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

Abstract 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 ϵ′(∼ 1 λ 2 ) or ϵ″(∼ ρ 1 ) . For NbN a behaviour consistent with the BCS-formalism is found. For DyBa 2 Cu 3 O 7−δ , 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 ϵ″, even down to 60 K.