Uranium $L_3$ and $M_4$ X-ray absorption spectra, taken in a high-energy resolution fluorescence detection mode on a heavy-fermion antiferromagnet ${\mathrm{UPd}}_2{\mathrm{Cd}}_{20}$, display two absorption edges, each associated with a different valence state, namely $5f^2$ and $5f^3$. Due to a single type of U position in the crystal structure it is strongly suggestive of slow $f^2\text{−}{f}^3$ fluctuations on a timescale longer than the characteristic time of the spectroscopy technique. Unlike $4f$ systems, in which such fluctuations lead to a destruction of magnetism due to the involvement of one nonmagnetic state, the $f^2$ and $f^3$ states in U are both possibly magnetic, allowing for magnetic ordering in the fluctuating state. A valence fluctuation state has been speculated in the past in the context of U intermetallic compounds, but it was not corroborated by a microscopic technique. This evidence can bring more light into emergent phenomena observable in U systems at the verge of $5f$ localization.

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• Electronic structure
• Exotic phases of matter
• Localization
• X-ray absorption spectroscopy