Non-uniform columnar defect implantation in YBCO coplanar resonators for the control of vortex-induced microwave dissipation and nonlinearity

This paper reports on 4.2 GeV Au ion irradiation of YBCO coplanar resonators aimed at controlling vortex-induced microwave dissipation and nonlinearity by means of a suitable distribution of columnar defects. Two procedures were used: (i) uniform irradiation at a given dose and (ii) non-uniform irradiation, obtained by means of a moving-target technique, producing a tailored profile of columnar defect density. The last configuration is needed in order to attain an optimal compromise between defect-induced enhancement of vortex pinning and increase of dissipative scattering. The resonators were characterized before and after irradiation in order to evaluate penetration depth and surface impedance, by means of a data analysis also suitable to account for the substrate properties. It turns out that in the case of uniform irradiation the induced damage, attributed to the addition of dissipative regions, always prevails over the improvement due to vortex pinning. In contrast, in the case of the tailored dose profile, significant damage is absent, while the onset of non-linearity is shifted towards higher circulating power. The need to control vortex-induced effects is confirmed by magneto-optical analysis that puts in evidence that vortex penetration occurs even at low values of transverse dc magnetic field and temperature.