Sun Gang, Xu Limei, Giovambattista Nicolas
International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China.
Department of Physics, Brooklyn College of the City University of New York, Brooklyn, New York 11210, USA.
J Chem Phys. 2017 Jan 7;146(1):014503. doi: 10.1063/1.4973348.
In the case of fragile liquids, dynamical properties such as the structural relaxation time evolve from Arrhenius at high-temperatures to non-Arrhenius at low temperatures. Computational studies show that (i) in the Arrhenius dynamic domain, the liquid samples regions of the potential energy landscape (PEL) that are insensitive to temperature (PEL-independent regime) and the relaxation is exponential, while (ii) in the non-Arrhenius dynamic domain, the topography of the PEL explored by the liquid varies with temperature (PEL-influenced regime) and the relaxation is non-exponential. In this work we explore whether the correlation between dynamics and PEL regimes, points (i) and (ii), holds for the Fermi-Jagla (FJ) liquid. This is a monatomic model liquid that exhibits many of the water anomalous properties, including maxima in density and diffusivity. The FJ model is a rather complex liquid that exhibits a liquid-liquid phase transition and a liquid-liquid critical point (LLCP), as hypothesized for the case of water. We find that, for the FJ liquid, the correlation between dynamics and the PEL regimes is not always present and depends on the density of the liquid. For example, at high density, the liquid exhibits Arrhenius/non-Arrhenius (AnA) dynamical crossover, exponential/non-exponential (EnE) relaxation crossover, and a PEL-independent/PEL-influenced regime crossover, consistent with points (i) and (ii). However, in the vicinity of the LLCP, the AnA crossover is absent but the liquid exhibits EnE relaxation and PEL regime crossovers. At very low density, crystallization intervenes and the PEL regime crossover is suppressed. Yet, the AnA dynamical crossover and the EnE relaxation crossover remain. It follows that the dynamics in liquids (AnA and EnE crossovers) are not necessarily correlated with the changes between the PEL regimes, as one could have expected. Interestingly, the AnA crossover in the FJ liquid is not related to the presence of the Widom line. This result may seem to be at odds with previous studies of polymorphic model liquids, and a simple explanation is provided.
对于易脆液体，诸如结构弛豫时间等动力学性质在高温下遵循阿仑尼乌斯定律，而在低温下则呈现非阿仑尼乌斯行为。计算研究表明：（i）在阿仑尼乌斯动力学区域，液体样本在势能面（PEL）中对温度不敏感的区域（与PEL无关的区域），弛豫是指数型的；（ii）在非阿仑尼乌斯动力学区域，液体所探索的PEL地形随温度变化（受PEL影响的区域），弛豫是非指数型的。在这项工作中，我们探究动力学与PEL区域之间的相关性，即上述（i）和（ii）点，对于费米 - 雅格拉（FJ）液体是否成立。这是一种单原子模型液体，展现出许多水的反常性质，包括密度和扩散率的最大值。FJ模型是一种相当复杂的液体，呈现出液 - 液相转变和液 - 液临界点（LLCP），正如对水的假设情况一样。我们发现，对于FJ液体，动力学与PEL区域之间的相关性并非总是存在，而是取决于液体的密度。例如，在高密度下，液体呈现阿仑尼乌斯/非阿仑尼乌斯（AnA）动力学转变、指数/非指数（EnE）弛豫转变以及与PEL无关/受PEL影响的区域转变，与上述（i）和（ii）点一致。然而，在LLCP附近，AnA转变不存在，但液体呈现EnE弛豫和PEL区域转变。在非常低的密度下，结晶作用介入，PEL区域转变受到抑制。不过，AnA动力学转变和EnE弛豫转变仍然存在。由此可见，液体中的动力学（AnA和EnE转变）不一定与PEL区域之间的变化相关，尽管人们可能会有这样的预期。有趣的是，FJ液体中的AnA转变与维德曼线的存在无关。这一结果可能看似与先前对多晶型模型液体的研究相悖，并给出了一个简单的解释。
