Specific heat and ac susceptibility studies of the superconducting phase diagram of Pr Os4 Sb12

We report on ac-susceptibility and heat-capacity measurements of the superconductor Pr Os4 Sb12 in magnetic fields. The resulting phase diagram reveals two distinguishable superconducting phases with transitions at the upper critical field Bc2 and a slightly lower field B c2 *. Between B c2 * and 0.7x B 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 Tc. The decrease of the discontinuity of the specific heat at Tc and the corresponding pronounced increase of the Ginzburg-Landau-Maki parameter κ2 indicate that the superconductivity is most probably not Pauli limited in a large temperature range.