Microscopic investigation of enhanced Pauli paramagnetism in metallic Pu2C3

A combined study of the structural and electronic properties of polycrystalline Pu₂C₃ is reported based on x-ray diffraction, specific heat, magnetic susceptibility, and ¹³C nuclear magnetic resonance (NMR). X-ray diffraction reveals a global noncentrosymmetric cubic lattice, with a nearest-neighbor C--C bond length of $r=1.38$ {}. ¹³C NMR measurements indicate that the global cubic symmetry is locally broken, revealing two unique carbon environments. Magnetic susceptibility suggests enhanced Pauli paramagnetism, and specific heat reveals a moderately large electronic Sommerfeld coefficient γ=450.16em0exmJ0.16em0exmolPu^-10.16em0exK^-2, with a Wilson ratio RW≈1.3 further indicating moderate correlations. ¹³C nuclear spin-lattice relaxation rate (1/T₁) and Knight shift (K) measurements find metallic Korringa behavior (i.e., T₁TK²=const.) with modest ferromagnetic spin fluctuations at low temperature. Taken together, the data point to a delocalized nature of a narrow $5f$-electron band with electronic correlations. Our data provide prime evidence for a plutonium-based metallic system with electronic correlations, which sheds new light on the understanding of complex paramagnetism in actinide-based metallic compounds.