Partial gap opening on the Fermi surface of the noncentrosymmetric superconductor Mo ₃Al ₂C

We report the results of 27Al nuclear magnetic resonance and specific-heat measurements of the noncentrosymmetric superconductor Mo ₃Al ₂C in the normal state. The phase transition at ∼130 K is inferred from a distinct change in the NMR spectrum, a clear peak in the spin-echo decay rate, and an anomaly in the specific heat. At ∼130 K, the Knight shift and the spin-lattice relaxation rate 1/T ₁ exhibit a significant decrease, which is associated with a reduction in the conduction electron density. These results suggest most probably charge-density wave ordering at ∼130 K. Furthermore, the decrease in 1/T ₁ below the superconducting transition temperature suggests that superconductivity emerges on the partially quenched Fermi surface.