1202.0049 (Dale R. Harshman et al.)

Concerning the superconducting gap symmetry in YBa2Cu3O7-δ,
YBa2Cu4O8, and La2-xSrxCuO4 determined from muon spin rotation in mixed
states of crystals and powders    [PDF]

Dale R. Harshman, Anthony T. Fiory

Muon spin rotation ({\mu}+SR) measurements of square-root second moments of
local magnetic fields {\sigma} in superconducting mixed states, as published
for oriented crystals and powder samples of YBa2Cu3O7-{\delta} ({\delta}
{\approx} 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 {\sigma} 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
{\sigma}, irreproducible data and strong H dependence of apparent transition
temperatures are also evident. Strong intrinsic pinning suppresses thermal
anomalies in c-axis components of {\sigma}, which reflect nodeless gap
symmetries in YBa2Cu3O7-{\delta} and YBa2Cu4O8. For YBa2Cu3O7-{\delta}, 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 {\mu}+SR and surface-sensitive measurements are discussed.

View original: http://arxiv.org/abs/1202.0049