Anomalous Spin-lattice Relaxation in Plutonium Carbide probed by NMR

Actinide carbides have attracted technological interest in the search for a good nuclear fuel suitable for high burnup and efficient operation, and the understanding of mixed U-Pu carbide fuel offers an important step forward.^[1] The most stable Pu-carbide compound Pu₂C₃ provides a good starting point for this. It is a paramagnet with cubic symmetry and does not order magnetically above liquid helium temperature.^[2] To date, there is only a few experimental reports on this compound,^[2-4] and its electronic properties are not fully understood. Here, we present our experimental investigation on Pu₂C₃ using nuclear magnetic resonance (NMR), X-ray diffraction, heat capacity, and resistivity. Our results demonstrate a heavy-fermion nature with well-defined Fermi-liquid behavior below 10 K. An anomalous peak appears in the ¹³C nuclear spin-lattice relaxation rate 1/T₁T at ~25 K that is accompanied by the appearance of a subtle shoulder in the powder NMR spectrum. NMR measurements as a function of magnetic field have been performed in an attempt to elucidate the origin of this feature.

[1] D. Manara, FR. Agarwal, Comprehensive Nuclear Materials 7, 155 (2020).

[2] G. Raphael, C. de Novion, Solid State Commun. 7, 791 (1969).

[3] J. L. Green, G. P. Arnold, J. A. Leary, N. G. Nereson, J. Nucl. Mat. 34, 281 (1970).

[4] T. Gouder, L. Havela, A. B. Shick, F. Huber, Physica B 403, 852 (2008).

Email: mhirata@lanl.gov