Degree of 5f electron localization in URu2Si2: Electron energy-loss spectroscopy and spin-orbit sum rule analysis

We examine the degree of 5f electron localization in URu2Si2 using spin-orbit sum rule analysis of the U N-4,N-5 (4d -> 5f) edge. When compared to alpha-U metal, US, USe, and UTe, which have increasing localization of the 5f states, we find that the 5f states of URu2Si2 are more localized, although not entirely. Spin-orbit analysis shows that intermediate coupling is the correct angular momentum coupling mechanism for URu2Si2 when the 5f electron count is between 2.6 and 2.8. These results have direct ramifications for theoretical assessment of the hidden order state of URu2Si2, where the degree of localization of the 5f electrons and their contribution to the Fermi surface are critical.