Effect of BaZrO3 Inclusions on the Microwave Surface Impedance of YBCOFilms in a Magnetic Field

Abstract

We perform measurements of high-frequency (similar to 48 GHz) microwavesurface impedance with an applied magnetic field in YBa2Cu3O7-delta (YBCO) laser-ablated films with various amounts of BaZrO3 ( BZO)sub-micrometric inclusions, up to 7 mol % concentration. BZO inclusionsare very effective in the reduction of the field-induced surfaceresistance in our experimentally accessible field range [ 0, 0.8] T. Attemperatures low enough, the application of a moderate (similar to 0.2T) field makes the YBCO/BZO films markedly less dissipative than pureYBCO. This result, examined in the light of the very high measuringfrequency ( very small vortex oscillation amplitude) shows that BZOinclusions are even more effective pinning centers than columnardefects. We study the dependence of the vortex parameters ( vortexviscosity, pinning constant) on the BZO concentration. We examine thecorrelation between the reduction of the microwave dissipation and theareal density of BZO-induced defects. We argue that the very improvedperformances in a magnetic field are due to individual pinning ofvortices on BZO inclusions.