缺陷铋烯拓扑态的机理和调控

Bismuthene, which is a kind of two dimensional topological insulators made of bismuth atoms, has attracted wide attentions for its novel non-dissipative edge states and large band gap. However, these fascinating properties might be covered or destroyed by defects. The critical point of topological phase transition of Bismuthene caused by the type, density and distribution of defects, as well as what kind of modulation could be used to recover the topological properties of Bismuthene with defects, or searching novel topological states, are still open questions. (1)In this program, the first-principle together with semi-empirical tight-binding models are employed to study the effects of type, size, distribution and local magnetic moments of defects on the topological edge states of Bismuthene, (2)to find atoms and functional groups that could be used for doping and decoration in order to eliminate the effects of defects to the bulk and edge band structures of Bismuthene, (3) to build new model to describe the effects of defects and decorations on the topological properties of Bismuthene and improve the methods of parameter fitting with the help of Wannier functions, and to explore other novel topological states like valley-polarization and quantum anomalous Hall effects, etc.. The studies of this program will promote the applications of Bismuthene in novel topological functional devices.

铋烯是由铋原子构成的二维拓扑绝缘体，因其新奇的无耗散拓扑边缘态和宽带隙而受到广泛关注。由于这些优良的特性容易被材料中存在的缺陷所掩盖，所以研究导致铋烯拓扑相变的缺陷类型、尺寸、分布，以及采用何种调控手段恢复其拓扑边缘态或找到新的拓扑态，成为亟待解决的问题。（1）本项目采用第一性原理与半经验紧束缚模型相结合的方法，研究缺陷的类型、尺寸、分布和局域磁矩对铋烯拓扑边缘态的影响机制；(2)寻找适当的原子和基团对缺陷铋烯进行填充和修饰，实现对其电子结构的有效调控，以消除缺陷对体态能隙和拓扑边缘态的不利影响；(3)建立描述缺陷和修饰调控铋烯拓扑态的新模型，借助Wannier函数来改进参数拟合方法，探索由缺陷或调控导致的谷极化和量子反常霍尔效应等新奇拓扑态。本项目将有助于推动铋烯在新型拓扑器件中的实验研究和实际应用。

铋烯是一种由铋原子构成的宽带隙二维拓扑绝缘体，自2017年被实验成功合成后一直受到广泛关注。然而其优良的特性可能受到合成及应用环境中出现的缺陷的影响，因此研究不同的掺杂或修饰情况下铋烯拓扑性质的变化规律，成为铋烯理论与实验中需要解决的问题之一。本项目针对铋烯中存在的原子替换和基团修饰对铋烯拓扑性质的影响机制进行了系统研究，并建立了一套描述基团修饰对铋烯的拓扑性质和谷极化性质影响规律的半经验等效模型。此外还对与项目相关的二维拓扑节线态半金属材料进行了拓展性研究。本项目取得了以下重要结果：(1)在12.5%均匀掺杂下，钼、铅、铜、锡、银和金原子会使铋烯呈现金属态，钨掺杂会使铋烯产生磁性并转化为陈数为3、带隙达到271 meV的量子反常霍尔效应绝缘体，而钼掺杂会使铋烯转变为拓扑平庸的磁性绝缘体，这些结果为实验合成铋烯中减少掺杂原子影响提供了理论依据；(2)证实了铋烯在各种不同取代比例和分布的氟化甲基修饰下均能保持其拓扑绝缘体特性，同时非对称修饰会引发体系自发电极化以及谷极化现象，说明氟化甲基修饰的铋烯具有在复杂化学环境中良好的拓扑鲁棒性以及在电极化材料和谷霍尔器件领域的潜在应用；(3)建立了能够描述修饰铋烯电子能带结构、电极化与谷极化特性的半经验等效模型；(4)在上述研究基础上，我们将研究内容拓展到二维拓扑半金属材料，首次设计了具有非水平镜面结构对称性的二维磁性节线态半金属材料MnNF薄膜和二维非磁性节线态半金属材料NiB2等新型拓扑材料，其节线态受到水平滑移面非点式对称性保护，特别是在MnNF中面外磁化方向能够进一步保护其在自旋-轨道耦合作用下保持零带隙，并可以在外电/磁场作用下打开带隙，这些研究成功地拓宽了二线节线态材料的研究范畴和保护机制。