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 $T^{*}\approx 100\mathrm{K}$. 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.

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• Density of states
• Electronic structure
• Kondo effect
• Magnetic susceptibility