拓扑晶体绝缘体SnTe的表面结构及其新奇性质研究

发布时间：2018-08-08 18:55

【摘要】：拓扑晶体绝缘体是一种新的物质相,它的拓扑性质受晶体对称性保护,并具有多个狄拉克表面态。基于第一性原理和量子输运计算,我们系统研究了拓扑晶体绝缘体SnTe的(111)表面和薄膜中的一些新奇性质。由于极性,理想的SnTe(111)表面原则上是不稳定的。因此我们首先研究了SnTe(111)表面的稳定性,发现在不同的生长条件下可以形成三种稳定的表面结构。表面电子结构计算发现它们具有两种定性不同类型的拓扑表面态:未重构和(√3×√3)重构的表面具有第一种类型的表面态,即四个狄拉克点位于四个时间反演不变动量点;(2×1)的表面重构引起表面布里渊区的折叠,使不同狄拉克谷产生相互作用,产生了新类型的表面态,即两个狄拉克点偏离了布里渊区中心的时间反演不变点。我们的研究表明,除了选择不同的表面方向还可以通过控制生长条件来产生不同类型的拓扑表面态。拓扑晶体绝缘体在其表面能带中具有偶数个狄拉克锥(多个谷)。我们系统研究了SnTe(111)表面狄拉克谷在应变下的演化,发现压缩应变使ˉΓ和ˉM谷的狄拉克锥产生不同程度的移动,甚至相反的移动;拉伸应变可以增强上下表面间的耦合,甚至使ˉΓ和ˉM谷产生不同大小的能隙。在SnTe(111)表面上,我们设计了一种应变异质结构,并发现通过动态施加局部压力,可以实现强的狄拉克费米子的谷过滤效应。这些结果显示,拓扑晶体绝缘体中可以实现应变的功能化应用和狄拉克谷电子学应用。拓扑材料薄膜的狄拉克费米子具有螺旋自由度。我们以拓扑晶体绝缘体SnTe的(111)薄膜为例,发现用适当的电场可以使薄膜的狄拉克费米子产生巨大的螺旋性劈裂。基于这些结果,我们对狄拉克费米子透过双栅极纳米结构的输运进行了计算,发现了一些螺旋性相关的特性,包括狄拉克费米子螺旋性的选择透射,螺旋性切换,螺旋性负折射以及双负折射。我们的结果为实现基于螺旋性的电子学应用提供了可能。
[Abstract]:Topological crystal insulator is a new material phase. Its topological properties are protected by crystal symmetry and have many Dirac surface states. Based on the first-principles and quantum transport calculations, we have systematically studied some novel properties on the (111) surface and thin films of the topological crystal insulator (SnTe). The ideal SnTe (111) surface is unstable in principle due to its polarity. Therefore, we first studied the stability of SnTe (111) surface and found that three stable surface structures can be formed under different growth conditions. Surface electronic structure calculations show that they have two types of qualitatively different topological surface states: unreconstructed surfaces and (3 脳 m2 3) reconstructed surfaces with the first type of surface states. That is, the four Dirac points are located at four time inversion invariant momentum points. (2 脳 1) the surface reconstruction results in the folding of the Brillouin zone, which results in the interaction of different Dirac valleys and the formation of new types of surface states. That is, two Dirac points deviate from the center of Brillouin zone in time inversion invariant points. Our results show that different types of topological surface states can be generated by controlling growth conditions in addition to choosing different surface directions. A topological crystal insulator has an even number of Dirac cones (multiple valleys) in its surface band. We have systematically studied the evolution of Dirac Valley on SnTe (111) surface under strain. It is found that compression strain makes the Dirac cones of Gamma and M valleys move in varying degrees, even opposite, and that tensile strain can enhance the coupling between the upper and lower surfaces. Even the energy gaps of different sizes are produced in the Gamma and Mian valleys. A strain heterostructure is designed on the surface of SnTe (111) and it is found that the strong Dirac fermion can be filtered by dynamic local pressure. These results show that the functional application of strain and the application of Dirac Valley electronics can be realized in the topological crystal insulator. The Dirac fermion of the thin film has helical degrees of freedom. Taking the (111) thin film of the topological crystal insulator SnTe as an example, it is found that the Dirac fermion of the thin film can be caused by a large helical splitting with a proper electric field. Based on these results, we have calculated the transport of Dirac fermions through double gate nanostructures and found some helicity related characteristics, including selective transmission of Dirac fermions helicity, helicity switching. Helical negative refraction and double negative refraction. Our results provide the possibility for the realization of helical electronic applications.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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