Flux dynamics of high-temperature superconductors

Flux pinning and motion in YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ crystals are studied by ac magnetic, SQUID and resistive methods. The ac response of YBCO is found to be very anisotropic for field orientations. For H//c, in typical fields, the ac response can be linear for vortex displacement up to about 50 nm. For H//ab, the ac response is nonlinear for small vortex displacement ($\u3c$0.5 nm). The large reversible vortex displacements for H//c suggest that the individual oxygen vacancies cannot be responsible for pinning. The extremely small irreversible vortex displacement for H//ab suggests that there is a very narrow-sized barrier for vortices moving across the layered structure. The linear-response ac susceptibility can be described by the formula which was used for normal-metal with a classical skin-depth. The nonlinear response can persist up to T$\sb{\rm c}$(H) (determined by the onset of dc diamagnetism) at high frequency and high amplitude, suggesting that the concept of vortex lines is relevant and pinning effect is present in high temperature regime where linear resistivity is measurable and magnetic hysteresis vanished. We identified the presence of peak-effect in YBCO crystals by an anomalous dip in temperature dependence of real part of the nonlinear ac susceptibility. We found that the peak-effect is more pronounced for H//ab than for H//c, and disappears for field below 1 kOe. The details of the peak-effect can be understood, if the peak-effect is due to the softening of the vortex-lattice. We conclude that the nonlocal effect of the tilt modulus C$\sb{44}$ cannot be responsible. Flux creep in YBCO is studied by a SQUID method. We found that the flux-creep characteristic is not logarithmic in time. A method of analyzing flux-creep data is given to distinguish various models. It is found that the time dependence can best fit data is a power-law M $\sim$ (1 + t/$\tau)\sp{-\alpha}$ or a stretched exponential M $\sim$ exp($-$(t/$\tau)\sp\beta)$. The relevance of flux creep to thermal activation and avalanche processes is discussed.