"S-triplet" order in superconductors without inversion symmetry: Li$_2$Pd$_3$B and Li$_2$Pt$_3$B

Abstract

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 $\lambda(T)$. Broken inversion symmetry admits antisymmetric spin-orbit coupling that in turn admixes spin-singlet and spin-triplet pairing. The triplet contribution is weak in Li$_2$Pd$_3$B, a BCS-like superconductor with an anisotropic gap. With increased spin-orbit coupling the spin-triplet component dominates 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. Our findings demonstrate that unconventional superconducting properties (e.g., line nodes) can originate from a conventional phonon pairing mechanism without the need for other (electronic) coupling mechanisms.