Low-energy optical phonons induce glassy-like vibrational and thermal anomalies in ordered crystals

Abstract

It is widely accepted that structural glasses and disordered crystals exhibit anomalies in their thermal, mechanical and acoustic properties as manifestations of the breakdown of the long-wavelength approximation in a disordered dissipative environment. However, the same type of glassy-like anomalies (i.e. boson peak in the vibrational density of states (VDOS) above the Debye level, peak in the normalized specific heat at T;10 K etc) have been recently observed also in perfectly ordered crystals, including thermoelectric compounds. Here we present a theory that predicts these surprising effects in perfectly ordered crystals as a result of low-lying (soft) optical phonons. In particular, it is seen that a strong boson peak anomaly (low-energy excess of modes) in the VDOS can be due almost entirely to the presence of low-energy optical phonons, provided that their energy is comparable to that of the acoustic modes at the Brillouin zone boundary. The boson peak is predicted also to occur in the heat capacity at low T. In presence of strong damping (which might be due to anharmonicities in the ordered crystals), these optical phonons contribute to the low-T deviation from Debye's T 3 law, producing a linear-in-T behavior which is typical of glasses, even though no assumptions of disorder whatsoever are made in the model. These findings are relevant for understanding and tuning thermal transport properties of thermoelectric compounds, and possibly for the enhancement of electron-phonon superconductivity.

Scale invariant solids

“…Our results suggest that as a generic property such a ratio decreases with increasing the dimensionless ratio T /m. We can think of the previous properties as the "melting" in our holographic system, which is very similar indeed to the phenomenology of amorphous solids and glasses as already hinted in previous literature [37,70,71].…”

“…The crossover is continuous and very analogous to what happens to certain extent in glasses and amorphous materials. The behaviour of the vibrational modes in these holographic systems has already produced important developments in the study of the latter [70,71].…”

Secondary relaxation in the terahertz range in 2-adamantanone from theory and experiments

“…Even more, we will demonstrate that the small differences in the VDOS between disordered M and ordered O phases are so subtle (just some optical modes shifted to lower energy for the disordered phase) that the dielectric susceptibility can be reproduced by using either of the two VDOS, despite the glassy features emerge due to the piling up of those optical modes (see also Ref. 35 ) at low-energy in the occupationally disordered M phase.…”

Zoology of solid & fluid holography — Goldstone modes and phase relaxation

“…It would be interesting now to try to use the framework discussed in a more phenomenological way to test the proposal of [74] explaining the electric transport properties of bad metals via the interplay of EXB and SSB and to investigate in more detail the relation between these models and the physics of the Boson peak and glassy dynamics [75][76][77]. These models could possibly be relevant for the study of phonons at quantum criticality, as recently discussed in the condensed matter community [78,79].…”