James Franck Institute, University of Chicago, Chicago, Illinois 60637, USA.
J Chem Phys. 2018 Jul 21;149(3):034503. doi: 10.1063/1.5026680.
We report the structure of transient fluctuations in the liquid phase of a two-dimensional system that exhibits several ordered phases with different symmetries. The density-temperature phase diagram of the system studied, composed of particles with a repulsive shouldered soft-core pair interaction, has regions with stable liquid and hexatic phases, a square solid phase, two separate hexagonal solid phases, and a quasi-crystalline phase with 12-fold symmetry. We have examined the character of the structured fluctuations by computing the same-time aperture cross correlation function of particle configurations in several fluid regions near to and far from phase transition lines. The two primary goals of our study are (1) determination if the spectrum of structures of the fluctuations in the liquid is broader than or limited to the motifs exhibited by the ordered phases supported by the system and (2) determination of the density domains in the liquid that support particular transient structured fluctuations. In the system studied, along a low-temperature isotherm in the temperature-density plane that intersects all the ordered phases we find that the liquid phase exhibits structured fluctuations with hexagonal symmetry near both liquid-hexatic transition lines. Along the same isotherm and in the stable liquid between the lower density hexatic-to-liquid and the higher density liquid-to-square solid transitions, we find that transient hexagonal ordered fluctuations dominate the liquid region near the hexatic-to-liquid transition and square ordered fluctuations dominate the liquid region near the liquid-to square solid transition, but both of these structured fluctuations occur at all densities between these transition lines. At a higher temperature, at phase points in the liquid above, but close to the density maximum of an underlying transition, there are ordered fluctuations that can be correlated with the structure of the lower temperature phase. Although it is expected that very close to a liquid-ordered phase boundary a structured fluctuation in the liquid will have the same symmetry as the ordered phase, it is not obvious that structured fluctuations in thermodynamic states deep in the liquid phase will be similarly restricted. The most striking result of our calculations is that no evidence is found in the liquid phase for structured fluctuations with other symmetries than those of the ordered phases of the system.
我们报告了二维系统液相中瞬态涨落的结构，该系统表现出具有不同对称性的多个有序相。所研究系统的密度-温度相图由具有排斥肩软芯对相互作用的粒子组成，具有稳定的液相和六方相、方形固相、两个独立的六方固相以及具有 12 重对称性的准晶相区域。我们通过计算几个靠近和远离相变线的流体区域中粒子构型的同时孔径交叉相关函数，研究了结构涨落的特征。我们研究的两个主要目标是：(1) 确定液体中涨落的结构谱是否比系统支持的有序相所展示的结构谱更宽或受其限制；(2) 确定液体中支持特定瞬态结构涨落的密度域。在所研究的系统中，在穿过所有有序相的低温等温线上，我们发现靠近液体-六方相转变线的液体相具有六方对称性的结构涨落。沿着相同的等温线和在较低密度六方相-液相和较高密度液相-方形固相转变之间的稳定液相中，我们发现瞬态六方有序涨落主导靠近六方相-液相转变的液体区域，而方形有序涨落主导靠近液相-方形固相转变的液体区域，但这些结构涨落在这些转变线之间的所有密度上都存在。在较高温度下，在接近但高于底层转变密度最大值的相点处，存在可以与较低温度相结构相关联的有序涨落。尽管预计在接近液相-有序相边界时，液体中的结构涨落将具有与有序相相同的对称性，但在热力学状态深处的液体相中，结构涨落是否会受到类似限制并不明显。我们计算的最显著结果是，在液相中没有发现具有系统有序相以外对称性的结构涨落的证据。
