Pronounced enhancement of the lower critical field and critical current deep in the superconducting state of PrOs${}_4$Sb${}_{12}$

Abstract

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 $I_c(T)$.

4 pages, 3 figures, submitted to Physical Review Letters. v2: small corrections to references, updated figure 3