Lawrence Berkeley National Laboratory

Resonant x-ray emission spectroscopy (RXES) was employed at the U LIII absorption edge and the Lα1 emission line to explore the 5f occupancy, nf, and the degree of 5f-orbital delocalization in the hidden-order compound URu2Si2. By comparing to suitable reference materials such as UF4, UCd11, and α-U, we conclude that the 5f orbital in URu2Si2 is at least partially delocalized with nf=2.87±0.08, and does not change with temperature down to 10 K within the estimated error. These results place further constraints on theoretical explanations of the hidden order, especially those requiring a localized f2 ground state.