Evidence for a two-stage melting transition of the vortex matter in Bi2Sr2Ca1Cu2O8+delta single crystals obtained by muon spin rotation

From muon spin rotation measurements on under- to overdoped Bi-2212 crystal s we obtain evidence for a two-stage transition of the vortex matter as a f unction of temperature. The first transition is well known and related to t he irreversibility line (IL). The second one is located below the IL and ha s not been previously observed. It occurs for all three sets of crystals an d is unrelated to the vortex mobility. Our data is consistent with a two-st age melting scenario where the intraplanar melting of the vortex lattice an d the interplanar decoupling of the vortex lines occur independently. [S003 1-9007(99)09359-X].