75As-NQR studies of superconducting filled skutterudites PrRu4As12 and LaRu4As12

We report As-75 nuclear quadrupole resonance (NQR) measurements of the filled skutterudite superconductors LaRu4As12 and PrRu4As12. The temperature dependence of the As-75-NQR line and the nuclear spin-lattice relaxation rate 1/T-1 show that Pr3+ has a crystal electric field (CEF) singlet ground state with a separate first excited state. In the superconducting states of PrRu4As12 and LaRu4As12, 1 IT, has a coherence peak just below the transition temperature and decreases exponentially at low temperatures. This behavior shows that the s-wave superconductivity occurs in both compounds.