Concerning the superconducting gap symmetry inYBa2Cu3O7 − δ,YBa2Cu4O8, andLa2 − xSrxCuO4 determined from muon spin rotation in mixed states of crystals and powders

Abstract

Muon spin rotation (μ+SR) measurements of square-root second moments of local magnetic fieldsσ in superconducting mixed states, as published for oriented crystals and powder samples ofYBa2Cu3O7 − δ (δ≈0.05),YBa2Cu4O8 andLa2 − xSrxCuO4 (x ∼ 0.15–0.17), are subjected to comparative analysis for superconducting gapsymmetry. For oriented crystals it is shown that anomalous dependences ofσ on temperatureT and applied fieldH, as-measuredand extracted a- and b-axial components, are attributable to fluxon depinning and disorder that obscurethe intrinsic character of the superconducting penetration depth. Randomaverages derived from oriented crystal data differ markedly from correspondingnon-oriented powders, owing to the weaker influence of pinning in high-qualitycrystals. Related indicators for pinning perturbations, such as non-monotonicH dependence ofσ, irreproducibledata and strong H dependence of apparent transition temperatures, are alsoevident. Strong intrinsic pinning suppresses thermal anomalies inc-axis components ofσ, which reflect nodelessgap symmetries in YBa2Cu3O7 − δ and YBa2Cu4O8. ForYBa2Cu3O7 − δ, thecrystal (a–b components, corrected for depinning) and powder data all reflect a nodeless gap (however,a–b symmetries remain unresolved for crystallineYBa2Cu4O8 andLa1.83Sr0.17CuO4).Inconsistencies contained in multiple and noded gap interpretations of crystal data, and observed differencesbetween bulk μ+SR and surface-sensitive measurements are discussed.