Penetration depth from far-infrared transmission in YBa2Cu3O7 thin films

We have deduced the penetration depth λ(T) on YBa2Cu3O7 thin films from transmission measurement in the range 100–550 GHz. Our data yield simultaneously the absolute value and the temperature dependence of λ(T). In lower-quality films, we report a usualT2 dependence, associated to a large value of λ(0) compared to the standard 1400Å value reported in the literature. In better-quality films λ exhibits a linear temperature dependence associated to a value of λ(0) compatible with the standard one. This suggests that theT2 variation is of extrinsic origin. Thin films of sufficiently high quality exhibit the same linear behavior as single crystals (with a slope of about 4Å/K). We also observe a dependence with the frequency of the penetration depth which is the signature of the rising inductive effect of the quasiparticles. This gives us direct access to the quasiparticle scattering time at low temperature, and stresses the potential interest of using far infrared synchrotron radiation to extend our frequency range.