In a supercooled liquid, the crossover temperature $T_c$ separates a high-temperature region of diffusive dynamics from a low-temperature region of activated dynamics. A molecular-dynamics simulation of all-atom, flexible $o$-terphenyl [Eastwood et al., J. Phys. Chem. B 117, 12898 (2013)] is analyzed with advanced statistical methods to reveal the molecular features associated with this crossover. The simulations extend to an α-relaxation time of 14 μs (272.5 K), two orders of magnitude slower than at $T_c$ (290 K). At $T_c$ and below, a distinct state emerges that immediately precedes an orientational jump. Compared to the initial, tightly caged state, this jump-precursor state has a looser cage, with solid-angular excursions of 0.054–0.0125 × 4π sr. At $T_c$ (290 K), rate heterogeneity is already the dominant cause of stretched relaxation. Exchange within the distribution of rates is faster than α relaxation at $T_c$, but becomes equal to it at the lowest temperature simulated (272.5 K). The results trend toward a recent experimental observation near the glass transition (243 K) [Kaur et al., Phys. Rev. E 98, 040603(R) (2018)], which saw exchange substantially slower than α relaxation. Overall, the dynamic crossover comprises multiple phenomena: the development of heterogeneity, an increasing jump size, an emerging jump-precursor state, and a lengthening exchange time. The crossover is neither sharp, nor a simple superposition of the high- and low-temperature regimes; it is a broad region that contains unique and complex phenomena.

中文翻译：

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在过冷的邻三联苯中，跃变前体态出现在交叉温度以下

在过冷液体中，交叉温度 ${Ť}_C$将扩散动力学的高温区域与激活动力学的低温区域分隔开。全原子，柔性的分子动力学模拟$Ø$-三苯基[Eastwood等。，J。Phys。化学 乙 117，12898（2013）]进行分析以先进的统计方法来揭示与此交叉相关的分子特征。模拟扩展到了14μs（272.5 K）的α松弛时间，比在γ-松弛时间慢了两个数量级。${Ť}_C$（290 K）。在${Ť}_C$在下方和下方，出现了一个独特的状态，该状态紧接在定向跳跃之前。与初始的紧笼状态相比，该跃变前体状态的笼子更松散，实心角偏移为0.054–0.0125×4πsr。在${Ť}_C$（290 K），速率异质性已经是拉伸松弛的主要原因。利率分布内的交换比α松弛时快${Ť}_C$，但在模拟的最低温度（272.5 K）下变为等于它。结果趋向于在玻璃化转变温度（243 K）附近进行最近的实验观察[Kaur等。，物理 启ë 98，040603（R）（2018）]，它锯交换基本上慢于α松弛。总的来说，动态交叉包含多种现象：异质性的发展，跳跃大小的增加，跳跃前体状态的出现以及交换时间的延长。交叉既不是尖锐的，也不是高温和低温状态的简单叠加。它是一个包含独特而复杂的现象的广阔地区。