Non-linear microwave surface impedance of epitaxial HTS thin films in low DC magnetic fields

We have carried out non-linear microwave (8 GHz) surface impedance measurem ents of three YBaCuO thin films in de magnetic fields H-dc (parallel to c a xis) up to 12 mT using a coplanar resonator technique. In zero de field the three films, deposited by the same method, show a spread of low-power resi dual surface resistance, R-res and penetration depth, lambda (T = 15 K) wit hin a factor of 1.9. However, they exhibit dramatically different microwave field, H-rf dependences of the surface resistance, R-s, but universal X-s( H-rf) dependence. Application of a de field was found to affect not only ab solute values of R-s and X-s, but the functional dependences R-s(H-rf) and X-s(H-rf) as well. For some of the samples the dc field was found to decrea se R-s below its zero-field low-power value.