μSR study of charge neutrally doped Nd 1−2 xCa xTh xBa 2Cu 3O 7− δ

Abstract

Equal amounts of Ca and Th were doped into the Nd 1−2 x Ca x Th x Ba 2Cu 3O 7− δ superconductor with 0≤ x≤0.10. X-ray diffraction showed a single orthorhombic phase for all samples except at x=0.10, suggesting a solubility limit below x=0.10. For increasing doping concentration x, the superconducting T c and the electrical resistivity, ρ at 275 K both vary strongly, with a linear decrease of T c and an increase of ρ. The muon spin relaxation rate, σ, follows the Uemura line for large x, and deviates in the direction of increasing σ-values for decreasing x. The approach of the data to the Uemura line was compared between different alloy systems by plotting the normalized distance to the Uemura line as a function of doping concentration. Ca–Th doped samples were found to approach the Uemura line about three times faster than for Ca–Pr doping. The same replacements in Y-123 confirm this result. The strong doping concentration dependence of σ in Ca–Th substituted samples is explained in terms of electronic disorder effects.