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arXiv · 2412.03335

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

Daria Szewczyk·Manuel Moratalla·Grzegorz Chajewski·Jonathan F. Gebbia·Andrzej Jeżowski·Alexander I. Krivchikov·María Barrio·Josep Ll. Tamarit·Miguel A. Ramos
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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.

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Daria Szewczyk, Manuel Moratalla, Grzegorz Chajewski, Jonathan F. Gebbia, Andrzej Jeżowski, Alexander I. Krivchikov, María Barrio, Josep Ll. Tamarit, Miguel A. Ramos. 2024-12-04. Specific heat at low temperatures in quasiplanar molecular crystals: Where do glassy anomalies in minimally disordered crystals come from?. https://doi.org/10.1103/physrevb.110.174204

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