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 Z(s)=R-s +j.X-s of high-quality YBaCuO thin films w ith comparable low power characteristics [R(res)similar to 35-60 mu Om ega and lambda(L)(15 K)similar to 130-260 nm]. The surface resistance R-s is found to increase, decrease, or remain independent of the micro wave field H-rf (up to 60 mT) at different temperatures and for differ ent samples. However, the surface reactance X-s always follows the sam e functional form. Mechanisms which may be responsible for the observe d variations in R-s and X-s are briefly discussed. [S0163-1829(98)0521 42-1].