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 imagin ary part of the dielectric function of superconducting thin films is o btained. This is done by fitting the Fabry-Perot resonance spectrum fo r the film plus substrate over a broad frequency range (110-180 GHz) u sing the full Fresnel equations and a two-fluid description for the di electric function. Depending on the thickness of the film, the transmi ssion is mainly determined by epsilon'(similar to 1/lambda(2)) or epsi lon''(similar to sigma(1)). For NbN a behaviour consistent with the BC S-formalism is found. For DyBa2Cu3O7-delta results on films of thickne sses 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 c onductivity below T-c, indicative of a strong reduction of the quasipa rticle scattering. For the 20 nm film the transmission is dominated by epsilon'', even down to 60 K. (C) 1997 Elsevier Science B.V.