Millimeter-wave complex conductivity of some epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films

The first complex conductivities (${\mathrm{\sigma }}^{\mathrm{*}}$) of epitaxial and partly oriented Y-Ba-Cu-O films have been measured at 60 GHz. For the epitaxial films, the real part of the conductivity, ${\mathrm{\sigma }}_1$, shows a maximum just below the superconducting transition. The qualitative behavior of ${\mathrm{\sigma }}^{\mathrm{*}}$ is not consistent with classical BCS calculations for a homogeneous superconducting film. Large variations in the temperature dependence of ${\mathrm{\sigma }}^{\mathrm{*}}$ among these films suggest that they are mixtures of normal and superconducting regions. A surface resistance of ${10}^{\mathrm{-}3}$ Ω at 38 K and a penetration depth of 0.4 μm at 55 K have been determined.