Effects of non-uniform columnar defect distribution on the microwave properties of Y-Ba-Cu-O coplanar resonators

This paper reports on nonuniform heavy ion irradiations of YBCO coplanar resonators aimed at controlling vortex-induced microwave dissipation and nonlinearity. The adopted moving-target irradiation technique allows producing tailored profiles of columnar defect density to get an optimal compromise between defect-induced enhancement of vortex pinning and addition of dissipative scattering. The same samples were characterized before and after irradiation in order to evaluate the surface impedance, by means of data analysis suitable to account also for the substrate properties. In the case of the modulated dose profile, matching the fundamental-mode field distribution along the resonator, the onset of nonlinearity is shifted toward higher circulating power, without increasing dissipation. Finally, the possibility to locally modify by irradiation the properties of the material to obtain the confinement of external perturbations, while maintaining the current continuity throughout the whole device, has been preliminarily tested.