Penetration depth and conductivity of NbN and DyBa2Cu3O7-delta thin films measured by mm-wave transmission

Using mm-wave transmission, information about both the real and imaginary part of the dielectric function of superconducting thin films is obtained. This is done by fitting the Fabry-Perot resonance spectrum for the film plus substrate over a broad frequency range (110-180 GHz) using the full Fresnel equations and a two-fluid description for the dielectric function. Depending on the thickness of the film, the transmission is mainly determined by epsilon'(similar to 1/lambda(2)) or epsilon''(similar to sigma(1)). For NbN a behaviour consistent with the BCS-formalism is found. For DyBa2Cu3O7-delta results on films of thicknesses ranging from 20 to 80 nm are presented. We find a T-2 dependence of the penetration depth at low temperatures and a strongly enhanced conductivity below T-c, indicative of a strong reduction of the quasiparticle scattering. For the 20 nm film the transmission is dominated by epsilon'', even down to 60 K. (C) 1997 Elsevier Science B.V.