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 possible in two different experimental set-ups: a surface impedance measurement on 4000 A thick films using a parallel plate resonator (10 GHz), and a far infrared transmission (100-400 GHz) measurement which requires thinner (1000 A) samples. The measurements show a reduction of the penetration depth slope dA/dT with improving quality of the thin films, which exhibit a low temperature scattering rate and residual surface resistance, comparable to single crystals. A linear fitting to the λ(T) experimental results yields dλ/dT=2 Å/K in both experiments in the low frequency limit.