Pronounced enhancement of the lower critical field and critical current deep in the superconducting state of PrOs4Sb12 -: art. no. 107002

We have observed an unexpected enhancement of the lower critical field H-c1(T) and the critical current I-c(T) deep in the superconducting state below T approximate to 0.6 K (T/T-c approximate to 0.3) in the filled skutterudite heavy fermion superconductor PrOs4Sb12. From a comparison of the behavior of H-c1(T) with that of the heavy fermion superconductors U1-xThxBe13 and UPt3, we speculate that the enhancement of H-c1(T) and I-c(T) in PrOs4Sb12 reflects a transition into another superconducting phase that occurs below T/T-c approximate to 0.3. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs4Sb12 near T/T-c approximate to 0.3 that correlate with the features we have observed in H-c1(T) and I-c(T).