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

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