Thermal electrodynamic mechanisms of a rise of current-voltage characteristic (IVC) of technical superconductors at magnetic flux creep

The processes of the formation of the macroscopic states of a superconducting tape induced by the transport current at magnetic flux creep have been studied. It has been shown that there are characteristic values of electric field intensity that are affected by a rate of current injection, properties of a superconductor, cooling conditions, and properties of a stabilizing matrix. These values are the basis of thermal electrodynamic mechanism, which determines the slope of a rise of IVC of technical superconductors. The conditions of formation of current instabilities have been studied taking into account a nonuniform temperature distribution over the cross section of a technical superconductor. The conditions of the existence of the IVC of technical superconductors have been formulated. These conditions allow for the stable heating of a superconductor as high as the critical temperature. The results of the carried out studies should be taken into account when measuring the IVC of superconducting materials and determining their critical parameters and the current of instability occurrence.     

Thermal stabilization of the resistive states of superconducting composites: Quasilinear approximation

The conditions of the occurrence and development of thermal instabilities in the composite superconductor with a continuously increasing current-voltage characteristic, which is described by the power equation, have been studied. The conditions for thermal stabilization have been analyzed in the general form using dimensionless variables that keep their invariance when varying. For the local temperature disturbance, the critical energies and velocities of its irreversible propagation have been calculated. It has been proved that composites superconductors can have stable states, when the ultimate currents can be higher or lower of the conventionally preset critical current of the composite. Furthermore, superconductivity destruction at supercritical currents takes place not in the form of a stepwise transition from the superconducting to normal state, but due to the formation of thermal and electric switching waves that propagate along the composite superconductor with a constant speed. The condition for full thermal stabilization has been formulated for the superconducting composites with a power current–voltage characteristic. The results of the numerical experiments have proved that the existing theory of thermal stabilization, which assumes a stepwise superconducting–normal transition, leads to the considerable limitation of the range of the stable currents, at which a superconducting state can be kept.     

Limiting stable states of high-Tc superconductors in the alternating current modes

The limiting current-carrying capacity of high-T _c superconductor and superconducting tape has been studied in the alternating current states. The features that are responsible for their stable formation have been investigated under the conduction-cooled conditions when the operating peak values of the electric field and the current may essentially exceed the corresponding critical values of superconductor. Besides, it has been proved that these peak values are higher than the values of the electric field and the current, which lead to the thermal runaway phenomenon when the current instability onset occurs in the operating modes with direct current. As a result, the stable extremely high heat generation exists in these operating states, which can be called as overloaded states. The limiting stable peak values of charged currents and stability conditions have been determined taking into account the flux creep states of superconductors. The analysis performed has revealed that there exist characteristic times defining the corresponding time windows in the stable development of overloaded states of the alternating current. In order to explain their existence, the basic thermo-electrodynamics mechanisms have been formulated, which have allowed to explain the high stable values of the temperature and the induced electric field before the onset of alternating current instability. In general, it has been shown that the high-T _c superconductors may stably operate in the overloaded alternating current states even under the not intensive cooling conditions at a very high level of heat generation, which is not considered in the existing theory of losses.     

Maximum allowable currents in YBa2Cu3O7 superconducting tapes as a function of the coating thickness,external magnetic field induction,and cooling conditions

Maximum allowable (ultimate) currents stably passing through an YBa₂Cu₃O₇ superconducting current-carrying element are determined as a function of a silver (or copper) coating thickness, external magnetic field induction, and cooling conditions. It is found that if a magnetic system based on yttrium ceramics is cooled by a cryogenic coolant, currents causing instabilities (instability onset currents) are almost independent of the coating thickness. If, however, liquid helium is used as a cooling agent, the ultimate current monotonically grows with the thickness of the stabilizing copper coating. It is shown that depending on cooling conditions, the stable values of the current and electric field strength preceding the occurrence of instability may be both higher and lower than the a priori chosen critical parameters of the superconductor. These features should be taken into account in selecting the stable value of the operating current of YBa₂Cu₃O₇ superconducting windings.     

Thermal mechanisms responsible for the irreversible degradation of superconductivity in commercial superconductors

Conditions for the irreversible propagation of thermal instabilities in commercial superconductors subjected to intense and soft cooling have been formulated. An analysis has been conducted using two types of the superconductor’s I–V characteristics, i.e., an ideal I–V characteristic, which assumes a step superconducting-to-normal transition, and a continuous I–V characteristic, which is described by a power law. The propagation rate of thermal instabilities along the superconducting composite has been determined. Calculations have been made for both subcritical and supercritical values of the current. It has been shown that they propagate along a commercial superconductor in the form of a switching wave. In rapidly cooled commercial superconductors, the steady-state rate of thermal instability propagation in the longitudinal direction can only be positive because there is no region of steady stabilization. It has been proved that, in the case of thermal instability irreversible propagation, the rise in the commercial superconductor temperature is similar to diffusion processes that occur in explosive chain reactions.     

Resistive states of the composite superconductors at magnetic flux creep

The effect of magnetic flux creep on the formation of resistive states of the composite superconductors has been studied taking into account their self-heating. The obtained results have been compared with the calculations carried out using the existing thermal stabilization theory, which is based on the model of a stepwise transition from the superconducting to normal state. It has been shown that, over a wide range of the superconductor temperature, this model leads to overrated effective electric resistances of the composite. As a result of its stable self-heating, the notions on the critical current, which determine the maximum transport current and on the resistive transition temperature, the higher of which in the transport current begins dividing between the superconductor and matrix, a loss a physical sense at magnetic flux creep, are used in the existing thermal stabilization theory. As a result, the limits of the theory of thermal stabilization of the composite superconductors can be extended if the theory has been used to describe stable sates at currents, which are higher than the conditionally defined critical current of the composite.     

Electrodynamic stabilization conditions for high-temperature superconducting composites with different types of current-voltage characteristic nonlinearity

The current instability is studied in high-temperature superconducting current-carrying elements with I–V characteristics described by power or exponential equations. Stability analysis of the macroscopic states is carried out in terms of a stationary zero-dimensional model. In linear temperature approximation criteria are derived that allow one to find the maximum allowable values of the induced current, induced electric field intensity, and overheating of the superconductor. A condition is formulated for the complete thermal stabilization of the superconducting composite with regard to the nonlinearity of its I–V characteristic. It is shown that both subcritical and supercritical stable states may arise. In the latter case, the current and electric field intensity are higher than the preset critical parameters of the superconductor. Conditions for these states depending on the properties of the matrix, superconductor’s critical current, fill factor, and nonlinearity of the I–V characteristic are discussed. The obtained results considerably augment the class of allowable states for high-temperature superconductors: it is demonstrated that there exist stable resistive conditions from which superconductors cannot pass to the normal state even if the parameters of these conditions are supercritical.     

Current instability mechanisms in liquid-coolant-cooled high-temperature superconductors

The stable current distribution in Bi₂Sr₂CaCu₂O₈, Bi₂Sr₂Ca₂Cu₃O₈, and YBa₂Cu₃O₇ high-temperature superconductors depending on conditions of cooling thereof by liquid cryocoolants—helium, hydrogen, and nitrogen, respectively—is studied. It is shown that the current instability mechanism may change in going from one coolant to another. Consequently, stable states may be disturbed, first, when the conditions of cooling the superconductor surface change from nucleate boiling to film boiling. Such a thermal mechanism of stable current state disturbance is observed largely when a superconductor is cooled by liquid helium. Second, even for the nucleate boiling of a liquid coolant, current instability may result from the stable formation of the voltage-current characteristic of the superconductor. This type of injected current stability disturbance is most likely when a superconductor is cooled by liquid nitrogen. Criteria for determining a current instability mechanism in relation to the properties of the superconductor and coolant are given.     

Theoretical analysis of coherent electron transport in structures containing multiband superconductors with different types of superconducting pairing

Coherent electron transport in structures with multiband superconductors described by models of intraorbital (the s _± model) and interorbital superconducting pairing has been theoretically considered. Conductivities of junctions of a single-band normal metal with superconducting pnictides for these pairing models have been calculated. Temperature and phase dependences of the Josephson current through junctions containing a conventional Bardeen-Cooper-Schrieffer superconductor and a superconducting pnictide have been calculated within the considered pairing models taking into account temperature dependences of superconducting order parameters.     

Voltage–current characteristics and current instability conditions of a high-temperature superconductor in non-uniform temperature distribution

The influence of non-uniform temperature distribution in the cross section of a high-temperature superconductor (thermal size effect) on its voltage–current characteristic and the instability conditions of charged current is investigated. The boundary values of the electric field and the current above which the charged current is unstable are defined taking into account the size effect. It is shown that the calculated current of the instability determining the maximum allowable value of the charged current is reduced, if the thermal heterogeneity of the electrodynamics states is taken into consideration in the theoretical analysis of the stability conditions. As a result, the limiting stable values of the electric field and current depend nonlinearly on the thickness of the superconductor, its critical properties as well as on the external cooling conditions. Therefore, the current of instability will not increase proportionally to the increase in the thickness of superconductor or its critical current density at the intensive cooling conditions.     

Macroscopic states induced in superconducting media by a transport current under flux creep

The physical features of the formation of macroscopic states of superconducting composites consisting of a superconductor and a coating under flux creep are discussed. It is demonstrated that there exist characteristic electric field strengths depending on the properties of the superconductor, cooling conditions, and characteristics of the stabilizing coating, which affect the intensity of the E-I characteristics of the superconducting composites. Analysis shows that the measurements of the critical properties of superconductors can be accompanied by a nonuniform electric field distribution over the composite cross section and high stable superheating of the superconductor, which do not lead to superconductivity breaking.     

Andreev states and shot noise in bicrystal junctions of cuprate superconductors

Experimentally observed features of the electrical and noise characteristics of bicrystal junctions of cuprate superconductors, such as linearity of the critical current density versus square root of the junction transparency and increase in the spectral density of shot noise for small bias voltages (below the superconducting gap), indicate that the superconducting current in cuprate bicrystal junctions is determined by the passage of quasi-particles through a potential barrier at the superconductor boundaries. This process involves bound states appearing as a result of multiple Andreev reflections in superconductors with dominant wavefunction components of the d _x ² ? _y ² symmetry type. At the same time, interpretation of the experimental current-phase and current-magnetic field curves requires that the character of faceting at the bicrystal junctions would be also taken into account.     

A.C. loss and related effects in type II superconductors

This review gathers together information important to the understanding of superconducting materials under a.c. conditions, and points out some features which are at present not too well understood. The idea is to try to build a consistent picture of the performance of superconductors rather than to present a historical account of the subject.

The basic behaviour of fluxons in the bulk of type II superconductors can be explained by the critical state model, but some of the details of flux pinning and the method of flux motion are in doubt. Flux instabilities under d.c. conditions have received a lot of attention and so have the methods of stabilization, but instabilities under a.c. conditions have not been studied in so much detail, and the problem of stabilization is more difficult than for f.c.

Generally surface effects are less well understood than bulk effects, partly because there are a number of possible phenomena to contend with. A.C. loss in the Meissner state can be explained by field enhancement and flux penetration at peaks in the surface. For fields between H _c3 and H _c2 present solutions of the Ginzburg-Landau equations for the currents in the surface sheath do not give sufficient agreement with experiment and some alternative explanation, such as flux pinning, may be necessary. There are a number of effects between H _c2 and H _c1, but the most important are Meissner currents and flux pinning.     

Kinetics of an irreversible transition to a normal state by a multiconductor superconducting composite

A model that takes into account the discrete character of a multiconductor superconducting medium is used to study the characteristic features of transition processes which occur when a region of normal conductivity appears. This analysis is based on numerical simulations of the propagation of a normal zone inside a superconducting region consisting of rectilinear superconductors, with a model for the kinetics of the irreversible loss of the superconducting properties of a single-ply, large radius winding. The thermophysical aspects of the irreversible propagation of a thermal perturbation within the transverse cross section of a multiconductor superconducting medium are analyzed. The major features of the phenomena taking place during the loss of superconductivity by a discrete region are formulated as functions of the conditions for heat transfer between its elements, the character of the perturbation, the magnitude of the flowing current, and the conditions for stabilization. The influence of longitudinal heat conduction on the magnitude of the transverse propagation velocity of a normal zone is examined. Zh. Tekh. Fiz. 69, 40–51 (May 1999)     

Spin Hall magnetoresistance in Nb/Y3Fe5O12 hybrids

We investigate the spin Hall magnetoresistance (SMR) in niobium (Nb) attached to Y₃Fe₅O₁₂ near the superconducting critical temperature (T_c) of Nb. The SMR vanishes after cooling the sample below T_c, and recovers if the temperature is raised. When a magnetic field larger than the critical field of Nb is applied, the SMR re‐emerges with an enhanced magnitude even if the temperature is below T_c. The experimental results demonstrate that the SMR could be completely suppressed by the coupling between superconducting condensation and spin–orbit interaction in superconductors. In addition to the fundamental physics on the charge–spin interactions in superconductors, our work adds a different dimension to superconducting spintronics. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

一个可能的P波型重费密子超导体——UBe13

在朗道的费密液体理论框架下,我们解释了重费密子超导体UBe₁₃的超导临界温度随压力变化的实验结果。所得结论是:PBe₁₃可能是一个P波型的重费密子超导体。同时也发现由于加压使得系统的f能级与费密能级的相对移动和f电子与导带电子杂化的增加对S波超导有利,而对P波超导不利。 关键词：     

<Emphasis Type="Italic">I–V</Emphasis> characteristics and current instability in a high-temperature superconductor with a nonuniform temperature distribution over the cross section

The effect of a nonuniform temperature distribution in the cross section of a high-temperature superconductor on the formation of its I–V characteristic and the conditions for conservation of a stable distribution of the input current are analyzed. The equations permitting us to determine the boundary values of the electric field and current above which current instability can be developed with allowance for the dimensional effect are derived. It is shown that the input current quench determining the maximal permissible value of the input current decreases if thermal inhomogeneity of electrodynamic states is taken into account in theoretical analysis of its stability conditions. It is found that the limiting admissible values of the electric field and current are nonlinear functions of the thermal resistance of the superconductor, its critical parameters, and external heat-exchange conditions. Therefore, intense cooling of the superconductor does not result in an increase in the current corresponding to the emergence of instability in proportion to the increase in its cross-sectional area or critical current density.     

Electronic phase transitions in the spinless Falicov-Kimball model

We have studied thedV/dI vs.V characteristics of point contacts between the heavy fermion superconductor URu₂Si₂ and the conventional superconductors Zn and NbTi. Contacts between URu₂Si₂ and Zn do not show Josephson effects; instead Andreev reflection type structures occur, which are related to both, the heavy fermion and the conventional superconductor. In contrast, contacts between URu₂Si₂ and NbTi become superconducting at low currents. A closed-loop setup with two NbTi contacts on URu₂Si₂ showed SQUID oscillations in a small magnetic field. Our data give evidence that the contacts should be described as superconductor/normal/superconductor junctions with a thick normal layer on the URu₂Si₂ side of the contact with proximity induced superconductivity in theN-layer in the case of NbTi. With such a model the occurrence or absence of superconductivity can be explained and also the suppression of Andreev-scattering which is frequently observed for contacts between heavy fermion superconductors and normal metals.     

The ratio of normal state to superconducting state of spin lattice relaxation rate of unconventional superconductors

In this research, we derive a simple expression for the ratio of normal state superconducting state of spin lattice relaxation rate of unconventional superconductors from the BCS weak-coupling equation. The unconventional superconductors we consider have three types of order parameters as d-wave, ³He A-phase and p-wave three-dimensional order parameter that had been done before by Parker and Haas [D. Parker, S. Haas, Physical Review B 75 (2007) 052501]. After using some numerical approximations and some boundary conditions, we can find the ratio of normal state to superconducting state of spin lattice relaxation rate in power series of temperature dependent order parameters and temperature. Our numerical calculations show the coherence peak below critical temperature clearly that are consistent with Parker and Haas [D. Parker, S. Haas, Physical Review B 75 (2007) 052501]. These results do not agree with the believed that the coherence peak is the only property of s-wave superconductor. However from our calculation, we can conclude that the unconventional superconductors can show the coherence peaks.     

Influence of the current redistribution in the matrix of a composite superconductor on propagation of the normal zone

Influence of the current redistribution in the stabilizing matrix of a composite superconductor with nonuniform distribution of the superconducting component over its cross section on the normal zone propagation is calculated. Analysis of the problem in dimensionless variables allows one to find parameters determining the import of the effect considered. Parametric relationships for the normal zone propagation velocity are obtained under cooling conditions inadequate for steady state stabilization.