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

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18. November 2020

Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U $5 f$ electrons in U $M_{2} {S i}_{2}$ (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 $5 f^{2}$ configuration with the $Γ_{1}^{(1)}$ and $Γ_{2}$ quasi-doublet symmetry. The amount of the U 5 $f^{3}$ configuration, however, varies considerably across the U $M_{2} {S i}_{2}$ 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 ${U P d}_{2} {S i}_{2}$ and ${U N i}_{2} {S i}_{2}$ , the availability of orbital degrees of freedom needed for the hidden order in ${U R u}_{2} {S i}_{2}$ to occur, as well as the appearance of Pauli paramagnetism in ${U F e}_{2} {S i}_{2}$ . A unified and systematic picture of the U $M_{2} {S i}_{2}$ 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 ${T h C r}_{2} {S i}_{2}$ structure.

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