Neutron scattering evidence on the nature of the boson peak

Literature and unpublished neutron spectra of seven classical glass formers in the boson peak region are evaluated in terms of eigenvalue densities. The boson peak translates into a true maximum of the eigenvalue density, lying about a factor of two higher than the boson peak eigenvalue and followed by a slow decrease towards higher eigenvalues. We interpret the data in terms of a crossover from sound waves at low eigenvalues to a more or less constant eigenvalue density at high eigenvalues. The Ioffe–Regel limit of strong sound wave damping lies at the crossover eigenvalue λ c , slightly higher than the boson peak. A four-parameter fit form based on the soft-potential model provides reasonable fits up to and including the beginning of the slow decrease. The parameters from the neutron data agree within their error bars with those determined from the low-temperature anomalies in the heat capacity and in the thermal conductivity. The results indicate that the strong scattering of sound waves in glasses is due to the interaction with the excess vibrational modes.