Millimeter and submillimeter wave surface impedance of YBa2Cu3O7-δ thin films with reduced low temperature absorption

The surface impedance of epitaxial YBa2Cu3O7-δ thin films prepared by high oxygen pressure dc sputtering as well as by excimer laser ablation on LaAlO3 has been measured at 87GHz by a cavity endplate replacement technique and between 100GHz and 1THz by transmission experiments. Some of our films with low dc-resistivity values (ρ(100K)=40-50μΩcm, ρ(300K)/ρ(100K)≥3) and improved microstructure exhibit besides the initial steep falloff at Tc a further gradual decrease of the surface resistance below about 50K by at least one order of magnitude, presently limited by the sensitivity of our measurement techniques. The dynamic conductivity determined from the surface impedance shows a maximum below Tc looking similar to a BCS coherence peak. Beyond the relevance for the understanding of the conduction mechanism our low loss films are important for millimeter- and submillimeter wave applications below Tc/2. (orig.)