Comparative Photoemission Study of Actinide (Am, Pu, Np and U) Metals, Nitrides, and Hydrides
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0986-OO01-02
Comparative Photoemission Study of Actinide (Am, Pu, Np and U) Metals, Nitrides, and Hydrides Thomas Gouder1, Alice Seibert1, Jean Rebizant1, Frank Huber1, and Ladislav Havela2 1 European Commission, JRC, Institute for Transuranium Elements, Karlsruhe, 76125, Germany 2 Charles University, Faculty of Mathematics and Physics, Prague, 121 16, Czech Republic
ABSTRACT Core-level and valence-band spectra of Pu and the other early actinide compounds show remarkable systematics, which can be understood in the framework of final state screening. We compare the early actinide (U, Np, Pu and Am) metals, nitrides and hydrides and a few other specific compounds (PuSe, PuS, PuCx, PuSix) prepared as thin films by sputter deposition. In choosing these systems, we combine inherent 5f band narrowing, due to 5f orbital contraction throughout the actinide series, with variations of the chemical environment in the compounds. Goal of this work was to learn more on the electronic structure of the early actinide systems and to achieve the correct interpretation of their photoemission spectra. The highly correlated nature of the 5f states in systems, which are on the verge to localization, makes this a challenging task, because of the peculiar interplay between ground state DOS and final-state effects. Their influence can be estimated by doing systematic studies on systems with different (5f) bandwidths. We conclude on the basis of such systematic experiments that final-state effects due to strong e-e correlations in narrow 5f-band systems lead to multiplet like structures, analogous to those observed in the case of systems with localized electron states. Such observations in essentially band-like 5f-systems was first surprising, but the astonishing similarity of photoemission spectra of very different chemical systems (e.g. PuSe, Pu2C3 and δ-Pu) points to a common origin, relating them to atomic features rather than material dependent density of states (DOS) features.
INTRODUCTION Study of the electronic structure of early actinides and their compounds is mainly targeted to the complex nature of the 5f states. Because of the proximity to the localizationdelocalization transition, strong electron-electron correlations represent an important energy scale [1]. This is especially true for Pu, which sits right at the localization threshold. Over the last 20 years, photoelectron spectroscopy has been used to obtain information on the density of states (DOS) of Pu [2, 3, 4, 5]. Yet, it is not a ground state technique, and in highly correlated systems, the actinide ion may be left in an excited state after the photoexcitation process and the spectral
function differs significantly from the ground state DOS [6]. Current interpretation of photoemission data is therefore controversial, and ranges from direct comparison with ground state DOS calculations [Ref. 7 and references therein] to approximated spectral density function in the DMFT calculation [1]. The understanding of actinides was not brought that far as e.g. for lanthanide
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