Molecular orbital magnet with strong spin orbit coupling

The microscopic understanding of magnetism in a solid state system begins with an understanding of its smallest magnetic units. With well-separated magnetic ions, this entails understanding how local magnetic moments arise from atomic energy scales such as Hund's coupling, crystal electric field and spin orbit interaction (SOI). The much smaller electronic hopping between magnetic ions is usually treated perturbatively and manifests as weak exchange interactions between local moments. However, this simple recipe breaks down when the inter-site hopping becomes comparable to the atomic energy scales. A canonical example is the 6H-perovskite consisting of Ir/Ru-dimers with face-sharing octahedra. The reduced distance between magnetic ions with a face sharing geometry greatly increases hopping and necessitates a molecular orbital description of the local magnetism. As I will review in this talk, Ir dimer systems with a 6H-perovskite structure have been studied extensively with spectroscopic techniques, with significant progress made in quantifying the strength of hopping compared to other local energy scales. On the other hand, a microscopic understanding of the 4d 6H-perovskite dimer systems remains completely elusive due to a lack of suitable probes. With recent advances in Ru-edge RIXS, we revisit this problem by directly measuring the electronic excitations in a number of Ru dimer systems with varying structure and valence. Similar to the Ir case, we confirmed that hopping is significant and cannot be ignored in the Ru dimers. More interestingly, we find that their electronic ground states are much more tunable with an overall reduction in atomic energy scales. Such tunability directly manifests as an electronic phase transition through the Ba3LnRu2O9 (Ln=In,Y,La) series with only a slight change in crystal structure which we directly observe using Ru M-edge RIXS. This work is expected to open up new directions in the study of molecular magnets with strong SOI.