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Hund\u2019s coupling, trigonal crystal-field splitting, and hopping for\n                    <jats:italic>a<\/jats:italic>\n                    <jats:sub>\n                      1\n                      <jats:italic>g<\/jats:italic>\n                    <\/jats:sub>\n                    and\n                    <jats:inline-formula>\n                      <jats:alternatives>\n                        <jats:tex-math>$${e}_{g}^{\\pi }$$<\/jats:tex-math>\n                        <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                          <mml:msubsup>\n                            <mml:mrow>\n                              <mml:mi>e<\/mml:mi>\n                            <\/mml:mrow>\n                            <mml:mrow>\n                              <mml:mi>g<\/mml:mi>\n                            <\/mml:mrow>\n                            <mml:mrow>\n                              <mml:mi>\u03c0<\/mml:mi>\n                            <\/mml:mrow>\n                          <\/mml:msubsup>\n                        <\/mml:math>\n                      <\/jats:alternatives>\n                    <\/jats:inline-formula>\n                    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\n                    <jats:sub>3<\/jats:sub>\n                    CeRu\n                    <jats:sub>2<\/jats:sub>\n                    O\n                    <jats:sub>9<\/jats:sub>\n                    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\n                    <jats:sub>3<\/jats:sub>\n                    CeRu\n                    <jats:sub>2<\/jats:sub>\n                    O\n                    <jats:sub>9<\/jats:sub>\n                    , both the crystal field and hopping lower the\n                    <jats:italic>a<\/jats:italic>\n                    <jats:sub>\n                      1\n                      <jats:italic>g<\/jats:italic>\n                    <\/jats:sub>\n                    orbitals. For spin-orbit coupling\n                    <jats:italic>\u03b6<\/jats:italic>\n                    = 0, Hund\u2019s coupling favors an\n                    <jats:italic>S<\/jats:italic>\n                    = 0 ground state for small hopping but\n                    <jats:italic>S<\/jats:italic>\n                    = 1 for realistic larger hopping. Finite\n                    <jats:italic>\u03b6<\/jats:italic>\n                    , even though small, finally yields a non-magnetic\n                    <jats:italic>J<\/jats:italic>\n                    = 0 state.\n                  <\/jats:p>","DOI":"10.1038\/s41535-026-00909-0","type":"journal-article","created":{"date-parts":[[2026,6,13]],"date-time":"2026-06-13T16:46:20Z","timestamp":1781369180000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Between Mott and cluster Mott: spin-orbit entangled dimer singlets in Ba3CeRu2O9"],"prefix":"10.1038","volume":"11","author":[{"given":"Lara","family":"P\u00e4tzold","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Anna","family":"Sandberg","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Henrik","family":"Schilling","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hlynur","family":"Gretarsson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Enrico","family":"Bergamasco","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Marco","family":"Magnaterra","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Petra","family":"Becker","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Paul H. 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