Concerning the Superconducting Gap Symmetry in YBa₂Cu₃O_7–δ, YBa₂Cu₄O₈, and La_2–xSr_xCuO₄ Determined from Muon Spin Rotation in Mixed States of Crystals and Powders

Concerning the Superconducting Gap Symmetry in YBa₂Cu₃O_7–δ, YBa₂Cu₄O₈, and La_2–xSr_xCuO₄ Determined from Muon Spin Rotation in Mixed States of Crystals and Powders, D. R. Harshman and A. T. Fiory [arXiv]

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 YBa₂Cu₃O_7–δ (δ ≈ 0.05), YBa₂Cu₄O₈ and La_2-xSr_xCuO₄ (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 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 underlying superconducting penetration depth.  Random averages derived from oriented crystal data differ markedly from corresponding non-oriented powders, owing to the 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 YBa₂Cu₃O_7–δ and YBa₂Cu₄O₈. For YBa₂Cu₃O_7–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 La_1.83Sr_0.17CuO₄.  Inconsistencies contained in multiple and noded gap interpretations of crystal data, and observed differences between bulk μ⁺ and surface-sensitive measurements are discussed.

Root second moment data for YBa2Cu3O7–δ specimens. The triangles represent the average of the single crystal field-oriented data. The diamonds and circles correspond to the non-oriented powder data sets "A” (H = 0.35 T) and “B” (H = 0.2 T), respectively. The solid and dashed curves through the data represent fits of our pinning model assuming a two-fluid form for the gap function. Inset: Plot of fitted "two-fluid" exponent α versus Tc for oxygen-deficient YBa2Cu3O7–δ. The dashed line denotes the trend.

D. R. Harshman and A. T. Fiory, J. Phys.: Condens. Matter 23, 315702 (2011).