Surface Impedance Measurements of YBa₂Cu₃O_7−x Thin Films in Stripline Resonators

Abstract

Microwave surface impedance of thin films of YBa₂Cu₃O_7−x deposited by a single-target off-axis magnetron sputtering technique was measured as a function of temperature, frequency and if magnetic field, H_rf using a stripline resonator. The measurements included determination of the penetration depth λ, equal to 0.167 µm the best film. The surface resistance R_s was measured over a frequency range of 0.4 to 20 GHz; the temperature range was 4.2 K to T_c, and the H_rf ranged from 0 to 225 Oe. The R_s at 4.2 K and 1.5 GHz is 4.5 × 10⁻⁶ Ω. We observed that R_s(H_rf) depends upon the conditions under which the films were deposited. Films deposited at low temperature show a linear dependence on H_rf and those deposited at high temperature show a weak dependence upon on H_rf. All films show a rapid rise in R_s at high values of H_rf.The shape of R_s(H_rf) is similar to that of Nb and NbN films measured in the same stripline geometry. We propose that R_s(H_rf) is related to the grain structure of the films. We have measured intermodulation products in these resonators. Films showing weak dependence of R_s on H_rf demonstrate smaller intermodulation effects. We discuss the implications for microwave device applications.