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 measurements of three YBaCuO thin films in dc magnetic fields H/sub dc/ (parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero dc field the three films, deposited by the same method, show a spread of low-power residual surface resistance, R/sub res/ and penetration depth, /spl lambda/(T=15 K) within a factor of 1.9. However, they exhibit dramatically different microwave field, H/sub rf/ dependences of the surface resistance, R/sub s/, but universal X/sub s/(H/sub rf/) dependence. Application of a dc field was found to affect not only absolute values of R/sub s/ and X/sub s/, but the functional dependences R/sub s/(H/sub rf/) and X/sub s/(H/sub rf/) as well. For some of the samples the dc field was found to decrease R/sub s/ below its zero-field low-power value.