Pb烯等重元素薄膜体系电子结构及拓扑特性的理论研究

Since monolayer graphene was fabricated successfully in experiments and quantum spin Hall (QSH) effects were demonstrated in it, topological insulators of two-dimensional materials have attracted considerable research interest in condensed matter physics. Their bulk energy bands are insulating, while the edge states are conducting and robust, protected by the topology of the bulk materials. These topological insulators are expected to have potentially significant applications in future low-dissipation electronic devices. Previous studies, however, primarily focus on graphene, silicene, and their derivatives etc. Due to the small spin-orbit coupling (SOC) in these systems, many interesting theoretical predicted topological effects and states cannot be carried out in experiments. In this project, we would like to investigate the electronic structures and topological behaviors of Pb-related heavy element monolayers, with strong SOC interactions, based on density-functional theory combined with theoretical model methods. For Plumbene monolayers, a tight-binding model will be first built to find the mechanism why the electronic states of Plumbene are totally different from those of the monolayers with the elements in the same IV-A group, such as graphene and silicene. The ‘interference effects’ of the topological states and the forming, tuning, and phase transitions of the topological states of the non-Dirac electrons will also be explored. Besides, we plan to solve the structural stability problem of Plumbene and investigate the Weyl semimetals by stacking the Plumbene monolayers through Van der Waals interactions. The research is planned to extend to Tl and Bi monolayers to find new lattice structures and new topological effects. The performance of this project will help understand deeply the topological states of two-dimensional materials and promote their developments in both theories and experiments.

自石墨烯在实验上成功制备及量子自旋Hall态在其中被预言后，两维拓扑绝缘体成为凝聚态物理的研究热点。这类体系体电子态绝缘而边缘是由体拓扑特性驱动的无能隙金属态，有望在低耗散电子器件有重要应用前景。然而前面研究主要集中在石墨烯、Si烯及其衍生体系等，由于自旋轨道耦合(SOC）作用小使很多理论预言的有趣拓扑效应不能在实验中实现。本项目拟聚焦Pb烯等重元素薄膜体系，通过密度泛函与理论模型结合方法研究这类具有强SOC体系的电子态和拓扑特性。构造出适合描述这类体系的哈密顿，对Pb烯特殊的、完全不同于同族薄膜体系电子态的行为进行机制挖掘；研究拓扑‘干涉效应’、非Dirac型拓扑态的形成、调控及相变；解决Pb烯结构稳定性问题；探索多层Pb烯堆叠后形成Weyl半金属。同时拟将研究拓展到Tl和Bi重元素体系、预言新结构及新拓扑效应。本项目实施将有助于人们深入理解二维材料拓扑特性、推动其在理论和实验上发展。

拓扑电子态因有望在自旋电子学、量子计算及低能耗电子器件等领域发挥巨大应用价值近期受到凝聚态物理和材料科学领域研究者们的广泛关注，它是一种新型量子物质态，可由来自相对论效应的自旋轨道耦合（SOC）作用引起。然而前面研究主要集中在石墨烯、Si烯及其衍生结构等，由于SOC作用小使得很多理论预言的拓扑效应不能在实验中实现。项目围绕具有强SOC的含重元素Pb,Bi,Tl薄膜体系利用第一性原理计算、Wannier函数方法、并结合理论模型探索了实现大能隙拓扑绝缘体及新颖拓扑态的机制和方法。通过构造的紧束缚模型项目首先提出了拓扑“耦合效应”，很好解释了Pb烯不同于同族的石墨烯、硅烯等的特殊电子态。研究发现具有正常能带序的二度简并电子态在考虑SOC后都会打开非Dirac拓扑非平庸能隙。基于Bi烯实现了多重Hall效应及强的谷极化效应（谷劈裂达500多meV，文献报道最大值）。为解决Pb等重元素薄膜体系结构稳定性问题,项目为这类体系寻找了系列晶格匹配、不影响样品拓扑特性、实验容易制备的衬底材料。在Pb烯单层堆叠的体系中发现了Dirac半金属及新奇的“沙漏型表面态”，通过Z2及拓扑费米弧表征确认体系的拓扑本质。基于重元素Bi设计了BiSb和BiAs单层，在界面处形成了罕见的量子自旋-谷Hall扭结（QSVHK）态。项目还在前面研究的基础上构造了崭新的含Pb等重元素稳定的单层或堆叠体系，在其中提出或预言了几种新型拓扑电子态，包括半谷、半Chern绝缘态、半Chern-Weyl半金属态、轴子态、拓扑态的“奇偶振荡”特性等。通过晶体场作用、SOC、对称性、范德华力等多作用竞争机制可较好理解得到的拓扑态及相变现象。项目研究结果表明具有强SOC的重元素对形成新奇拓扑电子态具有重要意义，项目成果深化了人们对拓扑电子态的认识，促进其在未来微电子器件、谷电子器件等方面的应用。

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