Journal Article

PUBDB-2026-01827

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Between Mott and cluster Mott: spin-orbit entangled dimer singlets in Ba$_3$CeRu$_2$O$_9$

Pätzold, L.Extern* ; Sandberg, A. ; Schilling, H.Extern* ; Gretarsson, H.DESY* ; Bergamasco, E.Extern* ; Magnaterra, M.Extern* ; Becker, P. ; van Loosdrecht, P. H. M.Extern* ; van den Brink, J.Extern* ; Hermanns, M. ; Grüninger, M. (Corresponding author)Extern*XFEL.EU*

2026
Nature Publishing Group [London]

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Please use a persistent id in citations: doi:10.1038/s41535-026-00909-0  doi:10.3204/PUBDB-2026-01827

Abstract: The hexagonal 4d ruthenates Ba$_3$MRu$_2$O$_9$ host structural dimers and exhibit a delicate balance of competing interactions. Hund’s coupling, trigonal crystal-field splitting, and hopping for a$_{1g}$ and e$^{\pi}_{g}$ orbitals all fall within a narrow energy window. This yields a series of possible ground states, ranging from the localized Mott limit with (anti-)ferromagnetic exchange coupling via orbital-selective behavior to the cluster Mott limit with quasimolecular orbitals that are delocalized over the two dimer sites. Using resonant inelastic x-ray scattering, we show that Ba$_3$CeRu$_2$O$_9$ with four holes per dimer resides in the intricate crossover regime between the localized Mott case and the quasimolecular limit. The spin-orbit entangled singlet ground state predominantly shows a Mott-like charge distribution with two holes per Ru site, but at the same time a dominant fraction of the holes occupies bonding orbitals. Furthermore, spin and orbital occupation contradict an exchange-based Mott scenario but agree with a cluster Mott approach. A quasimolecular trial wave function describes more than 70 % of the ground state. In this crossover regime, small changes of, e.g., the crystal field may strongly affect the character of electronic states. In Ba$_3$CeRu$_2$O$_9$, both the crystal field and hopping lower the a1g orbitals. For spin-orbit coupling ζ = 0, Hund’s coupling favors an S = 0 ground state for small hopping but S = 1 for realistic larger hopping. Finite ζ, even though small, finally yields a non-magnetic J = 0 state.

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Contributing Institute(s):

1. DOOR-User (DOOR ; HAS-User)
2. PETRA-S (FS-PETRA-S)

Research Program(s):

1. 632 - Materials – Quantum, Complex and Functional Materials (POF4-632) (POF4-632)
2. 6G3 - PETRA III (DESY) (POF4-6G3) (POF4-6G3)
3. FS-Proposal: I-20230838 (I-20230838) (I-20230838)

Experiment(s):

1. PETRA Beamline P01 (PETRA III)

Appears in the scientific report 2026

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