Mott insulators with spin-orbit entangled j=1/2 moments host intriguing magnetic properties. The j=1/2 wave function requires cubic symmetry, while a noncubic crystal field mixes j=1/2 and 3/2 character. Spectroscopic studies of 5d5 iridates typically claim noncubic symmetry, e.g., based on a splitting of the excited j=3/2 quartet. A sizable splitting is particularly puzzling in antifluorite-type K2IrCl6, a frustrated fcc quantum magnet with global cubic symmetry. It raises the fundamental question about the stability of j=1/2 moments against magneto-elastic coupling. Combining resonant inelastic x-ray scattering with optical spectroscopy, we demonstrate that the multi-peak line shape in K2IrCl6 reflects a vibronic character of the j=3/2 states rather than a noncubic crystal field. The quasimolecular crystal structure with well separated IrCl6 octahedra explains the existence of well-defined sidebands that are usually smeared out in solids. Our results highlight the spin-orbital-lattice entangled character of cubic K2IrCl6 with ideal j=1/2 moments.
@article{arxiv.2407.12133,
title = {Spin-orbital-lattice entanglement in the ideal j=1/2 compound K$_2$IrCl$_6$},
author = {P. Warzanowski and M. Magnaterra and Ch. J. Sahle and M. Moretti Sala and P. Becker and L. Bohatý and I. Císařová and G. Monaco and T. Lorenz and P. H. M. van Loosdrecht and J. van den Brink and M. Grüninger},
journal= {arXiv preprint arXiv:2407.12133},
year = {2025}
}