Probing the superconducting gap symmetry of PrRu${}_4$Sb${}_{12}$: A comparison with PrOs${}_4$Sb${}_{12}$

Abstract

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of PrRu${}_4$Sb${}_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density ${\rho }_s$ exhibit an exponential behavior for $T$ $<$ 0.5$T_c$, with parameters $\Delta$(0)/\textit{k}${}_B$\textit{T}${}_c$ = 1.9 and $\lambda (0)$ = 2900 Å. The value of $\Delta$(0) is consistent with the specific-heat jump value of $\Delta C/\gamma T_c$ = 1.87 measured elsewhere, while the value of $\lambda (0)$ is consistent with the measured value of the electronic heat-capacity coefficient $\gamma$. Our data are consistent with PrRu${}_4$Sb${}_{12}$ being a moderate-coupling, fully-gapped superconductor. We suggest experiments to study how the nature of the superconducting state evolves with increasing Ru substitution for Os.