SFB colloquium Grüninger

Double spin-orbit excitons and spin-orbit double excitons in alpha-RuCl₃ and Na₂IrO₃

Spin-orbit-assisted d⁵ Mott insulators with local j=1/2 moments are discussed as realizations of the Kitaev model which hosts Majorana fermion excitations. In promising candidate materials such as RuCl₃ and Na₂IrO₃, lattice distortions give rise to a mixing of j=1/2 and 3/2 character which may spoil the picture. For Na₂IrO₃, an alternative description in terms of quasimolecular orbitals with electrons delocalized over individual Ir6 rings has been suggested, challenging the j=1/2 scenario.

Using infrared and Raman spectroscopy, we address the character of the relevant states by studying the so-called spin-orbit exciton (i.e. excitations to the j=3/2 state) as well as excitations across the band gap. These excitations are directly connected to the microscopic processes relevant for the Kitaev coupling.  In RuCl₃, we observe single as well as multiple spin-orbit excitons, challenging the electronic structure reported in the literature. In Na₂IrO₃, the small spectral weight in the vicinity of the gap strongly supports the j=1/2 scenario. Moreover, we find a mixing of the spin-orbit exciton with electron-hole excitations, giving rise to a spin-orbit-exciton-related exciton.