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 $T \approx 0.6$ K ($T/T_{ c} \approx 0.3$) in the filled skutterudite heavy fermion superconductor PrOs$_4$Sb$_{ 12}$. From a comparison of the behavior of $H_{ c1}(T)$ with that of the heavy fermion superconductors U$_{ 1-x}$Th$_x$Be$_{ 13}$ and UPt$_3$, we speculate that the enhancement of $H_{ c1}(T)$ and $I_{ c}(T)$ in PrOs$_4$Sb$_{ 12}$ reflects a transition into another superconducting phase that occurs below $T/T_{ c} \approx 0.3$. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs$_4$Sb$_{ 12}$ near $T/T_{ c} \approx 0.3$ that correlate with the features we have observed in $H_{ c1}(T)$ and $I_{ c}(T)$.
Categories: cond-mat.supr-con