NONLINEAR MICROWAVE PROPERTIES OF EPITAXIAL HTS FILMS

The surface resistance R-s of epitaxial HTS films of YBa2Cu3O7-delta o n sapphire is studied experimentally at a frequency of 8.95 GHz in the presence of an additional powerful microwave signal of frequency 9.4 GHz ensuring the ac magnetic field amplitude up to 20 Oe on the film. It is found that the surface resistance R-s of the HTS film increases with the amplitude of the additional signal. The value of R-s of the H TS film virtually coincides with the value of the nonlinear surface re sistance of the film at the same amplitude of the ac magnetic field. I n other words, the surface resistances of the film for a weak signal i n the presence of an additional powerful signal and for the powerful s ignal alone are close. The observed phenomena are explained under the assumption that the HTS film is a Josephson medium containing various types of Josephson junctions. Under the influence of a powerful microw ave signal, the properties of this medium can vary due to switching of a part of the junctions, causing a transition of the medium to a new state which is manifested identically both for a weak and for a powerf ul signal. (C) 1997 American Institute of Physics.