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 $T\approx 0.6\mathrm{K}$ ($T/T_c\approx 0.3$) in the filled skutterudite heavy fermion superconductor ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$. From a comparison of the behavior of $H_{c1}(T)$ with that of the heavy fermion superconductors ${\mathrm{U}}_{1-x}{\mathrm{T}\mathrm{h}}_x{\mathrm{B}\mathrm{e}}_{13}$ and ${\mathrm{U}\mathrm{P}\mathrm{t}}_3$, we speculate that the enhancement of $H_{c1}(T)$ and $I_c(T)$ in ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ 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 ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ near $T/T_c\approx 0.3$ that correlate with the features we have observed in $H_{c1}(T)$ and $I_c(T)$.

• Received 10 September 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.107002