Duality of 5f electrons in UM2Si2

23. November 2020
The intricate interplay of band-formation and electron correlation effects in uranium heavy fermion compounds is subject of an ongoing debate. Here scientists from MPI CPfS in Dresden, University of Cologne, University of Erlangen, Heidelberg University, University of California at San Diego, Los Alamos National Laboratory, Institute of Low Temperature & Structure Research in Wroclaw, Van-der-Waals-Zeeman Institute in Amsterdam, and the German Synchrotron in Hamburg (PETRA-III/DESY) gained new and unexpected insight from advanced spectroscopies on isostructural members of the UM2Si2 family with different properties.

In many fascinating U intermetallic compounds, the intricate interplay of band formation and electron correlation effects of the U 5f electrons is an intellectual challenge that has not yet been mastered. The question is, should the modelling start with band theory or a local atomic approach? This has been particularly troublesome because neither the presence nor the symmetry of the active atomic-like states was so far experimentally accessible nor are there generally accepted numbers for the filling of the 5f shell.

A team of scientists from MPI CPfS in Dresden, University of Cologne, University of Erlangen, Heidelberg University, University of California at San Diego, Los Alamos National Laboratory, Institute of Low Temperature & Structure Research in Wroclaw, Van-der-Waals-Zeeman Institute in Amsterdam, and the German Synchrotron in Hamburg (PETRA-III/DESY) has carried out a comparative investigation of the electronic structure of isostructural members of the UM2Si2 family with very different ground state properties. The UM2Si2 compounds with M=Pd and Ni are antiferromagnets with large magnetic moments, that with M=Ru is the so-called hidden order compound, and that with M=Fe is a Pauli paramagnet.

The uranium O4,5 edge NIXS spectra show the same directional dependence (q||001 vs q||100) for the four compounds studied, meaning that they have the same ground state symmetry of the U 5f2 states.

Non-resonant inelastic x-ray scattering experiments beyond the dipole limit (an innovative and very unique spectroscopic method) reveal the presence of atomic-like multiplet states in all four compounds and that the compounds have the same singlet ground-state symmetry of the U 5f2 configuration in common. It is shown that a quasi-doublet consisting of the two singlets states G2 and G1 of majority Jz=|+4> + |-4>) can induce large-moment magnetism (M=Pd and Ni) as well as the hidden order. Bulk-sensitive hard x-ray photoemission shows a systematic increase of f-d hybridization and itineracy from M= Pd to Ni to Ru to Fe. Hence, local-moment systems (M=Ni, Pd) highlight the local atomic aspects of the electronic structure while Pauli paramagnets (M=Fe) rather reflect the importance of band formation but with bands that are strongly renormalized due to the remnant local aspect. URu2Si2, with its hidden order and superconductivity, is in-between these two regimes.

The above findings demonstrate the validity of the concept the dual nature of U 5f electrons for this set of U compounds. The experiments also explain why in URu2Si2 the hidden order disappears with pressure and why a large magnetic moment emerges.

AA, MS, AL-J, AS, LHT /CPfS

Hard X-ray photoemission spectroscopy reveals different 5f3 (delocalized) and 5f2 (localized)contributions along the UM2Si2 series (M=Pd, Ni, Ru, Fe). This reflects different degrees of f-d hybridization and allows to position the four compounds on the phase diagram.

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