【作者】 陈鑫；

【导师】 赵明文；

【作者基本信息】 山东大学 ， 凝聚态物理， 2016， 硕士

【摘要】 二维拓扑绝缘体(也称为量子自旋霍尔效应绝缘体)由于量子自旋霍尔效应(QSHE)绝缘态受时间反演对称性保护,避免了边缘输运中低能量的散射,这种奇特性质和在自旋电子学及量子计算中的应用前景吸引理论凝聚态物理的研究兴趣。为满足实现这种效应的应用,理论研究预言了很多材料。然而,人们目前只有HgTe/CdTe和InAs/GaSb量子阱可以在极低温下达到。寻找具有大带隙的拓扑绝缘体是增加实现温度的关键。拓扑绝缘体拓扑带隙的来源是自旋轨道耦合效应,因此体系自旋轨道耦合效应越强,越可能是具有大带隙的拓扑绝缘体。我们知道,自旋轨道耦合效应的来源是相对论效应,其耦合强度和电子的机械动量与电场强度成正比。在重元素原子组成的材料中,原子核电场强度很高,电子动量大。因此重元素材料中更容易找到具有大带隙的拓扑绝缘体。因此本文将目光放在了Ge/Sn上。本文借助第一性原理计算,证明了两种稳定的大带隙拓扑绝缘体,即哑铃形锡烷DB stanane和哑铃形锡锗化合物的氢化物DB Sn6Ge4H4。本文的结构如下：第一章是该论文的研究背景以及选题意义,包含三部分内容——首先介绍了自旋电子学的研究背景和自旋轨道耦合效应,得到重原子具有强自旋轨道耦合效应的结论；随后介绍了量子自旋霍尔效应绝缘体研究现状,包括量子自旋霍尔效应奇特的性质,实验上合成的量子自旋霍尔效应绝缘体(HgTe/CdTe量子阱)存在的应用障碍,以及目前理论提出的几类大带隙拓扑绝缘体；最后介绍了Ⅳ族元素材料哑铃形结构的实验、理论研究现状以及Ⅳ族二元化合物的研究现状。第二章介绍了该论文所采用计算方法,即第一性计算理论和计算工具。包括第一性计算的基本原理、密度泛函理论和一些大幅简化计算而对计算正确性影响较小的近似以及量子计算程序包VASP的介绍。第三章中,借助第一性原理计算,本文证明了稳定的具有哑铃形结构的氢化锡烯有312 meV (Γ点)和160 meV(体带隙)的拓扑非平凡带隙,拓扑性质来自s-px,y能带翻转,可由拓扑数Z2=1证明。纳米带表现出螺旋的无带隙的具有高费米速度的边界态,费米速度与石墨烯的比拟。非平凡的拓扑态对衬底是鲁棒的。实现这种材料是实现室温下的量子自旋霍尔效应的可能途径,且有助于构建高速自旋电子器件。第四章从哑铃形锡烯结构的能带规律出发,用对哑铃形锡烯进行掺杂的方式得到了一种直接带隙半导体——哑铃形锗锡化合物,并证明了氢化的哑铃形锗锡化合物是具有自旋轨道耦合带隙166 meV的大带隙拓扑绝缘体。拓扑非平凡性被拓扑数Z2=1证明,与氢化引入的锡原子的s-px.y能带翻转有关。这一工作提供了一个可能的能够实现备受期待的室温量子自旋霍尔效应的候选材料。第五章对本论文做了总结。更多还原

【Abstract】 Protected by time-reversal symmetry, low energy scattering of the edge states in two-dimensional (2D) TIs (also called quantum spin Hall insulators) is enjoined, resulting in dissipationless transport edge channels, which are quite promising for application in spintronics and quantum computations, and are now attracting interests of material science and theoretical condensed matter physics. To achieve this perpose, many works predicted various of materials with topological properties, but only HgTe/CdTe and InAs/GaSb quantum wells can achieve QSH effect at ultralow temperature. Searching for large-gap QSH insulators is the key to increase the operating temperature.The topological gaps of QSH insulators are origin from spin orbital coupling (SOC), so the system with strong SOC effect is more likely to be a large-gap QSH insulator. As we all know, SOC comes from relativistic effect, with the coupling strength proportional to electronic momentum and electric field strength. The electric field strength around a heavey atom is extremely strong, while the electronic momentum is also quite high. As a result, the SOC are always strong in the materials with heavy atoms. Therefore we concentrated our mind on Ge/Sn materials. By first-principle calculations, we demonstrated two stable large-gap QSH insulators, so called dumbbell stanane and dumbbell Sn6Ge4H4. This thesis contains fellowing sections:The first chapter is an introduction of this thesis, containing the background and the allurement of the research. First of all is the introduction of Spintronics and SOC, coming out with a conclusion that heavy atoms own stronger SOC effect compared with light atoms like C, N, O. Following is the paper research of QSH effect and QSH insulators, containing both theory works and experimental works. We indicated the obstacles in the further application of QSH insulators. At the last part, the research review for dumbbell-like group IV materials and group IV compounds, including experimental and theoretical works.Chapter two presents the calculation method this thesis based on, first-pinciples calculations. In this chapter, the basic theories and approaches of ab initio calculations and density function theory (DFT) are briefly introduced. What’s more, introduction of the software we used in the DFT calculations, VASP codes is also given at the last part.In Chapter three, using first-principles calculations, we demonstrate that the stable hydrogenated stanene with a dumbbell-like structure (DB stanane) has large topological nontrivial band gaps of 312 meV (Γ point) and 160 meV for bulk characterized by a topological invariant of Z2=1, due to the s-px,y band inversion. Helical gapless edge states appear in the nanoribbon structures with high Fermi velocity comparable to that of graphene. The nontrivial topological states are robust against the substrate effects. The realization of this material is a feasible solution for applications of QSH effect at room temperature and beneficial to the fabrication of high-speed spintronics devices.In Chapter 4, we proved that high temperature QSH effect may be reached in a hydrogenated germanium-tin (Sn6Ge4H4) dumbbell (DB) structure, where spin-orbit coupling (SOC) opens a bulk band gap of 166 meV. The topological nontriviality is related to the band inversion of s-px,y of Sn atoms induced by surface hydrogenation and can be characterized by a topological invariant of Z2=1. This work offers a promising candidate material for achieving the long-desired room-temperature QSH effect.The last section, Chapter 5 is the conclusion of this whole thesie.更多还原