THE FLUX-LINE-LATTICE IN HIGH-T(C) SUPERCONDUCTORS

The flux-line lattice in type-II superconductors has unusual nonlocal elastic properties which make it soft for short wavelengths of distortion. This softening is particularly pronounced in the highly anisotropic high-T(c) superconductors (HTSC) where it leads to large thermal fluctuations and to thermally activated depinning of the Abrikosov vortices. Numerous transitions are predicted for these layered HTSC when the temperature T, magnetic induction B, or current density J are changed. In particular, the flux lines are now chains of two-dimensional (2D) ''pancake vortices'' which may ''evaporate'' by thermal fluctuations or may depin individually. At sufficiently high T, ohmic resistivity rho(T, B) is observed down to J --> 0. This indicates that the flux lines are in a ''liquid state'' with no shear stiffness and with small depinning energy or that the 2D vortices can move independently. At lower T, rho(T, B, J) is nonlinear since the pinning energy of an elastic vortex lattice or ''vortex glass'' increases with decreasing J as predicted by theories of collective pinning and by ''vortex glass'' scaling.