Muon Spin Relaxation and Isotropic Pairing in SuperconductingPrOs4Sb12

Transverse-field muon-spin rotation measurements in the vortex-lattice of the heavy-fermion (HF) superconductor ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_{4}{\mathrm{S}\mathrm{b}}_{12}$ yield a temperature dependence of the magnetic penetration depth $\ensuremath{\lambda}$ indicative of an isotropic or nearly isotropic energy gap. This is not seen to date in any other HF superconductor and is a signature of isotropic pairing symmetry, possibly related to a novel nonmagnetic ``quadrupolar Kondo'' HF mechanism in ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_{4}{\mathrm{S}\mathrm{b}}_{12}$. The $T=0$ relaxation rate ${\ensuremath{\sigma}}_{s}(0)=0.91(1)\text{ }\text{ }\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$ yields an estimated magnetic penetration depth $\ensuremath{\lambda}(0)=3440(20)\text{ }\text{ }\AA{}$, which is considerably shorter than in other HF superconductors.