Nonlinear microwave surface impedance of YBCO films: latest results and present understanding

While passive high-temperature superconductors (HTS) microwave devices are finding application in wireless communication systems, the nonlinear surface impedance and intermodulation distortion (IMD) limit wider application. Understanding and controlling the nonlinear microwave response of HTS is important for narrow band, sharp cut-off receive filters, which are susceptible to intermodulation, and high-power transmit filters, which suffer from power-dependent absorption. The sources of these two related nonlinear microwave responses are not understood at present. We discuss the latest experimental results directed toward understanding the nonlinearities. Several experiments have ruled out some of the most likely origins. We discuss measurements of the impedance of films on bicrystals showing that low-angle (<5degrees) grain boundaries are not the cause of the nonlinearities and measurements of the surface impedance on unpatterned films in a dielectric resonator and subsequently patterned in a stripline resonator, indicating that flux penetration is not the sole cause of the nonlinearities. Our latest measurements of the nonlinear impedance and IMD of impurity doped films and of films with different levels of oxygen doping indicate that disorder in the Cu-O planes or chains strongly affects the nonlinear microwave properties. (C) 2002 Elsevier Science B.V. All rights reserved.