S-wave/spin-triplet order in superconductors without inversion symmetry: Li${}_2$Pd${}_3$B and Li${}_2$Pt${}_3$B

We investigate the order parameter of noncentrosymmetric superconductors Li${}_2$Pd${}_3$B and Li${}_2$Pt${}_3$B via the behavior of the penetration depth $\lambda (T)$. The low-temperature penetration depth shows BCS-like behavior in Li${}_2$Pd${}_3$B, while in Li${}_2$Pt${}_3$B it follows a linear temperature dependence. We propose that broken inversion symmetry and the accompanying antisymmetric spin-orbit coupling, which admix spin-singlet and spin-triplet pairing, are responsible for this behavior. The triplet contribution is weak in Li${}_2$Pd${}_3$B, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in Li${}_2$Pt${}_3$B allows the spin-triplet component to be larger in Li${}_2$Pt${}_3$B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of $\lambda (T)$. The experimental data are in quantitative agreement with theory.