Acoustic phonon dispersion of alpha-RuCl3

Acoustic phonons have recently been posited as playing an integral role in explaining the half-quantized thermal Hall effect in alpha-RuCl3. Therefore, we present much needed inelastic x-ray scattering measurements of its acoustic phonon dispersion, along with calculations using the frozen-phonon method. We also discuss a temperature study which conclusively shows a first-order structural transition to a non-C2/m space group at low temperature. Together these results are an important backbone for future theoretical and experimental studies of alpha-RuCl3.