Determination of the magnetic field penetration depth in superconducting yttrium barium copper oxide: Deviations from the Bardeen-Cooper-Schrieffer laws

Abstract

We have measured the low-field magnetization for a number of fine powders of ${\mathrm{Y}}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and applied a standard formula to determine the magnetic field penetration depth $\ensuremath{\lambda}(T)$ in the superconducting state. Consistent values of $\ensuremath{\lambda}(0)$ and similar temperature dependences $\ensuremath{\lambda}(T)$ are obtained for average particle diameters ranging from 1.1 to 9 \ensuremath{\mu}m. At low temperatures $\ensuremath{\lambda}(T)$ is not Bardeen-Cooper-Schrieffer-like, instead a ${T}^{2}$ law is observed. The implications of this result are discussed.