Elastic energy, thermal fluctuation and depinning of the vortex lattice in high-Tcsuperconductors

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Abstract
Anisotropy and large penetration depth lambda soften the flux-line lattice in high-Tc superconductors and strongly enhance its thermal fluctuations and pinning-caused distortions. The small coherence length xi means weak pinning energies (but not necessarily weak pinning forces). This facilitates thermally activated depinning of flux lines. At low current densities, thermal depinning means flux diffusivity whose exponential temperature dependence explains various experiments in a natural way without assuming a vortex-lattice melting transition. Linear response expressions are given for the complex, AC penetration depth, surface impedance, AC resistivity and AC susceptibility of films and cylinders which consider elastic pinning, viscous drag, and creep of vortices.