姜泰勒扭曲和轨道序诱导的二维铁磁性

Two-dimensional ferromagnetism induced by Jahn-Teller distortion and orbital order

随着体系维度的降低,材料内部的量子限制效应和电子关联作用会相应地增强,往往可以衍生一些新奇的物理特性.在钙钛矿材料中,姜泰勒扭曲和轨道序通常会引起丰富的电子关联行为.本文通过第一性原理计算、对称性分析和蒙特卡罗模拟等方法,对比研究了钙钛矿氟化物KCuF_(3)及其单层结构,揭示了钙钛矿二维化引起的晶格动力学、结构、电子及磁性质等方面的变化.结果表明,KCuF_(3)块体中出现的协作姜泰勒扭曲和面内交错轨道序可以维持到单层极限,导致单层为二维铁磁绝缘体.与块体相不同的是,在单层中姜泰勒扭曲模式作为原型相的软模出现,且单层的绝缘性不依赖于姜泰勒扭曲的出现,而是与电子关联效应的增强有关.本文为研究二维钙钛矿材料以及设计基于钙钛矿的二维铁磁体提供了指导和借鉴.

With the decrease of system dimension,the quantum confinement effect and electronic correlation interaction inside the material will be enhanced correspondingly,often resulting in some novel physical properties.Recently,the freestanding perovskite oxide films as low as the monolayer limit have been successfully prepared and can be transferred to any desired substrate,which provides a great opportunity for exploring the functional properties of two-dimensional perovskite.In perovskite materials,Jahn-Teller distortion and orbital order often cause a variety of correlated electronic behaviors.However,unlike van der Waals materials that retain their structural and chemical bonding characteristics when they are reduced to the monolayer limit,perovskite materials may undergo structural reconstruction when they are reduced to two dimensional structures.Therefore,what are the issues to be solved urgently are whether Jahn-Teller distortion and related effects exist in the perovskite monolayer limit,and whether two-dimensional perovskite can exhibit some new properties different from its bulk phase.In this work,perovskite fluoride KCuF_(3) and its monolayer have been comparatively studied by the first-principles calculation,symmetry analysis,and Monte Carlo simulation methods,revealing the change in lattice dynamics,structural,electronic,and magnetic properties caused by dimensionality reduction in perovskites.The results show that the cooperative Jahn-Teller distortion and the in-plane staggered orbital order occurring in the KCuF_(3) bulk can be retained to the monolayer limit.However,unlike the bulk phase,the Jahn-Teller distortion mode appears as a soft mode of the prototype phase in the monolayer,and the insulating property of the monolayer does not rely on the emergence of the JahnTeller distortion,but it is related to the enhancement of the electronic correlation effect.The staggered orbital order causes the nearest-neighbor exchange interaction to be ferromagnetic,resulting in the monolayer being a two-dimensional ferromagnetic insulator,different from the antiferromagnetic phase in the bulk.Monte Carlo simulations predict that the Curie temperature of the monolayer is about 5 K,which is much lower than the Néel temperature of the bulk phase,indicating that the disappearance of interlayer coupling leads to a significant reduction in the magnetic phase transition temperature.This work provides guidance and reference for studying the two-dimensional perovskite materials and designing the perovskite-based two-dimensional ferromagnets.