Two-phonon absorption spectra in the layered honeycomb compound α-RuCl₃

We performed infrared absorption spectroscopy for α-RuCl₃ with a two-dimensional honeycomb lattice of Ru³⁺ ions. In the mid-infrared range, we observed three absorption peaks corresponding to intra-atomic d–d transitions among Ru³⁺ ions in accordance with a previous report. In the far-infrared range, we observed conspicuous broad absorption structures near 600 cm⁻¹ in addition to several sharp single phonon peaks. Our first-principles calculations for the isolate sandwich model together with the factor group analysis revealed that the peak structures near 600 cm⁻¹ are absorptions related to a two-phonon process. Our results stimulate further study on the spin–lattice coupling in α-RuCl₃ as well as the optical detection of Majorana excitations in Kitaev spin liquids.