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

TL;DR

This paper studies the superconducting order parameter in noncentrosymmetric Li$_2$Pd$_3$B and Li$_2$Pt$_3$B, revealing how broken inversion symmetry and spin-orbit coupling influence singlet-triplet pairing and gap structure.

Contribution

It demonstrates how antisymmetric spin-orbit coupling induces mixed pairing states, leading to different gap behaviors in these two superconductors.

Findings

Li$_2$Pd$_3$B shows BCS-like penetration depth behavior.

Li$_2$Pt$_3$B exhibits linear temperature dependence of penetration depth.

Theoretical models agree quantitatively with experimental data.

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 $\lambda(T)$. 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.