MICROWAVE SURFACE-RESISTANCE OF EPITAXIAL YBA2CU3O7 THIN-FILMS AT 18.7 GHZ MEASURED BY A DIELECTRIC RESONATOR TECHNIQUE

We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistance R(s) of 1 x 1 cm2 superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE01delta oscillation mode the highly reproducible quality factor of about 10(5) results in a sensitivity of +/- 50 muOMEGA for R(s) measurements. The temperature dependence of R(s) can be measured up to values as high as 1 OMEGA. We have investigated several YBa2Cu3O7 thin films prepared by high oxygen pressure d.c. sputtering on LaAlO3 and NdGaO3. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivity rho(T) with rho(300 K)/rho(100 K) values of about 3.7. Those films show, besides the initial fall-off just below T(c), a further strong decrease of R(s) at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. For T << T(c)/2 these films exhibit an exp (-alpha T(c)/T) dependence of R(s) with alpha-values around 0.4. These observations may be explained by a superconducting energy gap with 2DELTA/kT(c) almost-equal-to 0.8 for charge carriers localized in the CuO chains for YBa2Cu3O7.