75As-NQR Studies of Filled-Skutterudites PrRu4As12 and LaRu4As12

We report 75 As nuclear quadrupole resonance (NQR) measurements of the filled skutterudite superconductors LaRu 4 As 12 and PrRu 4 As 12 . The temperature dependence of 75 As-NQR line and the nuclear spin–lattice relaxation rate 1/ T 1 show that the Pr 3+ has a crystal electric field (CEF) singlet ground state with a separate first excited state. In the superconducting state of PrRu 4 As 12 and LaRu 4 As 12 , 1/ T 1 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.