Effect of nanosize BaZrO3 inclusions on vortex parameters in YBa2Cu3O7-x

We report on the field dependence of the microwave complex resistivity data in YBa2Cu3O7-x/BaZrO3 films grown by PLD at various BaZrO3 content. The data, analyzed within a recently developed general framework for the mixed-state microwave response of superconductors, yield the field dependence of the fluxon parameters such as the vortex viscosity and the pinning constant. We find that pinning undergoes a change of regime when the BaZrO3 content in the target increases from 2.5 mol.% to 5 mol.%. Simultaneously, the vortex viscosity becomes an increasing function of the applied magnetic field. We propose a scenario in which flux lines are pinned as bundles, and a crossover from dilute point pins to dense c-axis correlated defects takes place between 2.5 and 5 mol.% in the BZO concentration. Our data are inconsistent with vortices occupying mainly the BaZrO3 sites at low fields, and suggest instead that vortices occupy both BaZrO3 sites and interstitials in the YBa2Cu3O7-x matrix, even at low fields.