Muon spin rotation study of perturbation of the crystalline-electric-field induced by interstitial muon in PrOs$_4$Sb$_{12}$ and PrRu$_{4}$Sb$_{12}$*

Muon spin rotation measurements of the temperature dependence of the positive muon Knight shift in single crystals of PrOs$\rm_{4}$Sb$\rm_{12}$ and PrRu$\rm_{4}$Sb$\rm_{12}$ reveal a linear scaling of the Knight shift with the bulk magnetic susceptibility at high temperatures. A small deviation from the linear relation appears in PrOs$\rm_{4}$Sb$\rm_{12}$ below $6.3$ K. However, a large magnitude of deviation is observed in PrRu$\rm_{4}$Sb$\rm_{12}$ below $32$ K. The deviation can be explained by the positive muon induced modification of the susceptibility of neighboring Pr$^{3+}$ ions due to a change of the crystalline-electric-field (CEF) splitting. The data indicate that this modification is much smaller in PrOs$\rm_{4}$Sb$\rm_{12}$ than in PrRu$\rm_{4}$Sb$\rm_{12}$. A model calculation based on CEF theory is in progress.