Concerning the superconducting gap symmetry in YBa2Cu3O7-δ, YBa2Cu4O8, and La2-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 fields σ in superconducting mixed states, as published for oriented crystals and powder samples of YBa2Cu3O7-δ (δ ≈ 0.05), YBa2Cu4O8 and La2-xSrxCuO4 (x ~ 0.15-0.17), are subjected to comparative analysis for superconducting gap symmetry. For oriented crystals it is shown that anomalous dependences of σ on temperature T and applied field H, as-measured and extracted a- and b-axial components, are attributable to fluxon depinning and disorder that obscure the intrinsic character of the superconducting penetration depth. Random averages derived from oriented-crystal data differ markedly from corresponding non-oriented powders, owing to weaker influence of pinning in high-quality crystals. Related indicators for pinning perturbations such as non-monotonic H dependence of σ, irreproducible data and strong H dependence of apparent transition temperatures are also evident. Strong intrinsic pinning suppresses thermal anomalies in c-axis components of σ, which reflect nodeless gap symmetries in YBa2Cu3O7-δ and YBa2Cu4O8. For YBa2Cu3O7-δ, the crystal (a-b components, corrected for depinning) and powder data all reflect a nodeless gap (however, a-b symmetries remain unresolved for crystalline YBa2Cu4O8 and La1.83Sr0.17CuO4). Inconsistencies contained in multiple and noded gap interpretations of crystal data, and observed differences between bulk μ+SR and surface-sensitive measurements are discussed.
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