Specific heat at low temperatures in quasiplanar molecular crystals: Where do glassy anomalies in minimally disordered crystals come from?

TL;DR

This study investigates the origin of glassy anomalies in minimally disordered quasiplanar molecular crystals through low-temperature specific heat measurements, revealing that molecular asymmetry influences thermal anomalies more than orientational disorder.

Contribution

It demonstrates that molecular asymmetry, rather than orientational disorder, is key to glassy anomalies in quasiplanar molecular crystals, and explores the correlation between the boson peak and Debye temperatures.

Findings

Density of two-level systems does not correlate with orientational disorder.

Molecular asymmetry influences the thermal anomalies.

Boson peak correlates with Debye temperature in many disordered crystals.

Abstract

We present low-temperature specific heat (Cp) measurements of a monoclinic P2_{1}/c crystal formed by quasiplanar molecules of tetrachloro-m-xylene. The dynamic disorder frozen at low-temperature of the asymmetric unit (formed by a half molecule) consists of reorientation around a three-fold-like axis perpendicular to the benzene ring. Such a minimal disorder gives rise to typical glassy anomalies, as a linear in contribution in Cp ascribed to two-level systems and a broad maximum around 6.6 K in Cp/T^3 (the boson peak). We discuss these results in the framework of other quasiplanar molecular crystals with different accountable number of in-plane molecular orientations We find that the density of two-level systems does not correlate with the degree of orientational disorder. Rather, it is the molecular asymmetry that seems to play a relevant role in the thermal anomalies. Furthermore, we discuss the suggested correlation between the boson peak and Debye temperatures. We find that a linear correlation between the boson peak and Debye temperatures holds for many -- but not all -- structural glasses and strikingly holds even better for some disordered crystals, including our studied quasiplanar molecular crystals.