Abstract

Understanding lattice vibrations in two-dimensional (2D) materials is essential for controlling thermal transport, mechanical response, and energy dissipation in nanoscale devices. However, several low-energy phonon modes in hexagonal boron nitride (hBN) remain inaccessible using conventional probes. Here we use helium-3 spin-echo spectroscopy to directly resolve the low-energy phonons at the surface of hBN. We measure the dispersions of the flexural and hybrid Rayleigh wave modes and extract the mechanical properties of a quasi-freestanding hBN monolayer. We simultaneously observe multiple surface-confined interlayer shear modes whose frequencies agree closely with theoretical predictions, and determine their intrinsic linewidths, revealing an order-of-magnitude reduction in phonon lifetimes with few-layer confinement. Finally, we show that the amplitudes of the observed phonon modes can be used to perform non-perturbative thermometry based on their equilibrium (Bose-Einstein) occupations. These results demonstrate the ability of helium atom scattering to access optically silent vibrational modes and provide a direct route to measuring intrinsic phonon dynamics in two-dimensional materials.