D. L. Yin

Low-frequency AC susceptibility due to the interplay of loss mechanisms in HTS

The analysis of low-frequency ac susceptibility is complicated because the hysteretic losses due to pinning and the eddy-current losses are not simply additive. We show that the electromagnetic response of both ideal and nonideal type-II superconductors can be well described with a common nonlinear response function. This function makes a consistent description of the crossover from the power-law E(J) of flux-creep to the ohmic flux-flow regime. We interpret the scaling behavior of ac susceptibility with this nonlinear equation.

Observation of post-deposition resistance relaxation during growth of semicontinuous metal films

Abstract Semicontinuous niobium and silver films were made in an ultra-high-vacuum (UHV) chamber and in-situ d.c. resistance measurements were performed. After interrupting the deposition, we investigated the immediate ageing phenomenon (relaxation) of the sample resistance on a time scale of about 10 min. Resistance increase and decrease were observed for niobium and silver samples, respectively. The intensity of the relaxation is sensitive to substrate temperature and film thickness. We suggest that edge diffusion and coalescence of islands due to thermomigration of the metal atoms are responsible for the resistance relaxation.

A possible explanation of the magnetic relaxation in high-Tc superconductors using the extended model of thermally activated depinning

Abstract Experimental data on flux creep in a variety of high Tc superconductors show a temperature dependent plateau in the magnetization decay S ≡ −d ln M(t) d ln (t) , which appears at odds with conventional flux-creep theories. Based on an extended model of thermally activated depinning, we derived a new formulation for the magnetic decay rate S which can successfully explain this plateau.

Nonlinear electrodynamic response of superconducting materials near transition

One of the important problems of applied superconductivity is to understand, well describe and control the electromagnetic response due to flux motion. We show that the transport properties of type-II superconductors can be well described by a nonlinear response function. In connection with the superconducting- normal state (S-N) resistive transition we find further a model R(T, H) equation which fairly agrees with the experimental data of MgB/sub 2/ as well as the untwined high critical temperature YBCO crystals. Instead of the constant R in the Ohms law of normal metals, combining with Maxwell equations, this nonlinear resistance function over wide range of temperature, field and current provides a useful basis for engineering design and analysis.

Nonlinear current-voltage characteristics of HTS conductor and its application to the operation of SFCL

A proper description of the nonlinear voltage-current relation of the superconductor is necessary for accurate analysis of the superconducting fault current limiters (SFCL). Discrepancies between the experimental results and the prediction of Beans critical state model were observed. Using a recently found unified nonlinear response function of type-II superconductors, the authors derive the current-voltage characteristic of metal-sheathed HTS conductor and compare it with experiments. Its relation to the operation of SFCL is also discussed.

Nonlinear I/spl sim/V characteristic and magnetic relaxation in high-T/sub c/ superconductors

Large-scale applications of high-temperature superconductors require large critical current density J/sub c/. The limit of utilization is determined by the practical resistivity criterion and the current voltage characteristic of a superconductor which is highly nonlinear in these materials. Various models with different barrier-current dependencies U(J) have been developed to describe the flux motion in them but questions still remain. We show that these models may be unified to a general materials equation which gives a consistent description of different regimes of flux motion. We compare this equation with recent pertinent experiments and find fair agreements.

Surface interaction and resistance relaxation of thin metal films on mica and fullerene substrates

Abstract In situ DC resistance measurements were performed on semicontinuous niobium and silver films, which were made on mica and fullerene substrates in an ultra-high-vacuum(UHV) chamber. Right after the interruption of the deposition, we investigated the changes(relaxation) of the sample resistance on a time scale of about 10 minutes. Resistance increase was observed for Nb/mica and Ag C 60 systems, and decrease for Ag/mica system. The relaxation is sensitive to substrate temperature and film thickness. We suggest that the edge diffusion and mergence of islands due to thermomigration of the metal atoms are responsible for the resistance relaxation. The intensity and direction of the relaxation reflect the interfacial activity of the metal/substrate system. The heterogeneities on substrate may also play an important role during this process.

Extended power law of nonlinear transport properties of superconducting materials

Publisher Summary This chapter explores the extended power law of nonlinear transport properties of superconducting materials. The nonlinear transport properties of superconductors near tile transition are usually described by the so called power law E/E c = (J/J c ) n . A wide-range resistive transition equation with the form of an “extended power law” is presented. This equation fits the experimental data of MgB 2 and high T c cuprates. For designing superconducting magnets, fault current limiters, cables and many other devices thorough knowledge of the electromagnetic response near the critical state is necessary. In principle this should be determined by tile Maxwell equations combined with a proper materials equation J(E,T,B). At present, power law E(J) characteristics of the form E = E c [J/J co (T, B)] n are often used. In this study it is showed that the E(J) isothermal characteristics for wider range including the crossover to Ohmic-like regimes have the general form of extended power law which well fits experimental data.

Hall effect in the mixed state of superconducting MgB2

Abstract On the basis of a theoretical model for the Hall effect within the framework of BCS superconducting mixed state and considering our longitudinal nonlinear response equation, we derived an analytical expression for the mixed state Hall resistivity of MgB2. This equation enables one to compare quantitatively the experimental data with the theoretical model. We also propose some possible scaling relations for the temperature and field dependencies of the Hall resistivity of MgB2.

Scaling behavior of anomalous Hall effect and longitudinal nonlinear response in high-Tc superconductors

Some interesting empirical scaling relations of anomalous Hall resistivity ρ x y which tend to be sample, field, and temperature independent are found. Based on existing theoretical models and considering the expression of longitudinal nonlinear response function, we derive a nonlinear equation in which the mixed state Hall resistivity can be expressed as an analytical function of magnetic field, temperature, and applied current. This equation enables one to compare quantitatively the experimental data with the theoretical model. The comparison between our theoretical curves and experimental data shows a fair agreement.