Effect of ${\rm BaZrO}_{3}$ Inclusions on the Microwave Surface Impedance of YBCO Films in a Magnetic Field

We perform measurements of high-frequency ( ~48 GHz) microwave surface impedance with an applied magnetic field in YBa₂Cu₃O_7-delta (YBCO) laser-ablated films with various amounts of BaZrO₃ (BZO) sub-micrometric inclusions, up to 7 mol % concentration. BZO inclusions are very effective in the reduction of the field-induced surface resistance in our experimentally accessible field range [0, 0.8] T. At temperatures low enough, the application of a moderate ( ~0.2 T) field makes the YBCO/BZO films markedly less dissipative than pure YBCO. This result, examined in the light of the very high measuring frequency (very small vortex oscillation amplitude) shows that BZO inclusions are even more effective pinning centers than columnar defects. We study the dependence of the vortex parameters (vortex viscosity, pinning constant) on the BZO concentration. We examine the correlation between the reduction of the microwave dissipation and the areal density of BZO-induced defects. We argue that the very improved performances in a magnetic field are due to individual pinning of vortices on BZO inclusions.