Muon-Spin Rotation Spectra in the Mixed Phase of High-Tc Superconductors : Thermal Fluctuations and Disorder Effects

We study muon-spin rotation (μSR) spectra in the mixed phase of highly anisotropic layered superconductors, specifically Bi2+xSr2−xCaCu2O8+δ (BSCCO), by modeling the fluid and solid phases of pancake vortices using liquid-state and density functional methods. The role of thermal fluctuations in causing motional narrowing of μSR lineshapes is quantified in terms of a first-principles theory of the flux-lattice melting transition. The effects of random point pinning are investigated using a replica treatment of liquid state correlations and a replicated density functional theory. Our results indicate that motional narrowing in the pure system, although substantial, cannot account for the remarkably small linewidths obtained experimentally at relatively high fields and low temperatures. We find that satisfactory agreement with the μSR data for BSCCO in this regime can be obtained through the ansatz that this “phase” is characterized by frozen short-range