Probing the superconducting gap symmetry of PrRu₄Sb₁₂: A comparison with PrOs₄Sb₁₂

We report measurements of the magnetic penetration depth λ in single crystals of PrRu₄Sb₁₂ down to 0.1 K. Both λ and superfluid density ρ_s exhibit an exponential behavior for T<0.5T_c, with parameters Δ(0)/k_BT_c=1.9 and λ(0)=2900Å. The value of Δ(0) is consistent with the specific-heat jump value of ΔC/γT_c=1.87 measured elsewhere, while the value of λ(0) is consistent with the measured value of the electronic heat-capacity coefficient γ. Our data are consistent with PrRu₄Sb₁₂ 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.