Understanding the boson peak in glasses and glassy polymers

The boson peak is the ubiquitously observed peak at THz frequencies in the normalized vibrational density of states (DOS) of glasses. Its name is due to the fact that, in early Raman scattering experiments (where the Raman intensity is proportional to the DOS times the Bose-Einstein occupation factor), the temperature dependence of the peak in the Raman intensity displays the same temperature dependence as the Bose function. This observation induced some researchers, since those early days, to think that the boson peak oughts to be temperature-independent. As a result, the current theoretical paradigm holds that the boson peak originates from purely "harmonic" disorder-induced scattering of phonons, in spite of several other studies (including experimental investigations) showing evidence of a strongly "anharmonic" origin of the boson peak. We will show, using numerical simulations on polymer systems and metallic glasses, and theoretical arguments, that the boson peak is instead strongly temperature-dependent provided that one correctly accounts for the role of temperature and instantaneous normal modes in molecular simulations of glasses. I will also show that evidence from different angles and systems points to the anharmonic origin of the boson peak in glasses, while earlier statements about the "harmonic" origin of the boson peak are mislead by the strong temperature dependence of the Bose function in Raman scattering (which effectively obscures all other temperature dependencies) and by simulations of well-relaxed glasses in the inherent structures, where instantaneous normal modes are neglected and where, therefore, the boson peak appears as invariant with temperature.