Abrupt enhancement of non-centrosymmetry and appearance of the spin-triplet superconducting state in Li_2(Pd_{1-x}Pt_{x})_3B beyond x=0.8

TL;DR

This study investigates how increasing Pt content in Li_2(Pd_{1-x}Pt_{x})_3B induces a transition from spin-singlet to spin-triplet superconductivity, linked to structural distortion and enhanced spin-orbit coupling.

Contribution

It reveals a critical composition beyond which the superconductor's pairing symmetry shifts, highlighting the role of local structural distortion and spin-orbit coupling in non-centrosymmetric superconductors.

Findings

Superconducting pairing changes from spin-singlet to spin-triplet at x=0.8.

Structural distortion increases abruptly above x=0.8.

Enhanced spin-orbit coupling correlates with pairing symmetry transition.

Abstract

We report synthesis, ^{195}Pt, ^{11}B and ^{7}Li NMR measurements, and first-principle band calculation for non-centrosymmetric superconductors Li_{2}(Pd_{1-x}Pt_{x})_{3}B (x=0, 0.2, 0.5, 0.8, 0.84, 0.9 and 1). For 0 \leq x \leq 0.8, the spin-lattice relaxation rate 1/T_1 shows a clear coherence peak just below T_c, decreasing exponentially at low temperature, and the Knight shift ^{195}K decreases below T_c. For x=0.9 and 1.0, in contrast, 1/T_1 shows no coherence peak but a T^3 variation and ^{195}K remains unchanged across T_c. These results indicate that the superconducting state changes drastically from a spin-singlet dominant to a spin-triplet dominant state at x=0.8. We find that the distortion of B(Pt,Pd)_6 increases abruptly above x=0.8, which leads to an abrupt enhancement of the asymmetric spin-orbit coupling as confirmed by band calculation. Such local structure distortion that enhances the extent of inversion-symmetry breaking is primarily responsible for the pairing symmetry evolution. The insight obtained here provides a new guideline for searching new NCS superconductors with large spin-triplet component.