Influence of structural disorder on low-temperature behavior of penetration depth in electron-doped high-TC thin films

To probe the influence of structural disorder on low-temperature behavior of magnetic penetration depth, λ(T), in electron-doped high-TC superconductors, a comparative study of high-quality Pr1.85Ce0.15CuO4 (PCCO) and Sm1.85Ce0.15CuO4 (SCCO) thin films is presented. The λ(T) profiles are extracted from conductance-voltage data using a highly-sensitive home-made mutual-inductance technique. The obtained results confirm a d-wave pairing mechanism in both samples (with nodal gap parameter Δ0/kBTC = 2.0 and 2.1 for PCCO and SCCO films, respectively), substantially modified by impurity scattering (which is more noticeable in less homogeneous SCCO films) at the lowest temperatures. More precisely, Δλ(T) = λ(T) − λ(0) is found to follow the Goldenfeld–Hirschfeld interpolation formulae Δλ(T)/λ(0) = AT2/(T + T0) with T0=ln(2)kBΓ1/2Δ01/2 being the crossover temperature which demarcates pure and impure scattering processes (T0/TC = 0.13 and 0.26 for PCCO and SCCO films, respectively). The value of the extracted impurity scattering rate Γ correlates with the quality of our samples and is found to be much higher in less homogeneous films with lower TC.