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

Abstract

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