Kinetics of the destruction of superconductivity by a current in lead wires

An experimental investigation of the kinetics of the destruction of superconductivity by a current in cylindrical lead wires is presented. The results show that if the current is about 10% larger than the critical value J_c calculated from Silsbee's hypothesis, the transition is essentially governed by electromagnetic damping and is well described by the isothermal theory of Rothen and Bestgen. If the current lies between J_c and 1.1J_c, the observed transition times are much longer than those predicted by the isothermal theory. The influence of thermal effects is discussed in detail and a calculation of the transition times close to J_c is given which leads to an improved agreement with experiment. The calculations are also compared with previous experimental results on tin specimens.     

Kinetics of the destruction of superconductivity by a current less than Silsbee's critical value

An experimental study of the destruction of superconductivity in cylindrical tin wires by currents less than Silsbee's critical value is presented. The experiments show that in this case the average motion of the boundary between the superconducting and nonsuperconducting regions takes place longitudinally at a constant velocity. The conditions that must be fulfilled in order to observe this type of propagation are discussed, and calculations of the time required to start the transition and of the velocity of the boundary are given.     

Change in magnetic induction lines during the current-induced destruction of superconductivity

Recent results of experimental investigations show that during the current-induced destruction of superconductivity in cylindrical samples a non-azimuthal component of the magnetic induction arises. This ‘autoparamagnetic effect’ is observable both in type I and type II superconductors.Assuming a helical form for the magnetic flux filaments the angle between the magnetic induction lines and the plane perpendicular to the Pb + In alloy sample axis is estimated in several cases. A conceptual explanation of the energy losses in the resistive state is presented.     

The nitrogen gas tension test. Part 2: failure mechanism

In this study, the mechanism of concrete failure in the nitrogen gas tension test was investigated through a series of experiments. First, the nitrogen gas tension test was carried out two types of specimens: solid cylinders and hollow cylinders. The test results clearly showed that there was no significant difference in the gas pressure at failure between the solid specimen and the hollow specimen. Since a tension crack occurring on the surface of the concrete specimen at a gas pressure almost equal to the tensile strength of the concrete might play a key role in understanding the failure mechanism, a failure criterion based on linear elastic fracture mechanics (LEFM) was consequently developed. The nitrogen gas tension test was newly carried out on cylindrical specimens with circumferential notches of various depths. Though LEFM was found to be useful in developing an understanding of the mechanism of concrete failure, the experimental results indicated that it was not really valid for specimens with notch depths deeper than some critical size (critical notch depth). However, based on the experimental observation that the concrete specimen failed at its tensile strength at notch depths smaller than the critical notch depth, a modified LEFM based failure mechanism was proposed taking into account the notch sensitivity of the concrete.     

Analysis of the current-induced breakdown of superconductivity

Recently, the breakdown of superconductivity has been investigated in thin-film samples with extreme temporal resolution. As a result, the voltage drop across the flux-flow path shows a temporal pattern of striking structure caused by migrating domains of magnetic flux. This reveals the dynamic character of the so-called current-induced resistive state. This voltage structure is analyzed on the basis of the extended Gibbs free-energy-barrier model, and quantitative agreement is achieved for the single-domain and the multidomain cases as well. In the single-domain case, the influence of fluctuations of the transport current on the time-resolved voltage is demonstrated. In the multidomain case, where more than one domain exists within the sample, a transient oscillation of the number of flux quanta per domain is revealed occurring immediately after the onset of flux flow. This size oscillation dies out after a small number of cycles and approaches the stationary value observed in previous experiments. Thus, a successful explanation of the temporal aspects of the breakdown of superconductivity is established, which holds from its very onset to the stationary regime of the current-induced resistive state.     

Low-frequency impedance of the direct current induced intermediate state in superconductors: I. The in-phase component

We have measured the in-phase component of the low-frequency ac impedance of the dc current-induced intermediate state in indium-lead alloy wires. The specimens were mounted in a specific configuration that allowed a proper calculation of their dynamic resistances following the theory developed by Gauthier and Rochon, which is based on London's theory of the intermediate state. The theoretical values are considerably larger than our measurements, particularly near the critical current. We have modified the Gauthier-Rochon theory, to a first approximation, to take account of the Baird-Mukherjee model of the intermediate state and we find reasonably good agreement between the modified theoretical values and our observations.This work has been supported by the Chief of Research and Development, Department of National Defence, Canada through his contribution No. 3610-320.     

Destruction of superconductivity in indium and thallium wires by a current

Several theoretical models have been put forward for the destruction of type-I superconductivity in a wire by the passage of a large enough current through it. In an attempt to discriminate between the various models, an experimental investigation has been carried out of the transition in one thallium wire and in a series of indium wires of different diameters and purity. The return of resistance in pure, thick thallium and indium wires is found to agree well with the predictions of the Baird-Mukherjee model. In thin wires the resistance increases with decreasing wire diameter, and the present experimental results are in reasonable agreement with the predictions made by Baird and Mukherjee who, following Kuper, have attributed the increase in resistance to the reflection of electrons at superconducting-normal interfaces.     

Absolute calibration in grazing incidence interferometry via rotational averaging

Interferometry in grazing incidence can be used to test cylindrical mantle surfaces. The absolute accuracy of the resulting surface profiles is limited by systematic wavefront aberrations caused in the interferometer, in particular due to an inversion of the test wavefront in an interferometer using diffractive beam splitters. For cylindrical specimens, a calibration method using four positions has therefore been investigated. This test is combined with another method of optical metrology: the rotational averaging procedure. The implementation for grazing incidence is described and measurement results for hollow cylinders are presented. The gain in accuracy is demonstrated.     

Continuous change from weak to strong coupling behavior in quasi-one-dimensional superconductors

The current-induced breakdown of superconductivity in whiskers from the In-Pb system has been investigated. For indium-rich alloys the characteristics show the well-known voltage step structure due to the appearance of phase-slip centers. From the linear portion following the first voltage step in the voltage-current characteristics at fixed temperatures, the quasiparticle diffusion length and the time-averaged supercurrent above the critical current have been calculated. In the intermetallic phase the interpretation becomes more involved. For pure lead there is no separation of the linear portions by voltage steps. A comparison with theoretical work is given.     

Study of phonon-injection-induced destruction of superconductivity in tin films

We have investigated the phenomenon of the destruction of superconductivity in thin film strips of tin by injecting direct currents through a transverse normal metal (silver) strip in direct contact with the superconducting strip. We observe that the critical current of the superconducting tin strip decreases continuously (in some cases with an intermediate jump discontinuity) with increasing injection current. From a careful consideration of various possible mechanisms, such as current-induced depairing, simple heating, and nonequilibrium superconductivity, we infer that the experimental observations could be mostly due to steady-state nonequilibrium phenomena associated with heat pumping from various locally heated regions.Research carried out at the Tata Institute of Fundamental Research.     

Kinetics of the current-induced phase transition in type I superconducting wires

We have measured the time required for the destruction of type I superconductivity in high purity indium wires by a current greater than the critical current. Our results are not in agreement with theoretical treatments presented by Pippard and by Rothen and Bestgen and we suggest possible inadequacies in the theoretical models.     

Current induced resistive state in cylindrical superconducting films. II. Two-dimensional mixed state of type-1 superconductors

We present an experimental study of the current induced destruction of superconductivity in films of type-1 superconductors. The most interesting aspect of these experiments is the possibility to observe and investigate different mechanisms of the superconductivity destruction depending on the ratio of the sample thickness d to the coherence length (T).If d/(T) < 4.5 we observe, above a critical current value, a resistive state throughout the section of the film, but in the case of d/(T) > 4.5 the destruction of superconductivity starts, as in bulk samples, with the formation of a normal shell along the outer surface of the cylindrical sample together with a layer of the two-dimensional mixed state at the inner surface. Investigations on films allowed for the first time to measure the thickness of the two-dimensional mixed state.On leave from Kapitza Institute for Physical Problems, 117334 Moscow, Russia.     

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.     

Vibration of infinite piezoelectric cylinders with anisotropic properties using cylindrical finite elements

Natural frequencies for free vibration of infinite piezoelectric cylinders are computed using finite elements that are formulated in cylindrical coordinates. The finite-element method is used to model the cross-section of the cylinder in r, theta coordinates using circular sectors. Material constants that are functions of theta are allowed to vary in each circular sector and are computed using standard tensor transformations. The accuracy of the finite-element formulation is verified using previous results for isotropic cylinders and axisymmetric piezoelectric cylinders. New results are tabulated for frequencies of free vibration of solid and hollow piezoelectric cylinders of LiNbO(3) of crystal class 3m. Displacements for typical mode shapes are illustrated graphically.     

A finite element analysis of the time-delay periodic ring arrays for guided wave generation and reception in hollow cylinders

A new guided wave transducer model, time-delay periodic ring arrays (TDPRAs), is proposed and investigated in this paper for guided cylindrical wave generation and reception in hollow cylinders with application interests focusing on non-destructive testing (NDT) of piping/tubing. A finite element simulation has been performed for axisymmetric guided-mode excitation and reception with TDPRAs. By arranging a proper configuration of the time-delay profile and the electric-connection pattern of a ring array, unidirectional excitation and reception of guided waves can be achieved. The numerical results are obtained for the first three axisymmetrical modes and are compared with respect to generation efficiency and mode selectivity. Parametric influences on the performance of TDPRAs are discussed, combining a 2-D phase velocity-frequency spectrum approach with the mode dispersion and displacement structure analyses. The identification of converted modes in guided cylindrical wave reflections with a flexible TDPRA receiver has also been studied through sample notch reflection.     

Dynamic resistance of the current-induced intermediate state in superconductors

We have measured the in-phase component of the low-frequency ac impedance of the dc current-induced intermediate state in wires of pure and impure type I superconductors. We find that this dynamic resistance of the intermediate state is greater than the normal state resistance of the wire. The linear response theory for the dynamic resistance developed by Gauthier and Rochon provides a reasonable qualitative understanding of our experimental results.This work has been supported by the Chief of Research and Development, Department of National Defence, Canada through his contribution No. 3610-320.     

On the london model of the intermediate state in current-carrying superconductors

The London picture for the destruction of superconductivity by a current is considered. Equipotential S regions are taken as being placed periodically along the wire, and the magnetic field at the phase boundary is taken to have the critical value. From a solution of the Laplace equation for the electric potential near the contact point of two S regions a relationship between the angular aperture of the conical S region and the period of the structure is found. A numerical solution is given for the phase boundary and for the resistance at large currents. The volume of the S phase is found to be much smaller than that in the original London model. As a consequence the resistance of the wire after destruction of the uniform S state differs from that for the uniform N state by only a few percent.     

Kinetics of the current-induced phase transition in type-I superconductors

An experimental investigation of the kinetics of the destruction of superconductivity by rectangular current pulses was made on cylindrical Sn and In wires of different purities and diameters. The study was based on the observation of the variation of the voltage along the sample during the transition. As shown by Rothen and Bestgen, only the timet ₁ between the beginning of the transition and the onset of the intermediate state can be determined with this method. The results show that if the currentI is sufficiently larger than the critical currentI _c (typicallyI1.15I _c ), then both shape of the voltage pulses and the transition times agree well with the predictions of the electromagnetic theory of these authors. The transition is, in that case, controlled by the electromagnetic damping associated with the penetration into the normal material of the magnetic field due to the current. IfI is close to _c , then the transition is much slower than predicted by the theory and seems to be essentially governed by thermal effects, i.e., absorption of latent heat, generation of Joule heat, and heat exchange with the helium bath; a small influence of superheating is observed in indium samples.This work was supported by the Fonds National Suisse de la Recherche Scientifique.     

Specific Heat of CeRhIn5: Pressure-Driven Transition from Antiferromagnetism to Heavy-Fermion Superconductivity

CeRhIn₅ is known to show an unusual transition at a critical pressure of ～15 kbar. Specific-heat data show a gradual change in the zero-field “magnetic” specific-heat anomaly from one typical of antiferromagnetic ordering at ambient pressure to one more characteristic of a Kondo singlet ground state at 21 kbar. However, at 15 kbar there is a discontinuous change from an antiferromagnetic ground state to a superconducting ground state, and evidence of a weak thermodynamic first-order transition. Above the critical pressure, the low-energy excitations are characteristic of superconductivity with line nodes in the energy gap, and, at intermediate pressures, of extended gaplessness.     

Probing the Upper Critical Field of κ-(BEDT-TTF)2Cu(NCS)2

Studies of heat conduction and microwave absorption in the vortex state of -(BEDT-TTF)₂Cu(NCS)₂ reveal some of the specific aspects of superconductivity in this system. Moreover they allow us to probe the destruction of bulk superconductivity by a magnetic field. The two set of results yield identical magnitudes for the upper critical field with a temperature dependence distinctly different from the one suggested by the onset of resistive transition.