Spin triplet superconducting state due to broken inversion symmetry in Li2Pt3B

We report B11 and Pt195 NMR measurements in noncentrosymmetric superconductor Li2Pt3B. We find that the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature Tc. With decreasing temperature (T) below Tc, the spin-lattice relaxation rate 1/T1 decreases with no coherence peak and is in proportion to T3. These results indicate that the Cooper pair is in the spin-triplet state and that there exist line nodes in the superconducting gap function. They are in sharp contrast to those in the isostructural Li2Pd3B which is a spin-singlet, s-wave superconductor, and are ascribed to the enhanced spin-orbit coupling due to the lack of spatial inversion symmetry. Our finding points to a new paradigm where exotic superconductivity arises in the absence of electron-electron correlations.