Dresden 2026 – scientific programme
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TT: Fachverband Tiefe Temperaturen
TT 25: f-Electron Systems
TT 25.4: Talk
Tuesday, March 10, 2026, 10:15–10:30, HSZ/0101
Observation of 5f valence fluctuations in UPd2Cd20 by high-resolution XAS — •Jindřich Kolorenč — Institute of Physics (FZU), Czech Academy of Sciences, Prague, Czech Republic
The regime of valence fluctuations, known from anomalous rare-earth systems, has not yet been identified by microscopic techniques in any uranium compound. It was only suggested in a few of them on the basis of thermodynamic measurements (such as the magnetic susceptibility) and their similarity to the behavior of rare-earth valence fluctuators. Such arguments, however, are hardly unequivocal. We present uranium L3 and M4 x-ray absorption spectra taken on UPd2Cd20, which is a heavy-fermion antiferromagnet. Although it has only one type of uranium site in its primitive cell, the spectra obtained in the high-energy-resolution fluorescence-detection mode display two edges, each associated with a different valence state, 5f2 and 5f3. Moreover, the shape of the M4 spectrum is well approximated by a combination of UCl3 (5f3) and UCl4 (5f2) spectra. This suggests 5f2↔5f3 fluctuations on a timescale longer than the characteristic time of the spectroscopy technique. Unlike 4f systems, in which such fluctuations typically lead to a destruction of magnetism due to one of the involved 4fn configurations being non-magnetic, the 5f2 and 5f3 states are both magnetic, allowing for a magnetic ordering in the fluctuating state.
This contribution is a result of a collaboration between Niigata University, Japan Atomic Energy Agency, Kyushu University, Japan Synchrotron Radiation Research Institute, Helmholtz-Zentrum Dresden-Rossendorf, ESRF in Grenoble, and Charles University in Prague.
Keywords: uranium; valence fluctuations; x-ray absorption; electronic structure calculations