STRIPLINE RESONATOR MEASUREMENTS OF ZS VERSUS HRF IN YBA2CU3O7-X THIN-FILMS

We report measurements of the surface impedance, Z(S), of YBa2Cu3O7-x thin films using a stripline resonator. The films were deposited on LaAlO3 substrates by off-axis magnetron sputtering. We obtained Z(S) as a function of frequency from 1.5 to 20 GHz, as a function of temperature from 4 K to the transition temperature (approximately 90 K), and as a function of the RF magnetic field from zero to 300 Oe. At low temperatures the surface resistance, R(S), of the films shows a very weak dependence on the magnetic field up to 225 to 250 Oe. At 77 K, R(S) is proportional to the square of the field. The penetration depth shows a much weaker dependence on the field than does R(S). At 1.5 GHz the surface resistance of the best films is 2 x 10(-6) OMEGA at 4 K and 8 x 10(-6) OMEGA at 77 K. We also discuss the origins of the magnetic field dependence of Z(S).