CORRESPONDENCE BETWEEN MICROWAVE AND SUBMILLIMETER ABSORPTIVITY IN EPITAXIAL THIN-FILMS OF YBA2CU3O7

We have measured the low-temperature loss in six epitaxial ab-plane films of the high-T(c) superconductor YBa2Cu3O7 over a factor of 2000 in frequency. Submillimeter measurements from 25 to 700 cm-1 were made at 2 K by a direct absorption technique in which the film acts as the absorbing element in a composite bolometric detector. Microwave measurements near 10 GHz (0.3 cm-1) were made on five of the same films by resonance techniques at 4 K. The approximately 0.4-mum-thick films were grown epitaxially on SrTiO3, LaAlO3, and MgO by off-axis sputtering and laser deposition. The absorptivities measured for all films studied are qualitatively similar, increasing smoothly with frequency, with no gaplike features below the well-known absorption edge at 450 cm- 1. A successful three-parameter fit is obtained for all of our films. This fit can be interpreted either in terms of a weakly coupled grain model or a homogeneous two-fluid model with residual normal conductivity. The fitting parameters correspond to a grain-penetration depth lambda(g) equal to the muon-spin-relaxation value of 140 nm, and to reasonable grain properties. They also give carrier densities in reasonable agreement with optically determined plasma frequencies and conductivities in agreement with a Kramers-Kronig analysis of the absorptivity data.