We report Sb-NQR results, which evidence unusual superconducting (SC) property in a filled-skutterudite compound PrOs4Sb12 with a SC transition temperature, Tc=1.8 K. The nuclear-spin--lattice relaxation rate, 1/T1 have revealed that Pr-derived 4f moments behave as if they are localized in a high temperature (T) above T0\sim 10 K. The observed NQR-line shift below T0 suggests the local redistribution of the charges associated with quadrupolar moments of Pr3+. The 1/T1 in the SC state has revealed an isotropic energy gap of \Delta /kBTc\sim 2.7. The absence of the coherence peak, but an exponential decrease in 1/T1T below Tc=1.8 K cannot be accounted for by either a conventional s-wave model or an anisotropic SC model with point- or line-node gap, pointing to a new class of unusual superconductivity.
PACS numbers: 71.27.+a, 75.30.Mb
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