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

TL;DR

This study reveals that in the ideal cubic compound K$_2$IrCl$_6$, the observed spectral features are due to vibronic effects and spin-orbital-lattice entanglement, not noncubic crystal fields, confirming stable j=1/2 moments.

Contribution

The paper demonstrates that vibronic coupling explains spectral features in K$_2$IrCl$_6$, challenging previous interpretations of noncubic crystal field effects in similar iridates.

Findings

Vibronic effects cause multi-peak spectral line shapes.

K$_2$IrCl$_6$ maintains cubic symmetry with stable j=1/2 moments.

Well-separated IrCl$_6$ octahedra enable clear vibronic sidebands.

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.