Universal origin of boson peak vibrational anomalies in ordered crystals and in amorphous materials

TL;DR

This paper presents a first-principles theory showing that the boson peak in vibrational spectra of both ordered crystals and amorphous materials arises from the interplay of elastic modes and damping, independent of disorder.

Contribution

It introduces a disorder-independent theoretical explanation for the boson peak, unifying observations in crystals and glasses based on elastic and viscous effects.

Findings

Explains boson peak in perfect crystals without disorder assumptions

Describes how vibrational spectra vary with atomic density

Matches recent experimental observations of vibrational anomalies

Abstract

The vibrational spectra of solids, both ordered and amorphous, in the low-energy regime, control the thermal and transport properties of materials, from heat capacity to heat conduction, electron-phonon couplings, conventional superconductivity etc. The old Debye model of vibrational spectra at low energy gives the vibrational density of states (VDOS) as proportional to the frequency squared, but in many materials the spectrum departs from this law which results in a peak upon normalizing the VDOS by frequency squared, which is known as the "boson peak". A description of the VDOS of solids (both crystals and glasses) is presented starting from first principles. Without using any assumptions whatsoever about the existence and nature of "disorder" in the material, it is shown that the boson peak in the VDOS of both ordered crystals and glasses arises naturally from the competition between elastic mode propagation and viscous damping. The theory explains the recent experimental observations of boson peak in perfectly ordered crystals, which cannot be explained based on previous theoretical frameworks. The theory also explains, for the first time, how the vibrational spectrum changes with the atomic density of the solid, and explains recent experimental observations of this effect.