低维铁电材料极化漩涡畴形成和演化的定量表征和判据

As a novel polarization structure found in low-dimensional ferroelectrics (LDF) in recent years, vortex domain can be utilized for exploring some potential engineering applications. In order to develop domain engineering technology and realize function design for LDF devices, it is necessary to establish vortex formation and evolution criteria, but the existing related studies are rare. In this proposal, it is proposed to characterize the generalized load and effect in the process of polarization domain evolution by the electric field curl and the toroidal moment respectively. With the combination of MD simulation based on the shell model and experimental research, formation and evolution behaviors of LDF under mechanical, electric or combined loads will be investigated. The mathematical expressions between the electric field curl and the toroidal moment will be given and their parameters will be determined by data fitting. The effects of sample geometry, temperature, defect and boundary conditions on polarization configuration and its evolution will be evaluated. The criteria of vortex domain formation and evolution will be established in the basis of the electric field curl or the toroidal moment. Finally, some domain engineering strategies will be given for manipulating polarization configuration. It is helpful for improving recognition and control of inhomogeneous polarization and its evolution, and will enrich the content of ferroelectric solid mechanics and domain structure dynamics.

漩涡畴是近年来在低维铁电材料中发现的一种新型极化构型，借此可以拓展许多潜在工程应用。建立漩涡畴形成和演化判据，是发展畴工程技术、实现低维铁电器件功能设计的前提，然而目前相关的研究才刚刚起步。本项目提出采用电场旋度和极化环量矩分别刻画极化畴演化过程中的广义作用和广义效应，利用基于壳模型的分子动力学模拟和实验相结合的方法，研究低维铁电材料在力、电以及力电组合载荷作用下漩涡畴的形成和演化行为，分别建立适合不同结构形式的电场旋度和极化环量矩之间的数学表达式，并且通过拟合确定参数，评价材料几何、环境温度、缺陷、边界条件等因素对极化畴构型及其演化行为的影响，基于电场旋度或极化环量矩构造漩涡畴形成和演化判据，总结通过内外部条件调控低维铁电材料极化构型的畴工程策略。这将有助于提高对低维铁电材料非均匀极化及其演化行为的认识和控制水平，同时丰富铁电固体力学和畴结构动力学的学科内涵。

采用极化旋度描述极化构型及其分布特征，观察极化构型在不同外部条件驱动下发生的演化，取得了一系列研究成果。针对漩涡构型的三维特征，采用轴线位置、长度、形状和断面尺寸作为重要参数，描述漩涡的分布、大小和形状等几何特征，利用基于核壳模型的分子动力学模拟方法，首次从三维角度系统观察了在受压缩的BaTiO3/SrTiO3铁电超晶格中，漩涡萌生、长大、运动、变形、断开或萎缩、以及湮灭的全过程。研究了双材料界面、空位和畴界等缺陷对漩涡构型形成、演化和湮灭行为的影响。在含空位缺陷的压电体中，发现了一种由空位诱导的新型特殊极化构型，总结了该极化构型的表现形式，以及不同形式的特殊极化构型形成的条件及其相互转化的关系。利用极化旋度作为特征参量，描述极化漩涡强度，定量分析了极化漩涡的稳定性及其对压电体巨电热效应等的影响，建立了极化构型失稳（即漩涡萌生、湮灭或转变为其他形式）的临界条件或判据。