We investigate the order parameter of noncentrosymmetric superconductors ${\mathrm{Li}}_2{\mathrm{Pd}}_3\mathrm{B}$ and ${\mathrm{Li}}_2{\mathrm{Pt}}_3\mathrm{B}$ via the behavior of the penetration depth $\lambda (T)$. The low-temperature penetration depth shows BCS-like behavior in ${\mathrm{Li}}_2{\mathrm{Pd}}_3\mathrm{B}$, while in ${\mathrm{Li}}_2{\mathrm{Pt}}_3\mathrm{B}$ 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 ${\mathrm{Li}}_2{\mathrm{Pd}}_3\mathrm{B}$, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in ${\mathrm{Li}}_2{\mathrm{Pt}}_3\mathrm{B}$ allows the spin-triplet component to be larger in ${\mathrm{Li}}_2{\mathrm{Pt}}_3\mathrm{B}$, 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.

• Received 20 December 2005

DOI:https://doi.org/10.1103/PhysRevLett.97.017006