Delocalization and occupancy effects of 5f orbitals in plutonium intermetallics using L3-edge resonant X-ray emission spectroscopy

Abstract

Although actinide (An) L 3 -edge x-ray absorption near-edge structure (XANES) spectroscopy has been very effective in determining An oxidation states in insulating, ionically-bonded materials, such as in certain coordination compounds and mineral systems, the technique fails in systems featuring more delocalized 5f orbitals, especially in metals. Recently, actinide L 3 -edge resonant x-ray emission spectroscopy (RXES) has been shown to be an effective alternative. This technique is further demonstrated here using a parametrized partial unoccupied density of states method to quantify both occupancy and delocalization of the 5f orbital in α-Pu, δ-Pu, PuCoGa 5 , PuCoIn 5 , and PuSb 2 . These new results, supported by FEFF calculations, highlight the effects of strong correlations on RXES spectra and the technique's ability to differentiate between f -orbital occupation and delocalization.

Avoided valence transition in a plutonium superconductor

“…PuCoGa 5 exhibits no localized magnetic moments (6), and like other plutonium systems, its 5f electrons cannot be treated as fully localized or fully itinerant (4). [Strong Curie-Weiss-like magnetic susceptibility was initially reported for PuCoGa 5 , consistent with a local moment. However, additional susceptibility measurements (including polarized neutron scattering) have not reproduced this result (6).…”

“…These valence fluctuations disappear when the superconducting gap opens at T c , suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa 5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa 5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high T c of PuCoGa 5 .…”

“…These systems reside close to an antiferromagnetic quantum critical point (9), where antiferromagnetic fluctuations are believed to mediate unconventional superconductivity. Because there is no evidence for PuCoGa 5 being in proximity to a magnetically ordered state (10), it is unlikely that magnetic fluctuations are the primary driver of its anomalously high T c of 18.5 K (11). Valence fluctuationswhere the total number of f electrons per ion fluctuates with the conduction band or the configuration of a fixed number of f electrons fluctuates between two or more nearly degenerate f states (sometimes known as orbital fluctuations)-have been proposed as a possible alternative mechanism for superconductivity in several heavy-fermion systems (12)(13)(14)(15).…”

“…In some cases, this valence degeneracy becomes unstable, leading to valence fluctuations and ultimately, a transition to a different valence state as a function of temperature, pressure, and/or doping (3). X-ray and photoemission spectroscopy (4,5), neutron form factor measurements (6), and theoretical calculations (7) ) orbitals all residing near the chemical potential and all partially occupied. PuCoGa 5 exhibits no localized magnetic moments (6), and like other plutonium systems, its 5f electrons cannot be treated as fully localized or fully itinerant (4).…”

Pressure-Resistant Intermediate Valence in the Kondo InsulatorSmB6

“…The RXES intensity I 2D ðE i ; E t Þ is fit using the KramersHeisenberg formula for photon-atom scattering [33,34]:…”