Unusual features in the nonlinear microwave surface impedance of Y-Ba-Cu-O thin films

Abstract

Striking features have been found in the nonlinear microwave (8.0 GHz) surface impedance (Formula presented) of high-quality YBaCuO thin films with comparable low power characteristics (Formula presented) and (Formula presented) The surface resistance (Formula presented) is found to increase, decrease, or remain independent of the microwave field (Formula presented) (up to 60 mT) at different temperatures and for different samples. However, the surface reactance (Formula presented) always follows the same functional form. Mechanisms which may be responsible for the observed variations in (Formula presented) and (Formula presented) are briefly discussed. © 1998 The American Physical Society.