Change of strength of vortex pinning in YBCO due to BaZrO_3 inclusions

We probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7 GHz) on YBa${}_2$Cu${}_3$O${}_{7-\delta }$ films with and without sub-micrometer BaZrO${}_3$ inclusions. We explore the temperature and field ranges 60 K$<T<T_c$ and 0$<{\mu }_{0}H<$0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, $\Delta \rho_1(H)+$\mathrm{i}$\Delta \rho_2(H)$,inYBa$_2$Cu$3$O${7-\delta}$\mathrm{/}BaZrO$_3$withrespecttopureYBa$_2$Cu$3$O${7-\delta}$\mathrm{.}$\Delta \rho_1(H)$ is slightly superlinear in pure YBa${}_2$Cu${}_3$O${}_{7-\delta }$ (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa${}_2$Cu${}_3$O${}_{7-\delta }$/BaZrO${}_3$ (suggesting a possible suppression of thermal activation as a consequence of BaZrO${}_3$ inclusions). These features persist up to close to $T_c$. We discuss our data in terms of the ratio $r=\mathrm{\Delta }{X}_s^{\mathrm{\prime }}(H)\mathrm{/}\mathrm{\Delta }{R}_s^{\mathrm{\prime }}(H)$ in the framework of the models for the microwave surface impedance in the mixed state. Large $r$ are found in YBa${}_2$Cu${}_3$O${}_{7-\delta }$/BaZrO${}_3$, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa${}_2$Cu${}_3$O${}_{7-\delta }$. We discuss the different field dependence of the pinning constant.