近日，南京大学孙建团队研究了层状受限固体中的相变和尺寸交叉。该研究于2025年4月21日发表在《美国科学院院刊》杂志上。

在受限原子系统中，固相的性质以及从二维（2D）层到三维（3D）体的有序-无序相变的交叉在很大程度上仍未得到解释。为此，研究组考虑在不同压力和温度下限制在石墨烯片之间的稀有气体和铝。利用晶体结构搜索方法和基于量子力学精度机器学习势的分子动力学，他们识别了偏离简单密堆积的多层受限固体的结构。

根据两步连续Kosterlitz–Thouless–Halperin–Nelson–Young理论，在加热时，研究组发现受限的2D单层会熔化。然而，多层固体在不连续地熔化成各向同性液体之前，会连续地转变为中间的层状六方相。这种中间相在这里研究的至少有12层。这种变化可以通过在熔化过程中随着层数的增加从2D拓扑缺陷向3D拓扑缺陷的交叉来定性理解。

附：英文原文

Title: Phase transitions and dimensional cross-over in layered confined solids

Author: Wang, Yong, Wang, Junjie, Yao, Ge, Fan, Zheyong, Granato, Enzo, Kosterlitz, Michael, Ala-Nissila, Tapio, Car, Roberto, Sun, Jian

Issue&Volume: 2025-4-21

Abstract: The nature of solid phases and cross-over of order–disorder phase transitions from two-dimensional (2D) layers to three-dimensional (3D) bulk in confined atomic systems remain largely unexplained. To this end, we consider noble gases and aluminum confined between graphene sheets at different pressures and temperatures. Using crystal structure search methods and molecular dynamics based on machine-learned potentials with quantum-mechanical accuracy, we identify structures of multilayer confined solids that deviate from simple close packing. Upon heating, we find that confined 2D monolayers melt according to the two-step continuous Kosterlitz–Thouless–Halperin–Nelson–Young theory. However, multilayer solids transition continuously into an intermediate layered-hexatic phase before melting discontinuously into an isotropic liquid. This intermediate phase persists at least up to 12 layers studied here. This change can be qualitatively understood based on the cross-over from 2D topological defects toward 3D ones during melting as the number of layers increases.

DOI: 10.1073/pnas.2502980122

Source: https://www.pnas.org/doi/abs/10.1073/pnas.2502980122