Specific heat and ac susceptibility studies of the superconducting phase diagram of PrOs4Sb12

We report on ac-susceptibility and heat-capacity measurements of the superconductor PrOs4Sb12 in magnetic fields. The resulting phase diagram reveals two distinguishable superconducting phases with transitions at the upper critical field B-c2 and a slightly lower field B-c2(*). Between B-c2(*) and 0.7xB(c2)(*) the ac-susceptibility data shows a region with enhanced pinning properties characterized by an extended peak effect. The heat-capacity data reveal an extremely strongly coupled superconductivity with a considerable contribution of heavy quasiparticles. This unusual strong-coupling behavior originates in a sharp increase of the superfluid density at T-c. The decrease of the discontinuity of the specific heat at T-c and the corresponding pronounced increase of the Ginzburg-Landau-Maki parameter kappa(2) indicate that the superconductivity is most probably not Pauli limited in a large temperature range.