From antiferromagnetic and hidden order to Pauli paramagnetism in U M ₂ Si ₂ compounds with 5 f electron duality

Abstract

Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U5felectrons 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 U5f2configuration with theΓ1(1)andΓ2quasi-doublet symmetry. The amount of the U 5f3configuration, however, varies considerably across the UM2Si2series, 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 inUPd2Si2andUNi2Si2, the availability of orbital degrees of freedom needed for the hidden order inURu2Si2to occur, as well as the appearance of Pauli paramagnetism inUFe2Si2. A unified and systematic picture of the UM2Si2compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centeredThCr2Si2structure.

Isoelectronic perturbations to f - d -electron hybridization and the enhancement of hidden order in URu 2 Si 2

“…As small amounts of Os are introduced into CeRu2−x Osx Si2, the Kondo temperature increases to TK ∼ 10 2 K for x = 0.1 (89,91). These changes in the hybridization of the Ce-4f and s, p, and d electrons across the CeFe2Si2, CeRu2Si2, and CeRu2−x Osx Si2 systems appear to be consistent with the changes in the 5f -d -electron hybridization in other reports (92) and with our calculations across the UM2Si2 series with M = Fe, Ru, and Os.…”

“…For these systems, there is little or no variation across the series in the number of d -band electrons near the Fermi energy EF that are available for hybridization. The degree of f -d -electron hybridization is largely dependent on the density of states at the Fermi level such that any significant variation in the number of d electrons near EF would have an effect on the hybridization (45,92). Furthermore, nonisoelectronic substitutions for Ru with elements such as Rh and Re that are effectively electron (or hole) donors would shift the Fermi level away from the degenerate crossing of the hybridized bands, thereby stabilizing the FS in the paramagnetic phase.…”

Magnetic Properties under Pressure in Novel Spin-Triplet Superconductor UTe2

“…41) Recently, the dual nature of 5f electrons in U compounds is nicely proved microscopically by high energy spectroscopy experiments. 42) A small valence change under pressure in UTe 2 has already been emphasized 19) and directly shown by x-ray spectroscopy. 27) A strong indication for the mixed valence state with a dominant 5f 3 (U 3+ ) configuration has been recently reported in a Core-level spectroscopy study.…”

“…For the 4f systems it is equivalent of the 4f 2 configuration, such as Pr 3+ . For UTe 2 , the dual localized and itinerant character of the 5f electrons 42,49) seems the origin of the observed drastic change of the magnetic anisotropy with pressure; at high temperature signature of the localized character is marked by the nice Curie-Weiss behaviors in the all pressure range. In summary, our experiments reveal that the anisotropic magnetic susceptibility at ambient pressure in UTe 2 changes to the quasi-isotropic behavior under pressure.…”

High-energy resolution X-ray spectroscopy at actinide M4,5 and ligand K edges: what we know, what we want to know, and what we can know

“…X-ray Raman scattering (XRS) spectroscopy or non-resonant inelastic X-ray scattering (NIXS) are the new methods to study low energy absorption edges using hard X-rays [120][121][122][123][124][125] . The use of hard Xrays makes this technique fundamentally interesting for the actinide science, since K edges of ligands can be probed by hard X-rays without vacuum conditions [126][127][128][129][130] . Moreover, studies of radioactive samples, sealed in the container, can benefit from the high penetration depth of the hard X-rays.…”