The thermal, mechanical, and electronic performance of atomically thin semiconductors is governed by their low-energy phonons, yet the impact of atomic-scale disorder on these modes remains poorly understood. Here, we report the first measurement of acoustic phonon dispersions in a quasi-freestanding monolayer semiconductor (MoS2), using helium-3 spin-echo spectroscopy. We identify a defect-driven regime change at a critical wavevector, qc, marking the breakdown of continuum elastic behavior. At this length scale, the flexural mode transitions from continuum bending to defect-pinned standing waves, while the hybridized Rayleigh wave becomes vibrationally disordered in its dispersion and linewidth. We observe multiple defect-induced Van Hove singularities deep within the Brillouin zone and strongly suppressed acoustic group velocities, providing direct experimental evidence that four-phonon processes drive thermal transport in mono- and few-layer MoS2. These results offer a microscopic explanation for the anomalously low thermal conductivity widely observed in transition-metal dichalcogenides and demonstrate how atomic-scale disorder dictates energy flow in two-dimensional materials.
@article{arxiv.2503.14464,
title = {Defect-modified acoustic phonons in a single layer of MoS2},
author = {Aleksandar Radic and Boyao Liu and Akshay Rao and Sam Lambrick},
journal= {arXiv preprint arXiv:2503.14464},
year = {2026}
}