Sound modes broadening in quasicrystals

TL;DR

This paper presents a phenomenological model explaining vibrational mode broadening in quasicrystals, highlighting the role of optical mode hybridization and the limitations of traditional acoustic mode descriptions.

Contribution

It introduces a generalized Akhiezer mechanism for quasicrystals and analyzes mode broadening due to optical-acoustic hybridization at high wave vectors.

Findings

Optical modes in quasicrystals are nearly dispersionless.

Hybridization of acoustic and optical modes increases mode broadening.

Traditional acoustic mode descriptions break down at high wave vectors.

Abstract

We propose a simple phenomenological model to analyze vibrational characteristics of quasicrystals (QCs). The interpretation of the obtained recently data is based on the existence of almost dispersionless optical modes most probably related to the specific clusters which constitute the characteristic building blocks of any QC structure. We generalize to QCs the well - known Akhiezer mechanism, which in our case is related to a ''long wave'' disturbance of the quasicrystalline optical modes by the propagating sound modes. At higher wave vectors strong hybridization of acoustic and optical modes takes place, and it leads to a more steep broadening dependence on wave vectors, and besides the excitation can no longer be described as a single acoustic mode with a well defined wave vector.