Microwave dissipation in YBCO coplanar resonators with uniform and non-uniformcolumnar defect distribution

Abstract

The effects of uniform and non-uniform columnar defect implantation on thevortex-induced nonlinear behaviour of YBCO films at microwave frequencies areinvestigated by a coplanar resonator technique. Measurements have been performed as afunction of temperature, input power and small dc fields. A novel data analysis is proposedand results in a model-independent determination of the penetration depth and of thesurface impedance. The same samples were characterized before and after 4.2 GeV Au ionirradiation, producing nanosize columnar defects. In the first experiment (uniformirradiation) the induced damage, attributed to the addition of resistive regions,always prevails, in the investigated ranges of temperature, dc field and inputpower, over the improvement due to vortex pinning. In the second experiment wetried to modify the balance between these two competing effects, by implantingcolumnar defects only where needed: the irradiation dose was modulated along thelongitudinal direction, reaching its maximum value at the centre of the resonator,where the rf magnetic field is maximum. It turns out that significant damage isabsent, while the onset of nonlinearity is shifted towards higher circulating power.This results in the improvement of the power handling capability of the device.