Observation of the boson peak in a two-dimensional material

The boson peak is an excess in the phonon vibrational density of states relative to the Debye model. It has been observed in a wide range of amorphous materials, from inorganic glasses to polymers. Two-dimensional random matrix models and molecular dynamics simulations predict that the boson peak should also be present in amorphous two-dimensional materials, a notion that is of practical importance because it leads to an excess of heat capacity and influences transport properties. However, up until now, experimental observations in actual materials have not been possible due to the limited surface sensitivity of the methods usually applied to measure the boson peak. Here we present the experimental evidence of a boson peak in two-dimensional silica, through phonon spectra measured by means of inelastic helium-atom scattering. We identify the boson peak as a wavenumber-independent spectral maximum at a frequency similar to what has been observed in and predicted for bulk vitreous silica. Furthermore, we present a heterogeneous-elastic theory calculation in two dimensions, which shows how the vibrational coupling of the transversal and flexural shear vertical phonon modes produces the boson peak in two-dimensional materials at a frequency similar to that of the bulk, in agreement with our measurements. The boson peak refers to an excess in the phonon density of states seen in three-dimensional amorphous materials. Helium-atom scattering experiments have now revealed a boson peak in a two-dimensional material, too, at a frequency similar to that of the bulk material.

Automated summary

This study presents experimental evidence of a boson peak in two-dimensional silica and shows that the vibrational coupling of phonon modes produces the boson peak in two-dimensional materials at a frequency similar to that of the bulk. The boson peak has been observed in three-dimensional amorphous materials, and now it is also found in a two-dimensional material.