Muon-spin rotation spectra in the mixed phase of high-T-c superconductors:Thermal fluctuations and disorder effects

We study muon-spin rotation (mu SR) spectra in the mixed phase of highly an isotropic layered superconductors, specifically Bi2+xSr2-xCaCu2O8+delta (BS CCO), by modeling the fluid and solid phases of pancake Vortices using liqu id-state and density functional methods. The role of thermal fluctuations i n causing motional narrowing of mu SR line shapes is quantified in terms of a first-principles theory of the flux-lattice melting transition. The effe cts of random point pinning are investigated using a replica treatment of l iquid-state correlations and a replicated density functional theory. Our re sults indicate that motional narrowing in the pure system, although substan tial, cannot account for the remarkably small linewidths obtained experimen tally at relatively high fields and low temperatures. We find that satisfac tory agreement with the mu SR data for BSCCO in this regime can be obtained through the ansatz that this ''phase'' is characterized by frozen short-ra nge positional correlations reflecting the structure of the liquid just abo ve the melting transition. This proposal is consistent with recent suggesti ons of a ''pinned liquid'' or ''glassy'' state of pancake Vortices in the p resence of pinning disorder. Our results for the high-temperature liquid ph ase indicate that measurable linewidths may be obtained in this phase as a consequence of density inhomogeneities induced by the pinning disorder. The results presented here comprise a unified, first-principles theoretical tr eatment of mu SR spectra in highly anisotropic layered superconductors in t erms of a controlled set of approximations. [S0163-1829(99)08033-9].