SMALL-ANGLE NEUTRON-DIFFRACTION STUDIES OF VORTEX STRUCTURES IN HIGH-TEMPERATURE SUPERCONDUCTORS

We have used neutron scattering to provide direct information about flux structures in the bulk of crystals of the superconductor Bi2Sr2CaCu2O8. Its extremely high effective mass anisotropy, makes the flux lattice susceptable to melting and also to decomposition into 'pancake' vortices, which would give a more two-dimensional vortex structure. At low temperatures and fields the scattered intensity is consistent with a three dimensional flux-line structure. At higher fields and temperatures, the scattering from the flux lattice dissapears well below T-c. We can associate this dissappearance with the above changes in the vortex structure. We compare the neutron scattering results with macroscopic measurements of magnetisation.