新型二维电子材料中的拓扑态研究

As a novel quantum state of matter, topological insulator (TI) is one of the most attractive areas in condensed matter physics in recent years. In this project, we will focus on the controllable growth of high quality ultra-thin (Bi,Sb)2(Se,Te)3 film by molecular beam epitaxy. We will carry out in-situ surface analysis in UHV and transport measurements of the samples under the conditions of low temperatures and high magnetic fields, to explore the topological nature of our low dimensional electron system. The study includes: (1)Try the better doping method and obtain samples with high mobility and controllable topological state. For the ultra-thin films with different thickness, we will measure the existence of the edge state as the theory predicted, and observe the coupling between the top and bottom surface states and also a possible crossover from weak antilocalization to weak localization by the measurement of magnetoconductivity. (2)We will employ self-assembly growth of organic molecules on the surface of ultra-thin topological film and observe the doping effect of the molecules, which would tune the Fermi level with respect to the Dirac point of the topological insulators. (3)We will deposit magnetic organic molecules on the ultra-thin topological film, which would bring intrinsic magnetic order to the system, break the time reversal symmetry and open up an energy gap at the original Dirac point, inducing a range of striking phenomena. We will investigate the different self-assembly phase of the organic molecules with the corresponding modification of the topological surface states. Our study will solve several open problems in TIs, establish a new avenue for engineering topological materials, and is also helpful to discover the novel quantum states.

拓扑绝缘体，一种全新的量子物质形态，是当前凝聚态物理的研究热点。本项目中，我们将以分子束外延方法可控生长高质量拓扑绝缘体(Bi,Sb)2(Se,Te)3超薄膜为突破点，通过表面分析和极低温输运性质的测量，系统地进行以下研究：(1)探索更好的掺杂方法，得到高迁移率且拓扑态可调的样品。验证理论预言的不同厚度超薄膜中边缘态的存在，观测上下表面的耦合及反弱局域到弱局域的转变。(2)在超薄膜表面外延生长有机分子自组装有序结构，探索其掺杂效应对拓扑绝缘体费米面的调制。(3)在超薄膜上沉积磁性有机分子层，通过引入的长程铁磁序打破体系时间反演对称性，在原Dirac点打开表面带隙，并探索磁性有机分子的各种有序结构对拓扑表面态的影响。本项目将回答拓扑绝缘体研究中亟待解决的问题，为裁剪拓扑绝缘体电子结构提供有效途径，对构建新量子态具有重要意义。

拓扑绝缘体是当前凝聚态物理的研究热点。通过向拓扑绝缘体引入磁有序，打破时间反演对称性，使其表面态在狄拉克点附近打开一个能隙，可能出现诸多新奇的量子现象，具有非常重要的潜在应用价值。然而，该领域还存在相当多的问题亟待解决，包括如何抑制体态电子的介入、获得更加纯粹的表面态电子信息；如何降低磁性引入带来的无序；如何获得很强的磁近邻相互作用等。针对这些问题，在本项目中我们以分子束外延方法可控生长高质量拓扑绝缘体超薄膜为突破点，完成了以下几项工作：(1)优化了材料生长和器件加工工艺，得到高迁移率且拓扑态可调的Bi2Se3和(Bi,Sb)2Te3样品，为探测表面态电子性质及研究拓扑与磁的关联效应提供了坚实的材料基础。(2)通过对Mn掺杂Bi2Se3薄膜的系统研究，首次观察到磁性拓扑体系中的双组份反常霍尔效应。深入理解了磁性掺杂、拓扑绝缘体表面态和体态之间的复杂相互作用以及该体系中多种磁有序之间的竞争关系。(3)通过构造MnSe/(BixSb1-x)2Te3异质结，观察到不同磁插层界面结构对拓扑绝缘体的磁有序结构以及表面态输运特性的重要影响，这是磁延展体系的首个输运研究，为实现高温量子反常霍尔效应以及其它拓扑磁电效应提供了一个新的有效途径。