On the analysis of the vibrational Boson peak and low-energy excitations in glasses

TL;DR

This paper critically examines how reduction procedures affect the analysis of the Boson peak in glasses, revealing distortions in the vibrational density of states and linking spectral changes to external stimuli and theoretical models.

Contribution

It provides a detailed comparison between excess vibrational density of states and the frequency-reduced Boson peak, highlighting how reduction procedures distort spectral features and how external stimuli influence these features.

Findings

Reduction procedures distort the symmetry of the excess vibrational density of states.

The frequency and intensity of the Boson peak are sensitive to external stimuli.

Changes in the Boson peak can be explained by modifications in the excess density of states.

Abstract

Implications of reduction procedures applied to the low energy part of the vibrational density of states in glasses and supercooled liquids are considered by advancing a detailed comparison between the excess - over the Debye limit - vibrational density of states g(w) and the frequency-reduced representation g(w)/w^2 usually referred to as the Boson peak. Analyzing representative experimental data from inelastic neutron and Raman scattering we show that reduction procedures distort to a great extent the otherwise symmetric excess density of states. The frequency of the maximum and the intensity of the excess experience dramatic changes; the former is reduced while the latter increases. The frequency and the intensity of the Boson peak are also sensitive to the distribution of the excess. In the light of the critical appraisal between the two forms of the density of states (i.e. the excess and the frequency-reduced one) we discuss changes of the Boson peak spectral features that are induced under the presence of external stimuli such as temperature (quenching rate, annealing), pressure, and irradiation. The majority of the Boson peak changes induced by the presence of those stimuli can be reasonably traced back to simple and expected modifications of the excess density of states and can be quite satisfactorily accounted for the Euclidean random matrix theory. Parallels to the heat capacity Boson peak are also briefly discussed.