Superconducting Characteristics of Filled Skutterudites LaPt4Ge12 and PrPt4Ge12: Ge-73-NQR/NMR Studies

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We report on the superconducting characteristics of the filled skutterudites LaPt4Ge12 ($T_{\text{c}}=8.3$ K) and PrPt4Ge12 ($T_{\text{c}}=7.9$ K) by 73Ge-nuclear quadrupole resonance (NQR) at a zero field. In PrPt4Ge12, the nuclear spin–lattice relaxation rate $^{73}(1/T_{1})$ has revealed a coherence peak just below $T_{\text{c}}$, 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 LaPt4Ge12, the coherence peak in $^{73}(1/T_{1})$ is markedly suppressed owing to some damping effect of quasiparticles. We note that the $T_{\text{c}}$s of LaPt4Ge12 and PrPt4Ge12 being higher than that of other $R$Pt4Ge12 may be attributed to the larger density of states, not to some local anharmonic phonon modes derived from the possible rattling of $R$ ions.
2010-06-15