Residual losses in epitaxial thin films of YBa₂Cu ₃O₇ from microwave to submillimeter wave frequencies

We have measured the residual loss in five epitaxial a-b plane films of the high-T_c superconductor YBa₂Cu₃O₇. Microwave measurements near 10 GHz were made by resonance techniques at 4 K. Submillimeter measurements from ∼1.5 to 21 THz were made at 2 K by a direct absorption technique. We use a model of weakly coupled superconducting grains and a homogeneous two-fluid model to fit the data for each film below the well-known absorption edge at 13.5 THz. When the penetration depth determined from muon spin rotation measurements is used to constrain each model, the weakly coupled grain model is able to fit the measured absorptivities for all films, but the two-fluid model is less successful.