SMALL-ANGLE NEUTRON-SCATTERING STUDY OF THE FLUX-LINE-LATTICE IN A SINGLE-CRYSTAL OF BI2.15SR1.95CACU2O8+X

A flux-line lattice (FLL) was observed in a single crystal of Bi2.15Sr 1.95CaCU2O8+x (BSCCO) using small-angle neutron scattering methods. Th e sample has a superconducting transition at 85 K. The flux-line latti ce is observed to melt, evidenced by the rapid disappearance of diffra cted intensity as the temperature is increased above a field-dependent melting temperature. Diffracted intensity due to the vortex lattice a lso falls off as the applied field is increased. It is believed that t his is a manifestation of the transition of the three-dimensional flux lines into two-dimensional pancake vortices. The Bragg intensity of t he FLL peak is inversely proportional to the fourth power of the Londo n penetration depth (lambda L) Hence, the temperature (T) dependence o f the order parameter can be measured quite accurately from the intens ity of the Bragg spots at different temperatures. In BSCCO with an app lied field of 50 mT the measured T dependence appears linear. The low- T behavior is of great interest for an understanding of the underlying mechanism for superconductivity in these materials.