Electronic correlations associated with incipient magnetism have long been recognized as an important factor in stabilizing the largest atomic volume $\delta$ 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 $\delta$ phase reside in a pseudogapped state, accompanied by reductions in various physical properties below a characteristic temperature $T^\ast\approx$~100~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.

6 pages including 3 figure panels