Temperature and magnetic field effects on microwave intermodulation in YBCO films

The effects of temperature and dc field on third order microwave intermodulation distortion (IMD) measurements of three YBa₂Cu₃O_7-δ (YBCO) thin films grown on LaAlO₃ substrates are discussed. These films were patterned into coplanar resonators with a fundamental resonant frequency of around 5 GHz. The measurements were performed at temperatures of 15 K and 60 K, in zero dc magnetic field and also in an applied dc field of ∼ 10 mT. Good quality films exhibit 3:1 IMD scalings at low temperatures and low powers, decreasing to a 2:1 scaling at higher powers and temperatures. A dc magnetic field applied perpendicularly to good quality samples results in a 2:1 scaling across the whole range of power and temperature, though without any significant change in the IMD output power within the 2:1 scaling regime; this differs for poor quality samples, where the IMD output power can be increased by up to 20 dB by the application of a dc field.