Superconductivity in PrRu$_{4}$As$_{12}$ single crystals

Single crystals of the filled skutterudite compound PrRu$_{4}$As$_{12}$ were characterized by magnetization, specific heat, and electrical resistivity measurements. These measurements reveal the occurrence of superconductivity below $\sim 2.4$~K. The magnetic susceptibility exhibits behavior consistent with a Pr$^{3+}$ ninefold degenerate $J=4$ Hund's rule ground state multiplet split in a tetrahedral crystalline electric field with either a nonmagnetic singlet or doublet ground state. Fits to the specific heat data indicate an electronic contribution to the specific heat $\gamma$ of $\sim 70$~mJ/mol~K$^{2}$ and a Debye temperature $\Theta_{\mathrm{D}}$ of $\sim 344$ K. The value of $\Delta C/\gamma T_c \approx 1.53$ for PrRu$_{4}$As$_{12}$ is close to the weak-coupling BCS value of $1.43$. Electrical resistivity measurements in field were used to determine $H_{c2} $ as a function of temperature from which the zero temperature value of the orbital critical field $^{*}H_{c2}(0)$ was calculated and used to estimate the coherence length $\xi_{0}$ of PrRu$_{4}$As$_{12}$. In contrast to PrRu$_{4}$As$_{12}$, the compound PrOs$_{4}$As$_{12}$ displays two phase transitions at $2.2$~K and $2.3$~K in zero field, one of which is antiferromagnetic and the nature of the other is yet to be determined. PrOs$_{4}$As$_{12}$ also displays heavy fermion behavior with an enhanced electronic specific heat coefficient $\gamma$ on the order of $200$~mJ/mol~K$^{2}$ and a $\Theta_{\mathrm{D}}$ of only $260$~K. This research was supported by the U.S. DOE (No.~DE-FG02-04ER46105) and NSF (No.~DMR 0335173).