Ultralow Energy Measurement of Surface Phonons in MoS2

We present sub-10 meV measurements of surface phonons in MoS2 with μeV resolution. Dispersion curves near the centre of the Brillouin zone were measured using the HeSE (Helium-3 Spin-Echo) technique and reveal the presence of vibrational modes previously thought to be limited to monolayer MoS2 on the surface of the bulk. The low beam energy (~8 meV) of the HeSE provides ultimate surface sensitivity, only allowing the probe particles to interact directly with the valence electron density of a sample.<br/><br/>Additionally, we present the current lowest energy measurements of surface phonons in MoS2 as a benchmark for theoretical calculations of its electronic and vibronic properties.