We report measurements of the magnetic penetration depth $\lambda$ in single crystals of ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ down to 0.1 K, with the ac field applied along the $a$, $b$, and $c$ directions. In all three field orientations, $\lambda \sim T^2$ and superfluid density ${\rho }_s\sim T^2$ for $T<0.3T_c$. Data are best fit by the ${}^3\mathrm{H}\mathrm{e}$ A-phase-like gap with multidomains, each having two point nodes along a cube axis, and parameter ${\Delta }_0(0)/k_B{T}_c=2.6$, suggesting that ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$ is a strong-coupling superconductor with two point nodes on the Fermi surface. We also confirm the double transitions at 1.75 and 1.85 K seen in other measurements.

• Received 22 August 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.247003