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arXiv 0807.4699cond-mat.softcond-mat.stat-mech
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水的取向序参量和取向熵的时空关联

Space-Time Correlations in the Orientational Order Parameter and the Orientational Entropy of Water

Pradeep Kumar, Sergey V. Buldyrev, H. Eugene Stanley

摘要:

我们引入了局域四面体序参量$Q\equiv Q(r,t)$的空间相关函数$C_Q(r)$和时间自相关函数$C_Q(t)$。使用计算机模拟的TIP 5 P模型的水,我们调查$C_Q(r)$在一个广泛的区域的相图。首先,我们表明,$C_Q(r)$在高温$T>T_W\approx 250$ K下,在$r\approx 0.32$ nm处显示出负相关关系,在Widom线$T_W$以下变为正相关。此外,我们发现,在低温下,$C_Q(t)$表现出类似于自中间散射函数的两步时间衰减,并且相应的相关时间$τ_Q$显示出从$T>T_W$的非Arrhenius行为到$T<T_W$的Arrhenius行为的动态交叉。最后,我们定义了一个取向熵$S_Q$与{ \it本地}取向顺序的水分子,并表明,$τ_Q$可以提取$S_Q$使用模拟的亚当-吉布斯关系。

英文摘要:

We introduce the spatial correlation function $C_Q(r)$ and temporal autocorrelation function $C_Q(t)$ of the local tetrahedral order parameter $Q\equiv Q(r,t)$. Using computer simulations of the TIP5P model of water, we investigate $C_Q(r)$ in a broad region of the phase diagram. First we show that $C_Q(r)$ displays anticorrelation at $r\approx 0.32$nm at high temperatures $T>T_W\approx 250$ K, which changes to positive correlation below the Widom line $T_W$. Further we find that at low temperatures $C_Q(t)$ exhibits a two-step temporal decay similar to the self intermediate scattering function, and that the corresponding correlation time $τ_Q$ displays a dynamic crossover from non-Arrhenius behavior for $T>T_W$ to Arrhenius behavior for $T<T_W$. Finally, we define an orientational entropy $S_Q$ associated with the {\it local} orientational order of water molecules, and show that $τ_Q$ can be extracted from $S_Q$ using an analog of the Adam-Gibbs relation.

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