Penetration depth variation in high quality YBaCuO thin films

We report results obtained on state-of-the-art YBCO thin films prepared by pulsed laser deposition on LaAlO3 substrates, using samples as similar as p ossible in two different experimental set-ups: a surface impedance measurem ent on 4000 Angstrom thick films using a parallel plate resonator (10 GHz), and a far infrared transmission (100-400 GHz) measurement which requires t hinner (1000 Angstrom) samples. The measurements show a reduction of the pe netration depth slope d lambda/dT with improving quality of the thin films, which exhibit a low temperature scattering rate and residual surface resis tance, comparable to single crystals. A linear fitting to the lambda(T) exp erimental results yields d lambda/dT=2 Angstrom/K in both experiments in th e low frequency limit.