Alpha-plutonium's low-temperature elastic constants*

Using resonant-ultrasound spectroscopy, we measured alpha-plutonium's polycrystal elastic constants between 18 and 344 K. All elastic constants -- bulk, shear, extension, longitudinal moduli and Poisson ratio -- behave smoothly during cooling, indicating no significant phase transition: electronic, magnetic, or structural. Both principal elastic constants (bulk and shear) increase about 30{\%} upon cooling from 300 to 0 K, a large change among metals, which we attribute to 5$f$-electron delocalization. From the low-temperature elastic constants, we computed that the Debye temperature equals 205 K, exceeding significantly most previous estimates. From the bulk-modulus/temperature slope \textit{dB}/\textit{dT}, we computed that the Gruneisen parameter equals 5.1, intermediate among previous estimates using other approaches. Alpha-plutonium shows an unusually high shear-modulus/bulk-modulus ratio $G$/$B$, thus a low Poisson ratio: 0.18. Within 0.5{\%}, the Poisson ratio shows temperature invariance; its small negative slope being opposite expectation. Again, we attribute this exceptional behavior to 5$f$-electron localization.