrying thin superconducting film in the case of convective heat transfer on the free surface are considered, and their stability is investigated in a first approximation. It is shown that of the two equilibrium states, the state with the normalphase region of larger size is stable. In the limiting case of an infinitely long film, the stable two-phase equilibrium state tends to a spatially uniform normal state, and the unstable state remains localized. In a definite range of values of the system parameters, the relaxation time of such a formation can be fairly long, and it should be regarded as a quasistable equilibrium state. Zh. Tekh. Fiz. 68, 71–77 (June 1998)     

“Glassy” low-temperature thermal properties in crystalline superconducting ZrNb alloys

The general characteristics of the specific heat and thermal conductivity of glasses have been found in a quenched single crystal of superconducting ZrNb(20%). From consideration of the diffusionless dynamical fluctuations between the ω and β atomic configurations of the ZrNb alloy, an atomic tunneling picture at low temperatures is proposed. The degree of these “glassy” properties decreases in a quenched ZrNb(8%) crystal and also in the annealed ZrNb(20%) sample. These changes are explained by the formation of large ω domains, which do not favor rapid tunneling motion.     

Degradation of tungsten under the action of a plasma jet

The degradation of the surface and structure of single-crystal tungsten and sintered powder tungsten during the action of a pulsed plasma jet is studied. It is shown that the degradation of a tungsten target during the action of a plasma jet with an energy flux density of 0.25–1 MJ/m² is accompanied by surface evaporation and melting and the fracture of surface layers on scales of 150–250 μm. The results of a numerical simulation of the thermomechanical processes that occur in a tungsten target during the action of a plasma jet are presented. The degradation of tungsten during the action of a plasma jet is shown to proceed almost continuously from the action (evaporation, melting) to the times that are more than three orders of magnitude longer than the action time, which is caused by the thermomechanical processes occurring in the tungsten target. Moreover, the action of thermal stresses leads to structural and morphological changes throughout the sample volume, and these changes are accompanied by recrystallization in adiabatic shear bands.     

Cooling effect of thermal bias on a current-carrying nanodevice

We investigate the heat generation in a quantum dot (QD) coupled to two normal leads with different temperatures. It is found that heat in the QD can be conducted efficiently away through electron–phonon interaction in the QD when the QD is coupled stronger to colder lead than to the hotter one. As temperature of the colder lead is close to zero, the current through the QD peaks at the very QD level position, where the heat generation is zero, which helps to keep the stability of a working nanodevice. Then an ideal condition for nanodevice operation can be found.     

Stability of the superconducting state of a multiconductor current-carrying element during the induction of current in it

A model which permits estimation of the temporal variation of the temperature and the current in the components of a superconducting cable is proposed. The permissible currents which can be induced in a six-strand current-carrying element without destroying its superconducting properties are determined. It is shown that in the presence of inductive coupling between the conductors, the position of the cable component in which instability is initiated is not a regular function of the current induction rate if there is a spread in the nonlinearity parameters of the current-voltage characteristics of the components. Zh. Tekh. Fiz. 68, 39–44 (March 1998)     

Degradation of the second critical magnetic field in multilayered composites with nanodimensional layers of niobium and Nb-Ti superconducting alloy

Multilayered tapes containing layers of superconducting niobium alloys with 30 and 31 wt % Ti and separated by niobium layers were investigated. The layers were ∼140 to ∼10 nm thick. Effective pinning of superconducting vortex filaments was observed on the interlayer Nb-NbTi interfaces. It was established that second critical magnetic field H _c2 decreased as the layer thickness decreased. With thin layers, its magnitude depended on the orientation of rolling the tape with respect to the external magnetic field. Results are explained by the proximity effect.     

Components of the low-temperature heat capacity of rare-earth hexaborides

The temperature dependence of heat capacity C _p(T) was studied for nine rare-earth hexaborides MB₆(M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) at temperatures of 5–300 K. Using the correspondence principle for lattice heat capacities of isostructural compounds, the lattice contribution C ₁(T) and the excess contribution ΔC(T) to the heat capacity of the hexaborides were determined. The lattice heat capacity C ₁(T) is represented as the sum of the Debye contributions of the metal and boron sublattices: C ₁(T)=C _M (T)+6C _B(T). The Debye temperatures π_M and π_B of the metal and boron sublattices were determined. The anomalies in the excess heat capacity ΔC(T)=C _p (T)?C ₁(T) are related to the magnetic ordering effects, the Schottky contribution, and the Jahn-Teller effect.     

Giant low-temperature thermal resistance of ZnSeNi

The 200-fold increase in the thermal resistance of ZnSe at 15 K caused by doping of these crystals with nickel is accounted for by suppression of the thermal phonon contribution to thermal conductivity. As follows from our calculations, in order to reach such high thermal resistances, one has to suppress the contribution from phonons with frequencies lying in an interval not narrower than 100 K.     

Characteristics of current flow in composites made from a high-temperature superconductor and the low-temperature superconducting metal oxide Ba(PbBi)O3

Composites are prepared from a high-temperature superconductor with the 1-2-3 structure and the low-temperature oxide BaPb_3/4Bi_1/4O₃ with various volume concentrations of the high-temperature and low-temperature superconductors. The temperature dependences of the electrical resistivities and critical current densities of the composites are measured. It is shown experimentally that these composites comprise a synthetically created network of Josephson weak links (with a specified degree of coupling). Fiz. Tverd. Tela (St. Petersburg) 39, 418–424 (March 1997) Deceased.     

Mutual influence of voltage steps in the V–I characteristics of current-carrying superconducting tin whiskers

The mutual influence of the voltage steps in the V–I characteristics of tin whiskers near the superconducting/normal transition temperature was investigated by means of several potential probes along one sample. It is shown that the steps generated at one part of a whisker may enhance the critical currents related to the appearance of steps at other parts of the whiskers. Especially the width between the steps may become larger by the presence of other voltage steps.     

Enhanced thermal conductivity in nanofluids under the action of oscillating force fields

The thermal conductivity of nanoparticles colloidal suspensions, submitted to the action of an external force field has been calculated by nonequilibrium molecular dynamics simulations. For driven forces in the radio frequency and microwave ranges, we show that the thermal conductivity of nanofluids can be strongly enhanced without cluster formation.     

Theory of phase-slip processes in thin and dirty current-carrying superconducting wires: Deviations from local equilibrium

Oscillatory phase-slip solutions of a set of integrodifferential equations describing timedependent processes in dirty superconductors in the Ginzburg-Landau regime are found numerically very nearT _c . Deviations from local equilibrium improve the agreement with observedV?I curves.     

Piezoelectric properties of polymer-piezoelectric ceramic composites crystallized under the action of an electric-discharge plasma

The crystallization of a polymer-piezoelectric ceramic composite under the action of electric-discharge temperature and plasma is shown to cause strong oxidation of polymer chains, which increases the concentration of charge localization centers or the number of local levels in the quasi-bandgap of the polymer phase in the composite. In turn, this oxidation is accompanied by an increase in the interphase charges during electrothermal polarization, which favor effective polarization of piezophase domains and, hence, an increase in the piezoelectric properties. The degree of oxidation of the polymer phase is found to be controlled by the nature of the electric discharges used for composite crystallization. It is experimentally shown that piezoelectric composites should be crystallized under the action of a microdischarge that does not reach the streamer phase of its development. The optimum crystallization conditions are determined.