Nonlinear surface impedance of Y-Ba-Cu-O films in a DC magnetic field

We study the microwave power dependence of the surface impedance of YBCO epitaxial films in a DC magnetic field up to 0.7 T. We demonstrate that the surface impedance of our films in the linear regime is dominated by weak links at low DC field and by vortices at high DC field. However, the microwave power dependence of the surface impedance arises mostly from the hysteretic RF vortex penetration.