Superconducting characteristics of filled skutterudites LaPt ₄Ge₁₂ and PrPt₄Ge₁₂: ⁷³Ge-NQR/NMR studies

We report on the superconducting characteristics of the filled skutterudites LaPt₄Ge₁₂ (T_c = 8:3 K) and PrPt₄Ge₁₂ (T_c = 7:9 K) by ⁷³Ge- nuclear quadrupole resonance (NQR) at a zero field. In PrP₄Ge ₁₂, the nuclear spin-lattice relaxation rate ⁷³(1/T ₁) has revealed a coherence peak just below T_c, indicating that the superconductivity of PrPt₄Ge₁₂ is accounted for in terms of the conventional BCS regime. The result was consistently reproduced by not only an isotropic s-wave gap model but also an anisotropic s-wave gap model with a point node. In the conventional superconductor LaPt ₄Ge₁₂, the coherence peak in ⁷³(1/T ₁) is markedly suppressed owing to some damping effect of quasiparticles. We note that the Tcs of LaPt₄Ge₁₂ and PrPt₄Ge₁₂ being higher than that of other RPt4Ge12 may be attributed to the larger density of states, not to some local anharmonic phonon modes derived from the possible rattling of R ions.