Unusually strong coherent response from grain-boundary Josephson network in polycrystalline Pr$_x$Y$_{1-x}$Ba$_2$Cu$_3$O$_{7-\delta}$

By applying a highly sensitive homemade AC susceptibility technique to Pr$_x$Y$_{1-x}$Ba$_2$Cu$_3$O$_{7-\delta}$ polycrystals (with $x=0.0$, $0.1$ and $0.3$), we observed very sharp Fraunhofer type low-field periodic oscillations of the real part of the AC susceptibility which were attributed to Josephson vortices penetrating intergranular regions of grain-boundary Josephson network in our samples. Assuming the Lorentz type distribution of single-junction contact areas, we were able to successfully fit the experimental data.