Flux pinning and creep in very anistropic high temperature superconductors

Abstract

The effect of thermal fluctuations on collective flux-pinning and creeps is investigated for thin-film superconductors and layered superconductors with weak Josephson coupling between the layers in a field normal to the layers and much less than Bc2. Temperature and field dependences of the critical current jc in a two-dimensional (2D) system are obtained. The activation barriers for 2D flux-creep are shown to grow infinitely as U(j)∝j-μ at j⪡jc, which is characteristics for the vortex-glass state. At very small currents this behaviour is cut off by the plastic motion of edge-dislocation pairs which are either induced by disorder or thermally created, leading to linear current-voltage behaviour inhibiting the existence of a vortex-glass state in 2D systems. The Josephson coupling in layered superconductors changes the dimensionality of the vortex lattice. It is shown that a sufficiently large field when the lattice constant ao becomes less than the characteristic length of the interlayer coupling a0<r3D=(Rja0)12 or B >B2D=Ф0/R2J, (RJ=Г12sis the effective Josephson length, s is the interlayer spacing and Г=mz/m is the mass anisotropy), the fluctuations of the vortex lines become of 2D nature. This means in particular that 3D-lattice melting will take place at T=Tm, Tm is the dislocation-mediated melting temperature of a 2D vortex-lattice. The mixed state at B >B2D is studied and the possibility of different regimes of pinning and creep is demonstrated. The crossover from 2D to 3D pinning is found when the pinning length Rc exceeds r3D. The crossover conditions are derived and displayed in a schematic phase diagram.