Detection of nonlinear behavior by phase-resolved microwave harmonic mixing in high-temperature superconductors

Abstract

Investigations of the non-ohmic properties of conducting materials are a powerful tool in the elucidation of transport mechanisms. Phase-resolved microwave harmonic mixing is a very sensitive method for the detection of nonlinearities which avoids many of the disadvantages of other methods. A nonlinear current-voltage characteristic gives rise to a dc-current proportional to the deviation from linearity when microwave radiation of frequency (omega) and its second harmonic 2 (omega) are applied to a sample. We report on investigations of the nonlinear behavior of thin films of YBCO and TIBCCO at incident microwave power levels ranging from 0.4W to 40W. In films of high quality a mixing signal appears only in the temperature region of the superconducting transition. In contrast films of poor quality exhibit nonlinear behavior between 4.2K and room temperature. The dependence of the mixing signal on the incident power level was studied too. The mixing results are compared to those of harmonic generation experiments done by other groups. The possible mechanisms for the nonlinear behavior are discussed.