Visual understanding of the hidden-order transition inURu2Si2by high-resolution x-ray Compton scattering

We report the change of electronic structure associated with the hidden-order (HO) transition in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ through Compton scattering experiment on the (001) plane of the single crystal. The two-dimensional electron occupation number densities (EONDs) obtained at 14 and 20 K, which are the projection of Fermi volume onto the ${k}_{x}\ensuremath{-}{k}_{y}$ plane of the first Brillouin zone, clearly reflect a whole image of electronic structure in the HO and paramagnetic (PM) phases, respectively. The change in electronic structure is well described by theoretical EONDs derived from the result of a band calculation, where $\mathrm{U}\phantom{\rule{0.28em}{0ex}}5f$ electrons are treated as itinerant ones. We also evaluate the electron (hole) numbers in the HO and PM phases and, therefore, its change on the HO transition. The HO transition is deeply involved with a significant change in $5f$ electrons from partially localized to itinerant states through hybridization with conduction electrons, resulting in the reconstruction of Fermi-surface structure.