拓扑绝缘体及半金属的第一性原理研究

发布时间：2018-04-09 07:10

本文选题：拓扑电子材料　切入点：二维拓扑绝缘体　出处：《中国科学院大学(中国科学院物理研究所)》2017年博士论文

【摘要】：近几十年,拓扑电子材料领域的发展十分迅速,成为凝聚态物理领域里的一个热点话题。拓扑电子材料是对具有非平庸拓扑电子结构的一类材料的统称,按照具体的电子结构特征可以分为拓扑绝缘体、拓扑半金属和拓扑超导体等。拓扑电子材料因受到对称性的保护,所以它的一个非常重要的特点就是不受外界环境微扰的影响,这个特性也促使了拓扑电子材料在未来材料领域的广泛应用。在对拓扑电子材料的理论研究中,第一性原理计算方法具有非常重要的地位。随着第一性原理计算方法的发展,越来越多的拓扑电子材料被理论预言,之后又有越来越多的拓扑电子材料在实验上被制备和验证。在本论文的一系列工作中,我们主要利用第一性原理计算方法,得到了每一种材料各自的拓扑性质,也为实验工作提供了基础与方向标。首先,我们对一些理论知识及计算方法进行了详细的介绍,主要包括基于密度泛函理论的第一性原理计算、赝势理论、Wannier函数计算方法和简单的k·p模型分析方法。本论文的所有工作都是基于这些理论及方法进行的。其次,我们成功预言了氧化物ZrSiO为三维弱拓扑绝缘体,其单层结构则为二维拓扑绝缘体,并且我们发现具有相同晶体结构的一系列材料WHM(W:Zr,Hf或者La;H:第四或第五主族元素;M:第六主族元素)也有与ZrSiO类似的电子结构性质。在不考虑自旋轨道耦合(SOC)时,二维拓扑绝缘体ZrSiO的体能带在费米面附近形成闭合的nodal-line的形状;当加入SOC之后,nodal-line处会打开能隙,全局能隙能够达到30meV。通过能带投影分析,我们发现费米面附近形成交叉的两条能带的主要成分是Zr的dx2-y2轨道和Si的px+py轨道。进一步通过空间群不可约表示的分析,我们发现能带的交叉是由滑移面对称性保护的。除了 ZrSiO材料的理论预言,我们还与人民大学王善才教授实验组合作进行了 WHM家族中另外一个材料ZrSnTe的实验研究。通过角分辨光电子能谱(ARPES)并结合第一性原理计算的方法,我们验证了层状ZrSnTe确实具有二维拓扑绝缘体的性质。这一系列材料非常庞大,而且还与铁基超导LiFeAs具有相似的晶体结构,因此我们或许可以从中找到拓扑与超导的结合性质,比如拓扑超导体,所以值得进一步的研究。然后,我们又预言了CaP_3一些列材料为拓扑nodal-line半金属,它们需要忽略强度非常小的SOC效应。这一系列材料单纯的具有时间反演对称性和空间反演对称性,所以当这两个对称性不受到破坏时,就会形成闭合能带交叉环路的结构。拓扑nodal-line半金属的重要特征就是表面态为鼓膜状的能带,我们不仅通过wannier函数构造哈密顿量得到了这样的表面态,还通过一个简单的k·p模型更加简洁有效的得到了交叉环路。当考虑SOC时,环路打开能隙,CaP_3系列材料成为小能隙的强拓扑绝缘体。最后,我们对本论文做了一个简单的总结,并对我未来的工作发展做了进一步的展望。提出了发掘拓扑电子材料的一个高效的方法:通过对材料基因库的有效利用以及对拓扑电子材料的明显特征参数的计算进行高通量的搜索。这一方法加快了拓扑绝缘体的发现与应用,将会是未来拓扑绝缘体及半金属发展的一个重要方向。
[Abstract]:In recent years, the development of the field of topology of electronic materials is very rapid, become a hot topic in the field of physics. The topology of electronic materials is a general designation of a class of materials with non trivial topology of the electronic structure and electronic structure according to the characteristics of the concrete can be divided into topological insulator, topological semi metal and topological superconductors. Electronic materials protected by the topological symmetry, so a very important feature of it is not affected by external environment perturbation, this feature also contributed to the topology of electronic materials in the field of materials are widely used in the future. In the theoretical research on the topology of electronic materials, the first principle calculation method is very important position. With the development of first principle calculation method, the topology of electronic materials has been more and more theoretical predictions, then the topology more and more electronic materials are produced in the experiment Preparation and validation of a series of work. In this paper, we mainly use the first principle calculation method, the topological properties of each of the respective materials, but also for the experimental work provides the basis and direction. Firstly, we carried out a detailed introduction to the theory and calculation method, including calculation the first principle based on the density functional theory pseudopotential theory, Wannier function calculation method and a simple k p model analysis method. All the work of this paper is that based on the theories and methods. Secondly, we successfully predicted the oxide ZrSiO for three-dimensional weak topological insulator, the single layer structure for two-dimensional topological insulator, and we found a series of WHM materials have the same crystal structure (W:Zr, Hf or La; H: fourth or fifth main elements; M: sixth main group elements) also have electronic structure properties similar to ZrSiO. Without considering the spin orbit coupling (SOC), 2D topological insulator ZrSiO fitness zone formed closed nodal-line near the Fermi surface shape; after added SOC, nodal-line will open the gap, the global energy gap can reach 30meV. through the analysis of projection zone, we found near the Fermi level of the formation of two band the main component is Zr dx2-y2 cross orbital and Si px+py orbital. Through further analysis of space irreducible representations, we found that the cross band is composed of slip symmetry protection. In addition to the theoretical predictions of the ZrSiO material, we have with the people's University professor Wang Shancai made a combination of experimental WHM family in addition an experimental study on the properties of ZrSnTe. By angle resolved photoelectron spectroscopy (ARPES) combined with the method of first principle calculation, we verify the properties of layered ZrSnTe has 2D topological insulator. This A series of materials is very large, but also with the iron-based superconducting LiFeAs has similar crystal structure, so we may find the binding properties and superconductivity from topology, such as topology of superconductor, so it is worth further study. Then, we predicted the CaP_3 series of materials for the topology of nodal-line half metal, they need to ignore the SOC effect the intensity is very small. With time reversal symmetry inversion symmetry and the space of this series of simple materials, so when the two symmetry is not damaged, will form a closed loop band crossing structure. An important feature of nodal-line is topological semi metallic surface states for tympanic membrane shaped band, we not only by constructing the Wannier function Hamiltonian of surface states that are more concise and effective got cross loop through a simple k p model. When considering SOC, ring The road opened the gap, CaP_3 series materials become a strong topological insulator minigap. Finally, we make a brief summary of this paper, and for my future development is prospected. Propose an efficient method to explore the topology of electronic materials: calculated by effective use of the material gene base and obvious characteristic parameters on the topology of electronic materials for high flux search. This method can accelerate the discovery and application of topological insulator, will be an important direction for future development of the semi topological insulator and metal.

【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469

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