低维有机多铁材料的相变及磁电耦合机理研究

Multiferroics, defined for those multifunctional materials in which more than one primary ferroic orders coexist simultaneously, such as ferroelectricity, ferromagnetism, and ferroelasticity. And it has become one of the hottest topics of condensed matter physics and material science due to its huge potential application in novel devices, such as sensors, memories, spintronics devices and so on. Based on the organic multiferroics synthesized experimentally, we propose theoretical quantum models to investigate ferroelectric phase transition and magnetoelectric coupling mechanism for two kinds of organic multiferroics, such as organic charge transfer compounds and organic metal complex，by means of Green’s function theory. On the one hand, for organic charge transfer compounds, we study the distribution of charge and spin, the effect of external fields(such as electronic field, magnetic field and temperature) on their magnetic properties, ferroelectric properties and phase transition, and explain the microscopic mechanism of megnetoelectric coupling, considering the lattice, charge, spin and orbital degrees of freedom and the interaction among them. On the other hand, for organic metal complex, we discuss the mechanism of ferroelectricity induced by hydrogen bonding, and the effect of magnetic field(electronic field) on polarization(magnetization). In addition, we study the influence of doping and chemical modification on the electron distribution and lattice distortion, as well as the strength of magnetoelectric coupling, by means of ab initio calculations. This will provide theoretical guide for the design of organic multiferroics and applications in molecular devices, and promote the development of organic spintronics.

多铁材料是具有铁电性、铁磁性和铁弹性等多种铁有序的新型多功能材料，在传感器、信息存储、自旋电子学器件等方面有很大的应用前景，成为凝聚态物理和材料科学的热门课题之一。本项目基于实验上合成的有机电荷转移复合物和有机-金属配合物两种有机多铁材料对铁电相变和磁电耦合机理进行研究。一方面，对于有机电荷转移复合物，考虑晶格、电荷、自旋和轨道等自由度以及它们的耦合作用，分析体系的电荷和自旋分布，并研究外场(如电场、磁场和温度)对体系铁电性和磁性及相变的影响，阐明磁电耦合微观机理。另一方面，对于有机-金属配合物，研究氢键引发铁电序的内禀机制，并探讨磁场(电场)对体系电极化强度(磁化强度)的调控。此外，采用第一性原理方法讨论掺杂或者化学修饰对体系的电子分布和晶格畸变以及磁电耦合强度的影响，为有机多铁材料的设计及其在分子元器件上的应用提供理论指导，促进有机自旋电子学的发展。

近些年来，低维化合物由于其丰富的结构以及迷人的特性引起科学家的广泛关注，其物性研究已成为凝聚态物理与材料、化学学科交叉领域的前沿课题。研究发现掺杂和外场等手段可以诱导并调制低维材料的结构和性能，为人们设计和开发有机自旋电子学器件提供理论指导。本项目主要研究内容及成果有：(1)对钙钛矿材料的金属离子进行掺杂，揭示其对结构、带隙和性能的影响。首先结合有机无机钙钛矿容忍因子的范围，应用平均有效离子半径方法预测了有机无机钙钛矿光电转化太阳能电池材料的空位最大值。然后计算了热处理温度对钙钛矿材料B位空位最大值的影响，研究发现B位空位含量随着温度的增加逐渐减少。探讨了A位空位以及电场对有机无机钙钛矿太阳能电池的结构和物理性能的调控机制。A位空位和电场都可以改变材料的带隙，进而调控太阳能电池的光学性能。(2)器件的设计：将FeN4分子嵌入扶手椅型石墨烯纳米带，并利用氮掺杂的石墨烯纳米带电极，搭建出一种新型的自旋电子学器件。并且运用密度泛函理论结合非平衡态格林函数方法，实现对这种新型器件的非平衡的自洽计算，研究了其外加偏压下的输运性质。成功发现该器件高度的自旋极化率和负微分电阻效应。理论结果预测此器件具有制备多功能自旋电子学器件的潜力，可用作高效率的自旋过滤器、倍频器和自旋超快转换器等。(3)以有机电荷转移复合物TTF-BA为例，利用电势调节的自旋—晶格耦合模型，基于磁热效应和电热效应，提出等熵过程温度调制机制增强磁电耦合效应。进一步利用量子关联调控手段将转变温度调至室温TC = 300 K，作为最佳的操控温度，这打破了研究等温过程磁电耦合效应的常规传统思维。