TRR 360 Seminar:

RIXS interferometry: cluster Mott insulators and bond-directional excitations in Kitaev materials

Markus Grüninger

RIXS interferometry: cluster Mott insulators and bond-directional excitations in Kitaev materials

Prof. Dr. Markus Grüninger

Universität zu Köln

Cluster Mott insulators provide a platform for novel types of quantum magnets with unconventional magnetic moments. Electrons are localized on a cluster such as a dimer and occupy quasimolecular cluster orbitals. In cluster Mott insulators, RIXS interferometry reveals the symmetry and character of excited states, offering a powerful tool to study the electronic structure. We systematically investigate the quasimolecular properties of dimers, trimers, and tetrahedra in the 5d transition-metal compounds Ba₃MIr₂O₉, Ba₄NbIr₃O₁₂, and GaTa₄Se₈. Furthermore, we discuss the applicability of the cluster picture for 3d GaV₄S₈.

RIXS interferometry can also be applied to Kitaev materials such as Na₂IrO₃ and α-Li₂IrO₃ and shows distinctive fingerprints of Kitaev physics. Kitaev exchange restricts spin-spin correlations to nearest neighbors, giving rise to a dimer-like dynamical structure factor. Bond-directional exchange implies the existence of bond-directional excitations. Making use of polarization selection rules, RIXS reveals this bond-directional character of magnetic excitations in Na₂IrO₃.