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Abstract:
The interplay of band-formation and electron-correlation effects in uranium heavy fermion compounds is the subject of an ongoing debate. Here unexpected insight has been gained from advanced spectroscopies on isostructural members of the UM2Si2 family with different properties. The antiferromagnetic (M = Pd,Ni), hidden order (M = Ru), and Pauli-paramagnetic (M = Fe) compounds all exhibit atomic-like U 5f2 multiplet states with singlet–singlet (quasi-doublet) symmetry while the U 5f3 weight increases from Pd→Ni→Ru→Fe, indicating increasing itineracy. This reveals the dual nature of the U-5f electrons throughout the family; the local aspects persist in the hidden-order compound URu2Si2 and surprisingly even in the highly itinerant Pauli paramagnet UFe2Si2. This study gives guidelines for the theoretical treatment of U intermetallic systems.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.All data are available upon request.
