Manifestation of electron correlation effect in 5f states of uranium compounds revealed by 4d-5f resonant photoelectron spectroscopy

We have elucidated the nature of the electron correlation effect in uranium compounds by imaging the partial U 5f density of states (pDOS) of typical itinerant, localized, and heavy fermion uranium compounds by using the U 4d-5f resonant photoemission spectroscopy. The obtained U 5f pDOS exhibit a systematic trend depending on the physical properties of compounds. The coherent peak at the Fermi level can be described by the band-structure calculation, but an incoherent peak emerges on the higher binding energy side (less than or similar to 1 eV) in the U 5f pDOS of localized and heavy fermion compounds. As the U 5f state is more localized, the intensity of the incoherent peak is enhanced and its energy position is shifted to higher binding energy. These behaviors are consistent with the prediction of the Mott metal-insulator transition, suggesting that the Hubbard-U type mechanism takes an essential role in the 5f electronic structure of actinide materials.