Neutron scattering evidence on the nature of the boson peak

Abstract

Literature and unpublished neutron spectra of seven classical glass formers in the bosonpeak region are evaluated in terms of eigenvalue densities. The boson peak translates into atrue maximum of the eigenvalue density, lying about a factor of two higher than theboson 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 eigenvaluesto a more or less constant eigenvalue density at high eigenvalues. TheIoffe–Regel limit of strong sound wave damping lies at the crossover eigenvalueλc, slightly higher than the boson peak. A four-parameter fit form based on the soft-potentialmodel provides reasonable fits up to and including the beginning of the slowdecrease. The parameters from the neutron data agree within their error bars withthose determined from the low-temperature anomalies in the heat capacity andin the thermal conductivity. The results indicate that the strong scattering ofsound waves in glasses is due to the interaction with the excess vibrational modes.