Magnetic and spin-orbit exciton excitations in the honeycomb lattice compound RuBr3

Abstract

$\mathrm{Ru}{X}_{3}$ trihalides ($\mathit{X}=\text{halogen atoms}$) offer a prominent platform for exploring the Kitaev spin liquid. Using polarization-resolved Raman spectroscopy, we report a comparative study of lattice, magnetic, and electronic excitations on $\mathit{X}=\text{Cl}$ and Br. Our phonon Raman spectra show that ${\mathrm{RuBr}}_{3}$ retains a three-layer honeycomb lattice without structural transition, unlike $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ that undergoes a monoclinic-to-rhombohedral structural transition at $\ensuremath{\sim}150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition, both $\mathrm{Ru}{X}_{3}$ compounds commonly feature single and double spin-orbit excitons of almost identical energies, alluding to the realization of a ${j}_{\mathrm{eff}}=1/2$ state to the same degree. In contrast to $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ two-magnon scattering dominates over fractionalized excitations in ${\mathrm{RuBr}}_{3}$. Our results suggest that a Br-for-Cl substitution moves $\mathrm{Ru}{X}_{3}$ further away from a pure Kitaev phase despite enhanced $p\text{\ensuremath{-}}d$ hybridization.