Impurity Effect as a Probe for the Gap Function in the Filled   Skutterudite Compound Superconductor PrOs$_{4}$Sb$_{12}$: Sb-NQR Study
  We have carried out nuclear quadrupole resonance (NQR) measurements in the filled skutterudite compounds Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ ($x=0.1, 0.2$), in order to gain insights into the symmetry of the superconducting gap function. Upon replacing Os with Ru, the spin-lattice relaxation rate $1/T_1$ becomes proportional to temperature ($T$) at low $T$ far below $T_c$, and the magnitude of $(1/T_1T)_{low-T}$ increases with increasing Ru content. These results indicate that a finite density of states is induced at the Fermi level by the impurity, and thus suggest that there exist nodes in the gap function of PrOs$_{4}$Sb$_{12}$. 
Masahide Nishiyama, Takayuki Kato, Hitoshi Sugawara, Daisuke Kikuchi,   Hideyuki Sato, Hisatomo Harima, and Guo-qing Zheng