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

Using a very sensitive microstrip-resonator technique, we have measured (T) of c-axis YBa2Cu3O7- films on various substrates and compared the results to similar data on Nb and NbCN films. For YBa2Cu3O7-, (T) is found to be exponential and not power law in its temperature dependence (for T<Tc/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 2(0)/kBTc 2.5 for the minimum gap in Yba2Cu3O7-, yet show strong-coupling-like behavior at high temperatures. We discuss our results in the context of other experiments on the cuprate superconductors.