From antiferromagnetic and hidden order to Pauli paramagnetism in UM₂Si₂ compounds with 5f electron duality

Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U 5f electrons in UM₂Si₂ (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U 5f² configuration with the Γ1⁽¹⁾ and Γ₂ quasi-doublet symmetry. The amount of the U 5f³ configuration, however, varies considerably across the UM₂Si₂ series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in UPd₂Si₂ and UNi₂Si₂, the availability of orbital degrees of freedom needed for the hidden order in URu₂Si₂ to occur, as well as the appearance of Pauli paramagnetism in UFe₂Si₂. A unified and systematic picture of the UM₂Si₂ compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered ThCr₂Si₂ structure.