We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in ${\mathrm{Pr}\mathrm{}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ with $T_c=1.85\mathrm{K}$. The temperature ($T$) dependence of nuclear-spin-lattice-relaxation rate, $1/T_1$, and NQR frequency unravel a low-lying crystal-electric-field splitting below $T_0\sim 10\mathrm{K}$, associated with ${\mathrm{P}\mathrm{r}}^{3+}(4f^2)$-derived ground state. In the SC state, $1/T_1$ shows neither a coherence peak just below $T_c\mathrm{K}$ nor a $T^3$-like power-law behavior observed for anisotropic HF superconductors with the line-node gap. The isotropic energy gap with its size $\Delta /k_B=4.8\mathrm{K}$ seems to open up across $T_c$ below $T^{*}\sim 2.3\mathrm{K}$. It is surprising that ${\mathrm{Pr}\mathrm{}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ looks like an isotropic HF superconductor—it may indeed argue for Cooper pairing via quadrupolar fluctuations.

• Received 17 April 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.027001