Pseudogap in elemental plutonium

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 rare earths and neighboring actinides periodic table has largely remained a mystery. We show here using calorimetry measurements, together prior detailed measurements phonon dispersion, that $5f$ electrons reside pseudogapped state, accompanied by reductions various physical properties below characteristic temperature ${T}^{*}\ensuremath{\approx}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The small energy scale state implies plutonium are much closer threshold for localization magnetic order than suggested state-of-the-art electronic structure theory, revealing be arguably most strongly correlated elements.