Evidence for flux-lattice melting and a dimensional crossover in single-crystal Bi2.15sr1.85cacu2o8+Delta from muon spin rotation studies

The microscopic magnetic field distribution in the mixed state of single-crystal Bi 2.15 Sr 1.85 CaCu 2 O 8+δ has been measured using the muon spin rotation technique. A study of the line shapes as a function of temperature and magnetic field gives evidence for an abrupt change of flux-lattice structure which is attributed to the presence of a flux-lattice melting line in the B-T phase diagram. In addition, the data suggest the existence of a crossover field above which the flux structure is more 2D in nature