Measurements of the temperature dependence of the magnetic penetration depth in YBa2Cu3O7−δ superconducting thin films

Abstract

Using a very sensitive microstrip-resonator technique, we have measured λ(T) of c-axis ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films on various substrates and compared the results to similar data on Nb and NbCN films. For ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, λ(T) is found to be exponential and not power law in its temperature dependence (for T<${\mathit{T}}_{\mathit{c}}$/2) in the best films. Overall the data cannot be described by a single-gap BCS temperature dependence. The data at low temperature provide a lower limit of 2Δ(0)/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$≊2.5 for the minimum gap in ${\mathrm{Yba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, yet show strong-coupling-like behavior at high temperatures. We discuss our results in the context of other experiments on the cuprate superconductors.