准平面分子晶体的低温比热：最小无序晶体中的玻璃态异常源自何处？
Specific heat at low temperatures in quasiplanar molecular crystals: Where do glassy anomalies in minimally disordered crystals come from?
AI总结：
针对准平面分子晶体四氯间二甲苯的单斜晶系P2₁/c晶体，通过低温比热测量发现其最小取向无序引发玻璃态异常，揭示分子不对称性是关键因素，且玻色峰与德拜温度的线性关联在部分无序晶体中更显著。
摘要：
我们对四氯间二甲苯准平面分子形成的单斜晶系P2₁/c晶体进行了低温比热（Cp）测量。低温下冻结的不对称单元（由半个分子构成）的动态无序表现为绕垂直于苯环的类三重轴的重取向。这种最小无序引发了典型的玻璃态异常：Cp中存在归因于二能级系统的线性贡献，且Cp/T³（玻色峰）在6.6 K左右出现宽峰。我们将这些结果与其他具有不同平面内分子取向数的准平面分子晶体对比，发现二能级系统的密度与取向无序程度无关，而分子不对称性似乎在热异常中起关键作用；此外，玻色峰与德拜温度的线性关联适用于多数（非全部）结构玻璃，且在包括本研究晶体在内的部分无序晶体中更显著。
英文摘要：
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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