Superconductivity and crystalline electric field effects in the filled skutterudite series Pr(Os${}_{1-x}$Ru${}_x$)${}_4$Sb${}_{12}$

Abstract

X-ray powder diffraction, magnetic susceptibility $\chi (T)$, and electrical resistivity $\rho (T)$ measurements were made on single crystals of the filled skutterudite series Pr(Os${}_{1-x}$Ru${}_x$)${}_4$Sb${}_{12}$. One end of the series ($x=0$) is a heavy fermion superconductor with a superconducting critical temperature $T_c=1.85$ K, while the other end ($x=1$) is a conventional superconductor with $T_c\approx 1$ K. The lattice constant $a$ decreases approximately linearly with increasing Ru concentration $x$. As Ru (Os) is substituted for Os (Ru), $T_c$ decreases nearly linearly with substituent concentration and exhibits a minimum with a value of $T_c=0.75$ K at $x=0.6$, suggesting that the two types of superconductivity compete with one another. Crystalline electric field (CEF) effects in ${\chi }_{\mathrm{dc}}(T)$ and $\rho (T)$ due to the splitting of the Pr${}^{3+}$ nine-fold degenerate Hund's rule $J=4$ multiplet are observed throughout the series, with the splitting between the ground state and the first excited state increasing monotonically as $x$ increases. The fits to the ${\chi }_{\mathrm{dc}}(T)$ and $\rho (T)$ data are consistent with a ${\Gamma }_3$ doublet ground state for all values of x, although reasonable fits can be obtained for a ${\Gamma }_1$ ground state for $x$ values near the end member compounds ($x=0$ or $x=1$).

10 pages, 8 figures, submitted to Phys. Rev. B