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.     

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)     

Thermal effects and diamagnetic response of a current-carrying YBCO film

The temperature dependence of the dissipative part of the response to an ac magnetic field has been studied for an YBCO high-temperature superconductor film carrying a transport current. The response is found to exhibit a thermal hysteresis, whose magnitude depends on the transport current. The hysteresis is assigned to the current-carrying superconductor having two stable equilibrium states within a certain thermostat temperature interval. The existence of these states governed by specific features of the release and removal of heat.     

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.     

Vortex system dynamics and energy losses in a current-carrying 2D superconducting wafer

The dynamic processes occurring in the vortex system of a 2D superconducting wafer carrying transport current are investigated using the model of the vortex system of high-temperature superconductors. Calculations are performed by the Monte Carlo method. For the first time, the dynamics of magnetic field penetration in a current-carrying HTSC wafer is demonstrated and the energy losses associated with a change in transport current are calculated. It is shown that changes in the transport current amplitude and in the number of defects lead to a change in the energy liberation mechanism: hysteresis energy losses are replaced by the losses in the saturated layer.     

Resistive state of superconducting structures with fractal clusters of a normal phase

The effect of morphologic factors on magnetic flux trapping and critical currents in a superconducting structure, which presents a type II percolation superconductor with pinning centers, is considered. The role of pinning centers is played by fractal clusters of the normal phase. The properties of these clusters are analyzed in detail: their statistics is studied, the distribution of critical currents of depinning is found, and the depen-dences of the main statistical parameters on the fractal dimension are obtained. The effect of fractal clusters of the normal phase on the electric field caused by the motion of the magnetic flux after the vortices have been broken away from pinning centers is considered. The current-voltage characteristics of superconducting structures in a resistive state are obtained for an arbitrary fractal dimension. It is found that the fractality of the boundaries of normal-phase clusters forces magnetic flux trapping, thereby increasing the critical current.     

High current-carrying capability in c-axis-oriented superconducting MgB2 thin films

In high-quality c-axis-oriented MgB2 thin films, we observed high critical current densities ( J(c)) of approximately 16 MA/cm(2) at 15 K under self-fields comparable to those of cuprate high-temperature superconductors. The extrapolated value of J(c) at 5 K was estimated to be approximately 40 MA/cm(2). For a magnetic field of 5 T, a J(c) of approximately 0.1 MA/cm(2) was detected at 15 K, suggesting that this compound would be a very promising candidate for practical applications at high temperature and lower power consumption. The vortex-glass phase is considered to be a possible explanation for the observed high current-carrying capability.     

On the current distribution in a shielded superconducting film

The current distribution is investigated theoretically in a shielded superconducting film, which is a film deposited on an insulating superconducting ground plate. An exact result is obtained in the case of a thin and wide film on the basis of London's equation.     

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.     

Degradation of the current-carrying capacity of low-temperature superconducting composites under the action of thermal perturbations

The stability of transport current introduced into a niobium titanium superconducting composite subjected to an external pulsed thermal perturbation has been studied. Stable states have been theoretically analyzed by solving Fourier and Maxwell equations that describe the thermoelectrodynamic states of lowtemperature superconductors with flux creep. It has been shown that, if the transport current is permanently introduced, subcritical thermal perturbations, i.e., perturbations that do not take the composite to a normal state provided that the current does not exceed the quench current, may result in the appearance of unstable current states. The higher the energy of the external thermal perturbation, the lower the instability onset current. It has been found that the degradation of the current-carrying capacity of the superconducting composite is due to intense heat release inside the superconductor, which is initiated by the thermal perturbations, and depends on the current input rate, the instant of time the current input is terminated, and cooling conditions.     

Surface induced mixed state of a superconducting film

Presented in this paper is a theoretical analysis of a planar surface induced mixed state for a superconducting film in parallel applied field. An analytical solution of the internal magnetic field is obtained based on Saint-James and de Gennes' order parameter in a film. An expression of Gibbs free energy per unit volume without restriction of a geometry is derived from non-linear Ginzburg-Landau (GL) equation in terms of a renormalized GL parameter and a modified geometric factor. Based on the Gibbs free energy, a phase diagram of distinguishing a first and second order phase transition for a type I superconducting film is calculated. The numerical results for exact solutions of spatial variation of order parameter, current density and internal magnetic field in the film geometry in parallel applied field case are presented. Near the upper critical field, the first entry of an applied field in the film exhibits a laminar structure.     

Spectral cut-off in the efficiency of the resistive state formation caused by absorption of a single-photon in current-carrying superconducting nano-strips

We have studied supercurrent-assisted formation of the resistive state in nano-structured disordered superconducting Nb(N) films after absorption of a single optical to near-infrared photon. The efficiency of the resistive state formation has a pronounced spectral cut-off; corresponding threshold photon energy decreases with the bias current. Analysis of the experimental data in the framework of the refined hot-spot model suggests that the quantum yield for near-infrared photons increases with the photon energy. Relaxation of the resistive state depends on the photon energy making the phenomena feasible for the development of energy resolving single-photon detectors.     

Effective nonlinear susceptibility of a grained superconducting film

The effective nonlinearity of a thin grained superconducting film is calculated theoretically. It is assumed that the film has the form of a continuous superconducting matrix with superconducting grained inclusions of cylindrical structure with a generatrix perpendicular to the film surface. The current and field distributions are determined to calculate the effective nonlinearity; both the matrix and the grains are considered in the framework of the Ginzburg-Landau theory. The spacing between the grains is assumed to be much larger than their diameter so that the Maxwell-Garnet approximation can be used for calculating fields. The analytic formula derived for effective nonlinear susceptibility is analyzed for various values of physical parameters. It is shown that the effective nonlinearity exceeds the nonlinearity of the matrix and grains by a factor of several units when the grain size is much larger than the magnetic field penetration depth in the substance of the matrix and the conductivity of the grains is lower than that of the matrix.     

Nucleation and collapse of the superconducting phase in type-I superconducting films

The phase transition between the intermediate and normal states in type-I superconducting films is investigated using magneto-optical imaging. Magnetic hysteresis with different transition fields for collapse and nucleation of superconducting domains is found. This is accompanied by topological hysteresis characterized by the collapse of circular domains and the appearance of lamellar domains. Magnetic hysteresis is shown to arise from supercooled and superheated states. Domain-shape instability resulting from long-range magnetic interaction accounts well for topological hysteresis.