Pseudogap in elemental plutonium

Abstract

Electronic correlations associated with incipient magnetism have long been recognized as an important factor in stabilizing the largest atomic volume $\ensuremath{\delta}$ phase of plutonium, yet their strength compared to those in the rare earths and neighboring actinides in the periodic table has largely remained a mystery. We show here using calorimetry measurements, together with prior detailed measurements of the phonon dispersion, that the $5f$ electrons of the $\ensuremath{\delta}$ phase reside in a pseudogapped state, accompanied by reductions in various physical properties below a characteristic temperature ${T}^{*}\ensuremath{\approx}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The small characteristic energy scale of the pseudogapped state implies that the $5f$ electrons in plutonium are much closer to the threshold for localization and magnetic order than has been suggested by state-of-the-art electronic structure theory, revealing plutonium to be arguably the most strongly correlated of the elements.