Evolution of superconducting order in Pr(Os1−xRux)4Sb12

We report measurements of the magnetic penetration depth λ in single crystals of Pr(Os1−xRux)4Sb12 down to 0.1 K. Both λ and superfluid density ρs exhibit an exponential behaviour for the x≥0.4 samples, going from weak (x = 0.4,0.6) to moderate coupling (x = 0.8). For the x≤0.2 samples, both λ and ρs vary as T2 at low temperatures, but ρs is s-wave-like at intermediate to high temperatures. Our data are consistent with the presence of an additional nodal low-temperature phase at Tc 3<0.6 K, for small values of x.