B-T phase diagram of PrOs4Sb12 studied by low-temperature thermal expansion and magnetostriction -: art. no. 235108

We report low-temperature thermal expansion alpha(T) and magnetostriction lambda(B) measurements on the filled skutterudite PrOs4Sb12 in magnetic fields up to 18 T. This compound appears to be the first example of a Pr-based heavy-fermion (HF) superconductor. Superconductivity manifests in a double jump at T-c1=1.82 K and T-c2=1.72 K, as also observed in specific heat measurements. The pressure dependencies of the two transitions are calculated by means of the Ehrenfest relation. For Bless than or equal to3 T, the normal-state thermal expansion shows a pronounced minimum related to crystalline electric field (CEF) splitting of the Pr3+ ions. At low temperatures, the thermal expansion is dominated by a nuclear spin contribution of the Pr ions. Furthermore, anomalies in alpha(T) and lambda(B) for 4.4 Tless than or equal toBless than or equal to15 T and Tless than or equal to1.3 K have made it possible to determine the boundary of an ordered phase in the B-T plane that is presumably associated with quadrupolar order.