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

Striking features have been found in the nonlinear microwave (8.0 GHz) surface impedance Zₛ=Rₛ+j·Xₛ of high-quality YBaCuO thin films with comparable low power characteristics [Rᵣₑₛ~35--60μΩ and λL(15K)~130--260nm]. The surface resistance Rₛ is found to increase, decrease, or remain independent of the microwave field Hrf (up to 60 mT) at different temperatures and for different samples. However, the surface reactance Xₛ always follows the same functional form. Mechanisms which may be responsible for the observed variations in Rₛ and Xₛ are briefly discussed.