Magnetic penetration depth measurements on high-temperature superconducting thin films and their implications

Accurate measurements of the magnetic penetration depth λ of n-doped (Nd1.85Ce0.15CuO4−δ) and p-doped (YBa2Cu3O7−δ) high-temperature superconductors using a microscopic resonator are reported. The temperature dependence λ(T) in Nd1.85Ce0.15CuO4−δ 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 2Δ0/K B T c≈3.1 throughout the entire temperature range below T c. In contrast, measurements of λ(T) in YBa2Cu3O7−δ 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.