Two-phonon Absorption Spectra in the Layered Honeycomb Compound a-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.