Localization vs delocalization of 5f electrons in actinides: Dual nature and electron spectroscopies
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Localization vs delocalization of 5f electrons in actinides: Dual nature and electron spectroscopies
Gertrud Zwicknagl
Institut für Mathematische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
ABSTRACT Intermetallic compounds containing actinide ions exhibit a broad spectrum of different physical phenomena at low temperatures. The latter include heavy quasiparticles, unconventional superconductivity and various forms of magnetic ordering. The complex and sometimes enigmatic properties of these compounds derive from the strong correlations among the 5f electrons. Previous model calculations suggested that the intra-atomic Hund’s rule-type correlations may lead to partial localization which is reflected e. g. in the co-existence of itinerant 5f-derived heavy quasiparticles and local magnetic excitations. The conjectured "dual nature" of 5f electrons which is closely related to the question of the 5f valence of the actinide ions is not directly probed by ground state properties and the low-energy excitations. Here we present microscopic calculations for core-level spectroscopy emphasizing the consequences of strong intra-atomic correlations of the 5f shell. INTRODUCTION Actinide-based intermetallic compounds exhibit highly complex properties at low temperatures [1]. The anomalies observed in numerous observables originate from the strongly correlated electrons in the partially filled 5f shells of the actinide ions. The electronic properties of these materials are determined by the competition between the tendency towards delocalization of the 5f electrons via hybridization with extended conduction electrons and the tendency towards localization reflected in the formation of local moments. The central focus of the present study are the core-level spectra of U compounds. The key idea is to exploit the different screening behavior of localized and itinerant electrons to decide whether and to which extent the 5f -states can be considered as localized (atomic-like) or itinerant band-like. The theoretical work has been motivated by recent experiments [2] which showed a systematic variation of the spectra. Of particular interest is the question whether and how core-hole spectra can be used to identify the “dual character” of 5f electrons, i. e., the co-existence of both localized atomic-like and itinerant band-like 5f states [3, 4]. It has been shown that the dual model allows for a quantitative description of the renormalized
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quasi-particles - the heavy fermions - in UPd2 Al3 and UPt3 . The measured dH-vA frequencies for the heavy quasiparticle portions as well as the large anisotropic effective masses can be explained very well by treating two of the 5f electrons as localized [5, 6, 7, 8]. Experimentally, the co-existence of 5f-derived quasi-particles and local magnetic excitations in these compounds has been confirmed by recent neutron scattering and tunnelling experiments [9, 10]. At this point we would like to emphasize that the dual model for 5f electrons as sketched above
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