Evidence for Unconventional Strong-Coupling Superconductivity in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow><mml:mi mathvariant="normal">P</mml:mi><mml:mi mathvariant="normal">r</mml:mi><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">s</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi><mml:mi mathvariant="normal">b</mml:mi></mml:mrow><mml:mn>12</mml:mn></mml:msub></mml:math>: An Sb Nuclear Quadrupole Resonance Study

We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in Pr(Os4Sb12 with T(c)=1.85 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 T0 approximately 10 K, associated with Pr3+(4f(2))-derived ground state. In the SC state, 1/T(1) shows neither a coherence peak just below T(c) K nor a T3-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 K seems to open up across T(c) below T(*) approximately 2.3 K. It is surprising that Pr(Os4Sb12 looks like an isotropic HF superconductor-it may indeed argue for Cooper pairing via quadrupolar fluctuations.