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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 117; no. 48; pp. 30220 - 30227
• Main Authors: Amorese, Andrea, Sundermann, Martin, Leedahl, Brett, Marino, Andrea, Takegami, Daisuke, Gretarsson, Hlynur, Gloskovskii, Andrei, Schlueter, Christoph, Haverkort, Maurits W., Huang, Yingkai, Szlawska, Maria, Kaczorowski, Dariusz, Ran, Sheng, Maple, M. Brian, Bauer, Eric D., Leithe-Jasper, Andreas, Hansmann, Philipp, Thalmeier, Peter, Tjeng, Liu Hao, Severing, Andrea
• Format: Journal Article
• Language: English
• Published: United States 01.12.2020
• Subjects: uranium heavy fermions; X-ray spectroscopy; hidden order; strongly correlated electron systems
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2005701117

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 U M 2 S i 2 family with different properties. The antiferromagnetic ( M = Pd,Ni), hidden order ( M = Ru), and Pauli-paramagnetic ( M = Fe) compounds all exhibit atomic-like U 5 f 2 multiplet states with singlet–singlet (quasi-doublet) symmetry while the U 5 f 3 weight increases from Pd → Ni → Ru → Fe, indicating increasing itineracy. This reveals the dual nature of the U-5 f electrons throughout the family; the local aspects persist in the hidden-order compound U R u 2 S i 2 and surprisingly even in the highly itinerant Pauli paramagnet U F e 2 S i 2 . 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 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 5 f 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.