Low-temperature specific heat of real crystals: Possibility of leading contribution of optical and short-wavelength acoustical vibrations

TL;DR

This paper suggests that optical and short-wavelength acoustic vibrations in real crystals can cause a linear temperature dependence in low-temperature specific heat due to damping effects from defects, challenging the usual association with localized excitations.

Contribution

It introduces the idea that damping of optical and short-wavelength vibrations by defects leads to a linear specific heat contribution, which can dominate at low temperatures.

Findings

Damped optical and short-wavelength vibrations produce linear T-specific heat.

The crossover temperature T* depends on defect concentration as √N.

This linear contribution may be observable in experiments.

Abstract

We point out that the repeatedly reported glass-like properties of crystalline materials are not necessarily associated with localized (or quasilocalized) excitations. In real crystals, optical and short-wavelength acoustical vibrations remain damped due to defects down to zero temperature. If such a damping is frequency-independent, e.g. due to planar defects or charged defects, these optical and short-wavelength acoustical vibrations yield a linear-in-$T$ contribution to the low-temperature specific heat of the crystal lattices. At low enough temperatures such a contribution will prevail over that of the long-wavelength acoustical vibrations (Debye contribution). The crossover between the linear and the Debye regime takes place at $T^* \propto \sqrt N$, where $N$ is the concentration of the defects responsible for the damping. Estimates show that this crossover could be observable.