Abrupt enhancement of noncentrosymmetry and appearance of a spin-triplet superconducting state in Li₂(Pd_1-xPt_x) ₃B beyond x=0.8

We report synthesis, 195Pt, 11B, and 7Li NMR measurements, and first-principles band calculations for noncentrosymmetric superconductors Li₂(Pd_1-xPt_x)₃B (x=0, 0.2, 0.5, 0.8, 0.84, 0.9, and 1). For 0≤x≤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 195K decreases below T_c. For x=0.9 and 1.0, in contrast, 1/T₁ shows no coherence peak but a T3 variation and 195K 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 structure distortion that enhances the extent of inversion-symmetry breaking is primarily responsible for the pairing symmetry evolution. The insight obtained here provides a guideline for searching for noncentrosymmetric superconductors with a large spin-triplet component.