Anomalous local spin susceptibilities in noncentrosymmetric La2 C3 superconductor

We report on La139 and C13 nuclear magnetic resonance (NMR) study of noncentrosymmetric superconductor La2C3 with superconducting critical temperature Tc=11.8 K. In the normal state La139 NMR spectra show a characteristic quadrupole polycrystalline broadening yielding a quadrupole frequency νQ=3.1 MHz and a very large shift of ∼3000 ppm. A significant part of this shift arises from the Van Vleck-like susceptibility as a result of a sizable spin-orbit coupling. In the superconducting phase La2C3 shows abnormal behavior that dramatically deviates from the conventional s-wave superconductors. Namely, (i) La139 and C13 NMR shifts in the superconducting state do not change from their normal-state values, and (ii) the La139 spin-lattice relaxation rate is strongly enhanced below Tc. Such unconventional response in the local static and dynamic spin susceptibilities is discussed in terms of possible mixture of spin-singlet and spin-triplet Cooper pairs, which is promoted by the asymmetric spin-orbit coupling in the system without the center of inversion.