Disorder-driven intermediate state in the lattice melting transition of Bi2Sr2CaCu2O8+delta single crystals

Scanning Hall probe microscopy has been used to study high quality single crystals of the Bi2Sr2CaCu2O8+delta high-temperature superconductor over a wide range of temperatures at low fields. We observe pronounced stripes of high flux density which persist well above the melting line almost up to the critical temperature of the crystal. We show that these regions impose a disorder-driven intermediate state on the vortex lattice melting transition with a structure composed of alternating solid and liquid strips. We propose that the striping may reflect regions of slightly different oxygen doping level formed due to enhanced diffusion at the edges of atomic terraces during crystal growth. We are unable to resolve discrete vortices in the stripes above the vortex lattice melting line. We speculate either that the vortex liquid efficiently screens any pinned flux present or that spatial variations in the Gibbs free energy may be imposing a density modulation on the vortex liquid at these temperatures.