We report helium diffraction from graphene grown by chemical vapour deposition (CVD) using copper foil. This method reveals acoustic phonons, which are physically important to thermal conductance as well as a sensitive probe of graphene's interactions with the underlying substrate. Helium diffraction is made possible by the high quality of graphene produced by a recently reported "peel-off method". The graphene lattice parameter was found to remain constant in the temperature range between 110 and 500 K. The measured parabolic dispersion of the flexural mode along G M allows determining the bending rigidity k ¼ (1.30 ± 0.15) eV, and the grapheneeCu coupling strength g ¼ (5.7 ± 0.4) Â 10 19 N/ m 3. Unlike analytics employing atomic resolution microscopy, we obtain information on the atomic-scale quality of the graphene over mm length scales, suggesting the potential for Helium atom scattering to become an important tool for controlling the quality of industrially produced graphene.

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We have determined the electron–phonon interaction in type II Dirac semimetallic 1T-PdTe2 by means of helium atom scattering. While 1T-PdTe2 is isostructural with 1T-PtTe2, only the former is superconductor. The difference can be traced to the substantially larger value of the electron–phonon coupling in 1T-PdTe2, λ = 0.58, obtained from the Debye-Waller attenuation of the He specular peak. With this value and the surface Debye temperature, ΘD = 106.2 K, we have figured out the superconducting critical temperature, Tc = 1.83 K given by the BCS theory, which is in good agreement with Tc = (1.95 ± 0.03) K obtained with low-temperature scanning tunneling microscopy. The value of the effective mass related to ΘD indicates that the large electron–phonon coupling in 1T-PdTe2 is due to coupling, not only with the zone-center optical mode O2 at 9.2 meV, as proposed in a recent theoretical study, but also with the zone-boundary acoustic mode LA. Our results suggest that the topological states of a Dirac cone play a negligible role on the onset of superconductivity.