Spin-Orbit Coupling Versus Exchange Interaction in Actinide Metals
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1104-NN06-01
Spin-Orbit Coupling versus Exchange Interaction in Actinide Metals Gerrit van der Laan1,2, and Kevin Thomas Moore3 1 Diamond Light Source, Didcot, OX11 0DE, United Kingdom 2 Daresbury Laboratory, Warrington, WA4 4AD, United Kingdom 3 Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA, 94550 ABSTRACT The electronic structure of the actinide metals, Th, U, Np, Pu, Am, and Cm, is investigated using electron energy-loss spectroscopy (EELS) in a transmission electron microscope, together with many-electron spectral calculations. At the N4,5 edge, sum rule analysis gives the angular part of the spin-orbit interaction per hole, showing that while light metals (Th and U) follow LS coupling, heavier metals (Pu, Am, and Cm) follow intermediate coupling of the 5f states. The intermediate coupling is near the jj limit for Pu and Am, but strongly shifted towards the LS coupling limit for Cm. At the O4,5 edge many-electron spectral calculations show that the prepeak corresponds to a “forbidden” transition. INTRODUCTION Actinide materials are becoming increasingly important for energy and industry, particularly in next-generation nuclear reactors. Despite this rising awareness, there is a gaping lack in the general knowledge of the fundamental physics and materials science of many of these materials, even for the elemental metals. For accurate modeling of the behavior of these materials, a better understanding of the basic aspects of actinide metals, alloys, and materials is required through experiment and theory [1]. We discuss here the fundamental issue of the filling of the j = 5/2 and 7/2 levels in the 5f state, i.e., the type of angular-momentum coupling, across the series [2,3]. To this end, the electronic and magnetic structure of actinide metals, Th, U, Np, Pu, Am, and Cm, is investigated using electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) [4] and compared with many-electron spectral calculations. The N4,5 (4d → 5f ) and O4,5 (5d → 5f ) EELS edges are both of interest as they supplement each other particularly well. While at the N4,5 edge the core level spin-orbit interaction is much larger than the core-valence interaction, the relative magnitude of these two interactions is reversed at the O4,5 edge. A sum rule relates the branching ratio of the N4,5 edge in EELS, or x-ray absorption spectroscopy (XAS), to the expectation value of the angular part of the 5f spin-orbit interaction per hole [5]. Analysis of the branching ratio in the actinide metals gives an interesting trend for the spin-orbit interaction as a function of the f-count (number of 5f electrons per atom). It shows that while light metals (Th and U) follow LS coupling, heavier metals (Pu, Am, and Cm) obey intermediate coupling of the 5f states [6]. This intermediate coupling is near the jj limit for Pu and Am, but is strongly shifted towards the LS coupling limit for Cm [7].
The O4,5 EELS structure of the α phase of Th, U, Np, Pu, Am, and Cm metals exhibits a
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