Microscopic investigation of enhanced Pauli paramagnetism in metallic Pu${}_2$C${}_3$

Abstract

A combined study of the structural and electronic properties of polycrystalline Pu${}_2$C${}_3$ is reported based on x-ray diffraction, specific heat, magnetic susceptibility, ${}^{13}$C nuclear magnetic resonance (NMR), and band structure calculations. X-ray diffraction reveals a global noncentrosymmetric cubic lattice, with a nearest-neighbor C--C bond length of $r=1.38$ Å. ${}^{13}$C 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 $\gamma =45$ mJ mol${}_{\mathrm{Pu}}^{-1}$ K${}^{-2}$, with a Wilson ratio $R_W\approx 1.3$ further indicating moderate correlations. ${}^{13}$C nuclear spin-lattice relaxation rate ($1/T_1$) and Knight shift ($K$) measurements find metallic Korringa behavior (i.e., $T_{1}T{K}^2=$ const.) with modest ferromagnetic spin fluctuations at low temperature. Taken together, the data point to a delocalized nature of a narrow 5$f$-electron band with weak electronic correlations. Density functional theory band-structure calculations confirm the appearance of such narrow 5$f$ bands near the Fermi level. Our data provide prime evidence for a plutonium-based metallic system with weak electronic correlations, which sheds new light on the understanding of complex paramagnetism in actinide-based metallic compounds.

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