Low temperature specific heat of the heavy fermion superconductor   PrOs$_4$Sb$_{12}$
  We report the magnetic field dependence of the low temperature specific heat of single crystals of the first Pr-based heavy fermion superconductor PrOs$_4$Sb$_{12}$. The low temperature specific heat and the magnetic phase diagram inferred from specific heat, resistivity and magnetisation provide compelling evidence of a doublet ground state and hence superconductivity mediated by quadrupolar fluctuations. This establishes PrOs$_4$Sb$_{12}$ as a very strong contender of superconductive pairing that is neither electron-phonon nor magnetically mediated. 
R. Vollmer, A. Fai{\ss}t, C. Pfleiderer, H. v. L\"ohneysen, E. D.   Bauer, P.-C. Ho and M. B. Maple