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arXiv · 2407.12133

Spin-orbital-lattice entanglement in the ideal j=1/2 compound K$_2$IrCl$_6$

P. Warzanowski·M. Magnaterra·Ch. J. Sahle·M. Moretti Sala·P. Becker·L. Bohatý·I. Císařová·G. Monaco·T. Lorenz·P. H. M. van Loosdrecht·J. van den Brink·M. Grüninger
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Abstract

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 $5d^5$ 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 K$_2$IrCl$_6$, 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 K$_2$IrCl$_6$ reflects a vibronic character of the j=3/2 states rather than a noncubic crystal field. The quasimolecular crystal structure with well separated IrCl$_6$ 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 K$_2$IrCl$_6$ with ideal j=1/2 moments.

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P. Warzanowski, M. Magnaterra, Ch. J. Sahle, M. Moretti Sala, P. Becker, L. Bohatý, I. Císařová, G. Monaco, T. Lorenz, P. H. M. van Loosdrecht, J. van den Brink, M. Grüninger. 2024-07-16. Spin-orbital-lattice entanglement in the ideal j=1/2 compound K$_2$IrCl$_6$. https://doi.org/10.1103/physrevb.110.195120

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