六方晶系的同位素效应：基于氘相关运动的磁性证据
Isotope Effect on the Magnetic Properties of Hexamethylbenzene: Evidence of Magnetism Based on Correlated Motion of Deuterons
摘要：
最近发现，六元环C $_6$（CH $_3$）$_6$中甲基的周期性旋转运动的相关磁矩对其整体磁化率有贡献。然而，这些测量仅在含氢多晶样品上进行。我们报道了在所施加的磁场平行和垂直于分子基面的情况下，单晶C $_6$（CH $_3$）$_6$的磁化率测量。在前一种情况下，亚稳态行为附近的起始温度T $_{C-H}$ =118 K被确定，而在后者中，没有观察到异常的行为。在氘代六亚甲基C $_6$（CD $_3$）$_6$（其中D是氘）中观察到类似的异常，然而，T $_{C-D}$在132 K时出现14 K以上。此外，在42 K附近发现的峰值异常被认为是由于氘核相干量子隧穿的发生。近立方基态被认为是在分子固体中发生的更激进形式的Jahn-Teller效应的结果，其中晶格扭曲以去除质子的轨道简并以降低其能量。明显的磁各向异性和同位素效应提供了证据，除了电子，不仅质子，而且氘核建立强相关行为。
英文摘要：
The associated magnetic moments of the periodic rotational motion of methyl groups in hexamethylbenzene C$_6$(CH$_3$)$_6$ were recently identified to contribute to its overall magnetic susceptibility. Those measurements however, were only performed on hydrogenous polycrystalline samples. We report magnetic susceptibility measurements on single crystalline C$_6$(CH$_3$)$_6$ in the cases where the applied magnetic field is parallel and perpendicular to the molecular basal planes. In the former case, metastable behavior near the onset temperature T$_{C-H}$=118 K is identified while in the latter, no anomalous behavior is observed. Similar anomalies are observed in deuterated hexamethylbenzene C$_6$(CD$_3$)$_6$ (where D is deuterium), however, T$_{C-D}$ occurs 14 K higher at 132 K. In addition, a peak anomaly identified near 42 K is suggested to be due to the onset of coherent quantum tunneling of deuterons. The near cubic ground state is proposed to be the result of a more radical form of the Jahn-Teller effect occurring in a molecular solid where the lattice distorts to remove the orbital degeneracies of the protons to lower its energy. The apparent magnetic anisotropy and isotope effect provide evidence that apart from electrons, not only protons, but also deuterons establish strongly correlated behavior.
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