Effect of superconductive destruction in YBa2Cu3 δ bulk bridges under the action of strong Joule self-heating

Transition from superconductive to normal state and back into superconductive state in bulk single crystal YBaCuO bridges with twins induced by a strong transport current has been investigated. The current-voltage characteristics (CVC), the temperature and magnetic field dependences of resistance [resistive transition (RT)] in the regime of fixed current nearT _c were studied in detail. A sharp dynamic superconductive transition or switching over effect, nonlinear CVC, and hysteresis were observed. The CVC and RT characteristics can be explained by the thermal bistability phenomena at strong Joule self-heating of bridges, in particular, in the presence of internal local inhomogeneities in superconductor. Electrical characteristics of bridges which can be used as nonlinear elements for cryogenic electronics were obtained.     

Magnetic field dependence of transition current of NbTi mechanical PCS

This paper deals with magnetic field dependence of transition current of NbTi mechanical persistent current switch (PCS), which realizes zero contact resistance and consists of two contact pieces made of NbTi bulk, in parallel or perpendicular magnetic field. The transition current is defined as the quench current with which the contact resistance abruptly generates. It is found that the transition current decreases with the field strength. Using a theoretical model to explain the formation mechanism of a superconducting connection at the contact of the mechanical PCS, it is confirmed that the transition current varies according to J_C–B characteristics of the contact piece material NbTi. These findings prove that NbTi mechanical PCS has superconductive connection between the contact pieces in the closed state and can be operated in persistent current mode as a superconducting PCS.     

A melt process to superconductive fibres of Bi2Sr2CaCu2 oxide

A particulate precursor of superconductive Bi₂Sr₂CaCu₂ oxide dispersed in an aqueous poly(vinyl alcohol) solution, was shaped into fibres. The fibres fired at a T _max of 850 °C were very porous. Although they exhibited a Meissner on-set transition at 85 K, electrical resistance at 0.02 Acm^–2 at 77 K was still not zero. Exposing the porous fibres to a temperature slightly above their melting point densified them. The treated fibres had a sharp Meissner transition and could carry critical current (J _c) as high as 331 A cm^–2 at 77 K. They were also more resistant to the effect of a magnetic field than the fibre fired at 850 °C. Eighty percent of the treated fibre was found to be the superconductive 2 2 1 2 phase.     

Magnetic field-induced superconductivity in the ferromagnetic state of HoMo6S8

The Chevrel-phase HoMo₆S₈ is one of the few known ferromagnetic reentrant superconductors, having an upper superconductive critical temperatureT _c1 =1.78 K, a Curie temperatureT _m =0.75 K, and reentering into a normal ferromagnetic state belowT _c2 =0.64 K. After a brief survey of the characteristic properties of HoMo₆S₈, we present new results obtained by simultaneous resistance and magnetization measurements on a single crystal of HoMo₆S₈, performed in the whole temperature range 35mKT4.2 K and for several orientations of the easy magnetization axis with respect to the applied magnetic field. Magnetic field-induced superconductive transitions are observed well belowT _c2 in the ferromagnetic state. Those transitions can be understood as the first experimental observation of a purely electromagnetic effect earlier suggested by Ginzburg: if the internal field is antiparallel to the spontaneous magnetization, the magnetic induction may be reduced below its critical value and a superconductive transition may take place. It is possible if exchange interactions are negligible; our results concerning the anisotropy of the internal upper critical fieldH _c2 are consistent with that requirement. We also discuss the possible origins for the low-resistance state in zero field: it might be due to domain wall superconductivity.     

RF susceptibility of magnetron sputtered YBa[]Cu[]O[]−x films

We measured the complex susceptibility of two different quality magnetron sputtered YBa₂Cu₃O_7−x films in RF range. Temperature and static magnetic field dependence of complex magnetic susceptibility is a useful indicator of appearance of phase coherence in the film including fluctuation region above T_c. The characteristics showed a low frequency type behaviour at loss peak and diamagnetic drop at near transition temperature and normal state non-zero susceptibility. One of our samples displayed an enhanced diamagnetism in the normal state close to T_c, which was probably attributed to the combination of the normal skin effect and excess conductivity due to superconductive fluctuations. The low temperature susceptibility was also frequency dependent and qualitatively agreed with the vortex-glass scaling model.     

Iron-Based Layered Compounds: The Effect of Negative Interband Coupling

The experimental data relative to new superconductor LaFeAsO_1−x F _x can be analyzed in the framework of the two-band Eliashberg theory. With reasonable assumptions on input parameters of the theory such as electron–phonons and electron-antiferromagnetic spin density fluctuations spectral functions and the Coulomb pseudopotential, it is possible to calculate mean physical quantities such as critical temperature, superconductive gaps, superconductive differential conductance, and so on. In this model also the effect of disorder and magnetic impurities on the critical temperature is examined. The key ingredient is the negative interband coupling that can remarkably increase the critical temperature. This effect stems from the presence of an electron–phonon coupling constant smaller than the sum of the Coulomb pseudopotential and electron-antiferromagnetic spin density fluctuations coupling constant in the interband channel. The resulting superconductive state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets.      

Galvanomagnetic and thermomagnetic phenomena in the mixed state of high-temperature superconductors

The thermomagnetic phenomena in the mixed state of high-T _c superconductors are investigated in terms of the phenomenological three-fluid (superconductive and normal quasiparticles, and Abrikosov vortices) model. General relations are found between the various transport coefficients which are valid for the entire transition superconductive statemixed stated normal state.     

Analysis of the Current Noise Produced in Stationary Conditions in MgB2 Films at Different Stages of the Superconducting Transition

In previous papers (Mazzetti et al. in Phys. Rev. B 77:064516, 2008; Ponta et al. in Phys. Rev. B 79:134513, 2009) the mechanism of resistive transition of MgB₂ films has been investigated by the analysis of the non-stationary noise produced during the transition process. The developed model suggested that when the transition occurs very near to T _C at low bias current, a mixed state, consisting of normal and superconductive domains, takes place. A different model based on fluxoids dynamics, generated by the current and/or by an external field, is appropriate when the transition occurs at strong bias currents and temperatures much lower than T _C. The purpose of this paper is a further investigation of the transition process at low bias currents by analyzing the 1/f power spectra of the noise, taken in stationary conditions at different values of the specimen resistance. These spectra, when renormalized to represent the relative fluctuation of the specimen resistance, are practically coincident, a fact in agreement with the developed model.     

On the influence of magnetic field processing on the texture,phase assemblage and properties of low aspect ratio Bi2Sr2CaCu2Ox/AgMg wire

Abstract

Bi₂ Sr₂ CaCu₂ O_x/AgMg conductors are potentially important for many applications up to 20 K, including magnets for cryogen-free magnetic resonance imaging and high field nuclear magnetic resonance research. One promising approach to increased critical current density is partial-melt processing in the presence of a magnetic field which has been shown to enhance c-axis texturing of wide, thin tape conductors. Here, we report on low aspect ratio rectangular conductors processed in an 8 T magnetic field. The magnetic field is applied during different stages of the heat treatment process. The conductors are electrically characterized using four-point critical current measurements as a function of magnetic field and magnetic field orientation relative to the conductor. The superconductive transition and magnetization hysteresis are measured using a SQUID magnetometer. The microstructures are characterized using scanning electron microscopy and energy dispersive spectroscopy and analyzed using digital image processing. It is found that the presence of a magnetic field during split melt processing enhances the electrical transport and magnetic behavior, but that the anisotropy is not consistently affected. The magnetic field also affects development of interfilamentary Bi2212 bridges, and that this depends on the initial shape of the Bi2212 filament. At least two behaviors are identified; one impacts the oxide phase assemblage and the other impacts textured growth.     

Automatic and continuous system for measurement of superconductive transition temperatures using a dynamic technique

An automatic and continuous system for measuring superconductive transition temperatures, T_c of samples using a dynamic technique is described. It includes a dynamic cryostat temperature measurement circuit and three T_c measurement methods: resistance, frequency and bridge lock-in techniques. Using this system, samples of various forms and types may be rapidly assessed and many superconductive transition curves can be plotted automatically and simultaneously over the temperature range 5–40 K or above. Superconductive phases present in samples can be analysed. T_c measurement precision is better than ± 0.1 K.