X-ray absorption branching ratio in actinides: LDA+DMFT approach

To investigate the X-ray absorption (XAS) branching ratio from the core 4d to valence 5f states, we set up a theoretical framework by using a combination of density functional theory in the Local Density Approximation and Dynamical Mean-Field Theory (LDA+DMFT), and apply it to several actinides. The results of the LDA+DMFT reduces to the band limit for itinerant systems and to the atomic limit for localized f electrons, meaning a spectrum of 5f itinerancy can be investigated. Our results provides a consistent and unified view of the XAS branching ratio for all elemental actinides, and is in good overall agreement with experiments. Copyright c © EPLA, 2009 Understanding the physics of elemental actinide solids is an important issue for many-body physics as well as for applications in nuclear power generation. In the early actinides, the f electrons behave as waves delocalized through the crystal, while in the late actinides, the f electrons behave as particles localized around each atom. Plutonium is near the localization-delocalization edge separating these two regimes. X-ray absorption (XAS) from the core 4d to the valence 5f in conjunction with atomic physics calculations has been a powerful probe of the evolution of the valence and the strength of the spinorbit coupling across the actinide series. A large number of spectroscopies have been applied to this problem. For example, photoemission spectroscopy of Pu, has revealed a multiple-peak structure in the occupied part of the density of states [1–3]. A combination approach of Local Density Approximation (LDA) and Dynamical Mean-Field Theory (LDA+DMFT) has allowed the interpretation of these features in terms of f electrons which are delocalized at low frequencies with a mixed valence electron count [4]. Other interpretations of Pu spectroscopies using LDA+DMFT with other impurity solvers have been presented recently [5–7]. High-energy probes such as electron energy-loss spectroscopy (EELS) or X-ray absorption spectroscopy (XAS) constitute a different set of spectroscopies which have been intensively used to study the electronic structure (a)E-mail: [email protected] of the actinide series [8–14]. These experimental works, combined with theoretical calculations exploiting a powerful sum rule, and the electronic structure of the atom has yielded valuable insights on the degree of localization of valence 5f electrons in actinides [8–10] as well as the spin-orbit strength [8,10,11]. Here we address the computation of the branching ratio within DMFT. There are several motivations for this study. 1) While the atomic multiplet approach of ref. [12] describes very successfully the majority of the data in the late actinides, it is restricted to the case that the f electrons are strictly localized. It is therefore useful to extend this approach by embedding it in a more general method that captures the itinerant limit as well. 2) Many other spectroscopies of the actinides such as photoemission spectroscopies are not well described by a localized model. LDA+DMFT provides a unified picture which reconciles the results of high-energy and low-energy spectroscopies. In physical terms, DMFT can describe the low-energy part of the excitations as itinerant, and the high-energy excitations as localized. Different spectroscopies weight different parts of the spectra. It is useful to incorporate the XAS branching ratio in the general LDA+DMFT formalism to achieve a unified interpretation of high-energy and low-energy spectroscopies. 3) The branching ratio can be expressed as a ratio of two quantities, one involving the spin-orbit coupling and the other involving the f occupation. Therefore a proper theoretical interpretation of the experiments requires the evaluation of these two