Reduction of the field-dependent microwave surface resistance in YBa_2Cu_3O_7 with sub-micrometric BaZrO_3 inclusions as a function of BaZrO_3 concentration

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin films, where sub-micrometric BaZrO$_3$ particles have been incorporated. As a function of the BaZrO$_3$ content, we observe that the absolute losses slightly decrease up to a BaZrO$_3$ content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently, in that they are not monotonic with increasing BaZrO$_3$ concentration: at small concentration (2.5%) the field-induced losses increase, but large reduction of the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO$_3$ concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance we estimate vortex pinning-related parameters. We find that BaZrO$_3$ inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO$_3$ content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, thus leaving room for further improvements.