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

Understand

It is widely accepted that structural glasses and disordered crystals exhibit anomalies in the 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 $T\simeq10 K$ 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.