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We present measurements of the magnetic field dependent microwave surfaceresistance in laser-ablated YBa$_2$Cu$_3$O$_{7-\delta}$ films on SrTiO$_3$substrates. BaZrO$_3$ crystallites were included in the films using compositetargets containing BaZrO$_3$ inclusions with mean grain size smaller than 1$\mu$m. X-ray diffraction showed single epitaxial relationship betweenBaZrO$_3$ and YBa$_2$Cu$_3$O$_{7-\delta}$. The effective surface resistance wasmeasured at 47.7 GHz for 60$< T <$90 K and 0$< \mu_0H <$0.8 T. The magneticfield had a very different effect on pristine YBa$_2$Cu$_3$O$_{7-\delta}$ andYBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$, while for $\mu_0H=$0 only a reduction of$T_c$ in the YBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$ film was observed,consistent with dc measurements. At low enough $T$, in moderate fieldsYBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$ exhibited an intrinsic thin filmresistance lower than the pure film. The results clearly indicate thatBaZrO$_3$ inclusions determine a strong reduction of the field-dependentsurface resistance. From the analysis of the data in the framework of simplemodels for the microwave surface impedance in the mixed state we argue thatBaZrO$_3$ inclusions determine very steep pinning potentials.

Strong Reduction of the Field-Dependent Microwave Surface Resistance in YBCO with BaZrO_3 Inclusions