Anomalous local spin susceptibilities in noncentrosymmetric La 2 C 3 superconductor

We report on $^{139}\mathrm{La}$ and $^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) study of noncentrosymmetric superconductor ${\mathrm{La}}_{2}{\mathrm{C}}_{3}$ with superconducting critical temperature ${T}_{\mathrm{c}}=11.8$ K. In the normal state $^{139}\mathrm{La}$ NMR spectra show a characteristic quadrupole polycrystalline broadening yielding a quadrupole frequency ${\ensuremath{\nu}}_{\mathrm{Q}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}3.1$ MHz and a very large shift of $\ensuremath{\sim}3000$ 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 ${\mathrm{La}}_{2}{\mathrm{C}}_{3}$ shows abnormal behavior that dramatically deviates from the conventional $s$-wave superconductors. Namely, (i) $^{139}\mathrm{La}$ and $^{13}\mathrm{C}$ NMR shifts in the superconducting state do not change from their normal-state values, and (ii) the $^{139}\mathrm{La}$ spin-lattice relaxation rate is strongly enhanced below ${T}_{\mathrm{c}}$. 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.