The origin of the boson peak in network-forming glasses

An analysis using a simple model is presented with the aim of providing an explanation for the characteristics of low-energy excitations in glasses with directional bonds. The model predicts the appearance of three types of acoustic excitation: weakly, strongly, and mesoscopically localized modes, and, in addition, recovers the excess density of states at low energies. We claim that the latter two excitations constitute the two broad bands observed in Raman spectra, i.e. the lower band (the boson peak) consists of strongly localized modes, and mesoscopically localized modes contribute to the second higher band at around the Debye cut-off frequency omega(D).