Surface impedance measurements of YBa2Cu3O 7-x thin films in stripline resonators

The microwave surface impedance of thin films of YBa₂Cu ₃O_7-x is measured as a function of temperature, frequency, and RF magnetic field, H_RF, using a stripline-resonator technique. The films were deposited in situ by single-target off-axis magnetron sputtering. The frequency range was from 0.4 to 20 GHz, the temperature range from 4.2 K to T_c, and the RF magnetic field range from 0 to 30 Oe. The surface resistance R_S at 4.2 K and 1.5 GHz is 4×10^-6 Ω. The penetration depth is determined to be 0.167 μm in the best film, In some films, R_S shows a linear dependence on H_RF and rises rapidly at large fields, showing no evidence of saturation. In others, R_S shows weak dependence on H_RF before rising rapidly. These behaviors differ from those observed in bulk ceramic YBa₂Cu₃O_7-x and in unpatterned films measured in microwave cavities. However, the shape of R_S(H_RF) for the YBa₂Cu₃O_7-x films is similar to that of Nb and NbN films measured in the same stripline geometry. The authors also present measurements of the intermodulation products in the resonators and discuss the implications of the R_S and intermodulation measurements for microwave device applications