family with M=Fe, Pd, Ru, and Ni studied with x-ray spectroscopy*

We have carried out hard x-ray photoelectron spectroscopy (HAXPES) measurements at the U 4f core level and non-resonant inelastic x-ray scattering (NIXS) experiments at the U O_4,5 edge of UM₂Si₂ compounds that all form in the tetragonal ThCr₂Si₂ structure but exhibit different ground state properties: UFe₂Si₂ is a Pauli paramagnet; URu₂Si₂ is the famous hidden order compound [1]; UPd₂Si₂ and UNi₂Si₂ are antiferromagnets with T_N well above 100 K and sizeable ordered magnetic moments [2].
We have determined the degree of the 5f-electron localization (HAXPES [3]) as well as the symmetry of the 5f ground state wave function (NIXS [4] across these very different compounds. The possibility of singlet order in the Pd and Ni compounds, the dual nature f-electrons, as well as an effective Doniach phase diagram will be discussed.
[1] see e.g. references in J.A. Mydosh and P.M. Oppeneer in Rev. Mod. Phys. 83, 1301{1322 (2011) and in Phil. Magazine 94, 3642 (2014).
[2] see e.g. references in T. Endstra et al, PRB 48, 9595 (1993)
[3] see e.g. M. Sundermann et al, J. Elec. Spec. & Rel. Phen. 209, 1 (2016)
[4] see e.g. M. Sundermann et al. PNAS 113, 13989 (2016)