MAGNETIC PENETRATION DEPTH MEASUREMENTS ON HIGH-TEMPERATURE SUPERCONDUCTING THIN-FILMS AND THEIR IMPLICATIONS

Accurate measurements of the magnetic penetration depth lambda of n-doped (Nd1.85Ce0.15CuO4-delta) and p-doped (YBa2Cu3O7-delta) high-temperature superconductors using a microscopic resonator are reported. The temperature dependence lambda(T) in Nd1.85Ce0.15CuO4-delta films shows that the electrodynamics of this cuprate is well described in the framework of the BCS theory, with a single-valued finite-gap ratio 2 Delta(0)/K-B T-c approximate to 3.1 throughout the entire temperature range below T-c. In contrast, measurements of lambda(T) in YBa2Cu3O7-delta cannot be fitted to a BCS temperature dependence derived from a single energy gap. In the low-temperature region, a quadratic behaviour of the penetration depth is observed, instead of the usual exponential law. These peculiar features are discussed in the light of recent theoretical models predicting a different symmetry state for the high-T-c superconductors.