Electronic Structure of Compressed Americium Metal
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Electronic Structure of Compressed Americium Metal Jindrich Kolorenc,1 Alexander B. Shick,1,2 and Roberto Caciuffo2 1 Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-18221 Praha, Czech Republic 2 European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, D-76125 Karlsruhe, Germany ABSTRACT We report a theoretical investigation of changes in the electronic structure of americium metal due to applied pressure. We employ a variant of the LDA+DMFT method that takes into account not only the correlations among the 5f electrons, but also the feedback of these correlations on the rest of the system by means of an appropriate adjustment of the electronic charge density. We observe only minor modifications of the electronic structure in the compressed lattice, which is in accord with recent resonant x-ray spectroscopy experiments. INTRODUCTION It is well established that the six 5f electrons in elemental americium are localized, reside in a non-magnetic 7 F0 state, and do not significantly contribute to the chemical bonds [1], at least at ambient conditions. The situation may be different under pressure, since compression induces a series of structural changes of the Am lattice [2], and such a complex phase diagram is rare for s, p or d bonded crystals. The Am phase diagram with all its four phases (Am-I through Am-IV) was successfully reconstructed using the local-density approximation combined with the dynamical-mean-field theory (LDA+DMFT) [3]. The calculations predicted an increased occupancy of the 5f states and a 5 f 6 ,5 f 7 intermediate-valence configuration was argued to appear in the compressed phases. This suggestion was tested with the aid of the resonant x-ray spectroscopy (RXES), which, however, found no apparent signs of the 5 f 7 admixture being induced by the applied pressure [4]. Arguably, these RXES measurements, being sensitive primarily to the 6d states, provide only an indirect probe of the 5f electrons, and hence the conclusions could not be as firm as would be desired. Nevertheless, the very same experimental technique has successfully detected the intermediate-valence configurations of the 4f electrons in a number of rare-earth compounds, for instance in SmS [5], which indicates that if there is any 5 f 7 weight in Am, it is only small. Recently, it was shown that the Am phase diagram is well described also by LDA supplemented with extra orbital-polarization terms [6], which does not predict any pressureinduced changes of the 5f valency. Nevertheless, this theory is not entirely satisfactory, since it does not recover the experimental photoemission (PE) spectra and, even more importantly, it leads to a spin-polarized ground state, which is difficult to reconcile with experimental data. Here we revisit the electronic structure of Am using a particular variant of the LDA+DMFT method [7]. We aim to visualize how strong effects of the intermediate-valence origin can be expected in the valence-band spectra, since, despite the conclus
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