Thermal expansion of the skutterudite superconductor PrOs4Sb12

The recently discovered Pr-based skutterudite superconductor PrOs4Sb12 was studied by means of low-temperature measurements of the thermal expansion coefficient a along the cubic (100) direction of a single crystal sample in magnetic fields up to 8T. Two superconducting phase transitions were found with critical temperatures T-c1 = 1.84K and T-c2 = 1.71 K. Their hydrostatic pressure dependencies calculated using the Ehrenfest relation are -250+/-50 mK/GPa and -450+/-70 mK/GPa, respectively. For B greater than or equal to 5 T, we observe a thermodynamic phase transition which is presumably due to the onset of quadrupolar order. The transition temperature shifts with increasing B towards higher temperatures. The strongly enhanced value of the Gruneisen parameter Gamma provides clear evidence for the occurrence of a heavy-fermion normal state. The pronounced change of Gamma at T-c provides evidence for the formation of Cooper pairs out of the heavy quasiparticles.