K.X. Chen

STRUCTURE MODIFICATION OF BiSrCaCuO SUPERCONDUCTORS DUE TO Pb-SUBSTITUTION

Pb-substitution to BiSrCaCuO superconductors has modified the crystallographic structure, in addition to the enhancement of the superconducting properties.

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.

COMPLEX SUSCEPTIBILITY AND CRITICAL CURRENT OF HIGH-TEMPERATURE SUPERCONDUCTORS

The ac complex susceptibility χ′−iχ″ of several bulk samples with very high critical current density Jc have been measured. The results show that these samples are all polycrystalline bulk material, the peak of χ″ still related to losses within “the weak link structure” of the intergrain material. But the intergrain coupling strength of the high Jc samples is much stronger than ordinary high-Tc superconductors. A parameter β=(Tc2(0) −Tc2(h0))/h02/3 has been proposed to characterize the coupling strength.

Thermal diffusion of vortices and self-organized criticality

Abstract A logarithmic barrier, U(J) ∝ ln ( Jc J ) , is often observed in transport and magnetic relaxation experiments. We show that this kind of current dependency is a natural result of a unified materials equation for type-II superconductors. With finite barrier and T → 0, this equation coincides with the nonlinear diffusion equation describing one-dimensional sandpile with self-organized criticality.

Nonlinear voltage response due to thermal noise of Josephson junction

Abstract The effects of thermal fluctuations on the Josephson effect in a junction with finite capacitance are studied in the framework of two variables Fokker-Planck equation of Brownian motion. A general analytic stationary solution is obtained. Comparison with experimental data of YBCO junctions as well as BSCCO single crystals is shown with satisfactory agreement.

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 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.

Kq, Scaling of ac susceptibility and the nonlinear response function of high-temperature superconductors

The amplitude-dependent ac susceptibility of high-temperature superconductors is shown to obey some empirical scaling relations. We try to analyze this behavior by extending a dc nonlinear response function of mixed state to the ac cases. The derived equations for critical current and ac susceptibility ξ(T) agree with the scaling relations of experimental data.

Flux-charge analogy and nonlinear response function in HTSC and 2D charge systems

Abstract The physics of flux lines in HTSC pinned by strongly correlated defects can be mapped onto charges in two dimensions (2D). Considering the viscous dissipation of moving vortices during hopping, we find a nonlinear response function. This function is compatible with so far suggested different model barriers U ( J ) and able to make a consistent description of the vortex system near transition. A comparison with the scaling behavior of the measured isothermal current-voltage curves with YBCO samples shows fair agreement. This nonlinear response function also shows an empirical fit with I–V behavior of some 2D charge systems.

A nonlinear response function for the scaling behavior of vortex matter near the transition temperature

Abstract We show that the electromagnetic response of high-temperature superconductors with inhomogeneties or defects as pinning centers can be well described by a universal nonlinear response function. This function is compatible with the different model barriers U ( J ), suggested so far. It is able to give a consistent description of the vortex system near the transition temperature. A comparison with the scaling behavior of the experimental measured isothermal current–voltage curves with YBCO samples shows reasonable agreement.