二维材料中的拓扑电子态和超导特性

发布时间：2018-05-20 08:08

本文选题：二维材料 + 拓扑绝缘体　；参考：《山东大学》2017年博士论文

【摘要】：石墨烯(graphene)是一种由sp2杂化轨道特点的碳原子构成的具有六边形蜂巢结构的二维晶体材料。伴随着石墨烯的成功剥离,以石墨烯为代表的二维材料成为材料科学和凝聚态物理等领域的研究新热点。石墨烯在费米能级附近的电子能带结构呈现线性的能量一动量色散关系,因其满足描述相对论粒子的狄拉克方程,这种能带结构被称为狄拉克锥。石墨烯中的狄拉克锥为二维材料的基础和应用研究提供了一个理想的模型,进而促进了自旋电子学和超导现象等领域的发展。基于石墨烯晶格的Haldane模型和Kane-Mele模型是描述量子反常霍尔态和量子自旋霍尔态的最早理论模型,为二维材料中拓扑电子态的后续研究打下了重要的理论基础;通过应变和电荷掺杂可以有效地调节石墨烯中电子—声子耦合强度,这为提高二维超导材料的超导转变温度提供了新的途径,推动了超导现象在新型二维材料中的研究进展。另外,狄拉克锥特点的能带结构使石墨烯本身具有优异的导热导电性,促进了对其他新型二维狄拉克材料的探索和研究。在二维材料拓扑电子态和超导特性的研究领域,最大程度的增大拓扑非平庸带隙和超导能隙是研究的重点之一。寻找实验上合成的、具有更大能隙的二维拓扑绝缘体和超导体有利于在更高的工作温度下实现上述奇异的电子行为,从而为相应电子态的实际应用提供材料选择。同时,探究二维拓扑电子态和超导特性的产生和调控机制,对于提高拓扑带隙和超导能隙或者产生新奇的量子现象具有重要意义,从而为设计、然后制备出具有更优异性能的二维材料提供理论指导。本论文以一系列新型二维材料为研究对象,采用基于密度泛函理论的第一性原理计算方法,对自旋—轨道耦合所导致的拓扑非平庸性和电子—声子耦合所导致的超导特性展开了系统的理论预测和调控。所取得的代表性成果如下:1、揭示了二维类石墨烯氮化碳材料(Graphitic Carbon Nitrides)中电子能带结构的调控规律,并成功证明了量子反常霍尔态在这种轻质元素材料中存在的可能性。针对实验上以Heptazine为结构单元合成的二维类石墨烯氮化碳材料(g-C3N4),探讨了空穴掺杂和拉伸应变对电子自旋极化和磁性耦合方式的调控规律。在此基础上,预言了两种具有自旋极化零带隙半导体(Spin-Gapless Semiconductor)性质的二维类石墨烯氮化碳材料:g-C10N9和g-C14N12。这两种材料具有稳定的铁磁耦合基态,较强的自旋—轨道耦合效应在费米能级处打开了陈数(Chern number)为-1的拓扑非平庸带隙,比石墨烯的拓扑带隙大三个量级,对应着更高的工作温度。因此,量子反常霍尔效应有望在这种不含金属的轻质元素材料中实现,为氮化碳材料的应用打开了新的领域。2、金属原子和有机配体组成的二维金属—有机材料(Metal-Organic Frameworks,MOF),具有丰富的晶体结构构型和新奇的物理特性。本论文针对具有Kagome晶格的二维有机拓扑绝缘体材料:HTT-Pt,探究了电荷掺杂对这种二维材料的电子自旋极化和拓扑特性的调控规律。从理论上预言了改变电子掺杂浓度可以导致HTT-Pt发生从普通绝缘体到量子自旋霍尔绝缘体、再到量子反常霍尔绝缘体的转变。同时,重金属元素铂(Pt)使HTT-Pt具有较强的自旋—轨道耦合作用,拓扑非平庸带隙可以达到～42.5 meV。因此,通过适当的电子掺杂,HTT-Pt有望在较高温度下实现量子自旋甚至是量子反常霍尔效应。3、得益于近年来实验纳米技术手段的提高,关于二维材料或者原子层厚度材料超导特性的研究被建立起来,并且取得了巨大的进步。基于实验上合成的、具有优异导电性的金属—有机材料:Cu-BHT,我们首次从理论上预言了超导特性在MOF中存在的可能性。二维单层Cu-BHT的超导转变温度(T。)约为4.43K,高于相应三维块体材料的转变温度(Tc≈1.58K)。对于这种异常的温度—维度变化规律,我们也进行了相应的讨论。对于单层Cu-BHT,铜原子和硫原子的低频振动模式与费米能级附近恰当的电子态之间具有较强的耦合作用;当单层Cu-BHT堆垛成三维块体材料时,较强的层间相互作用破坏了上述电子态,从而削弱了电子—声子耦合强度。这与界面效应等方式提高二维材料超导转变温度的机制不同,对进一步提升二维超导材料的超导能隙具有重要意义。4、基于实验上合成的C36富勒烯薄膜,成功预言了一种由准sp2-sp3轨道杂化碳原子形成的二维狄拉克材料:ph-graphene。与石墨烯类似,碳原子的pz轨道在费米能级处形成了自旋简并的狄拉克锥,费米速度可以达到2.8×105m/s。氢原子吸附可以有效调控其电学性质,包括产生电子自旋极化和打开带隙等。同时,ph-graphene具有"柔软"特性,面内刚度(In-plane Stiffness)不到石墨烯的十分之一,因此有望在柔性电子学领域得到应用。
[Abstract]:Shi Moxi (graphene) is a two-dimensional crystal material with hexagonal honeycomb structure composed of carbon atoms characterized by SP2 hybrid orbits. With the successful stripping of graphene, the two-dimensional material, represented by graphene, has become a new hot spot in the field of material science and condensed matter physics. The band structure presents a linear energy momentum dispersion relation, which satisfies the Dirac equation describing the relativistic particles. This band structure is called the Dirac cone. The Dirac cone in the graphene provides an ideal model for the basis and application of two-dimensional materials, and thus promotes the fields of spintronics and superconductivity. Development. The Haldane model and Kane-Mele model based on the graphene lattice are the earliest theoretical models describing the quantum anomalous Holzer state and the quantum spin Holzer state, which lays an important theoretical basis for the subsequent study of the topological electronic states in the two-dimensional material; the electron phonon coupling in graphene can be effectively regulated through the strain and charge doping. Strength, which provides a new way to improve the superconducting transition temperature of two dimensional superconducting materials, promotes the research progress of superconducting phenomenon in the new two-dimensional material. In addition, the band structure of Dirac cone has made Shi Moxi itself have excellent thermal conductivity and conduce to the exploration and research of his new two-dimensional Dirac material. The research field of the topological electronic state and superconducting properties of two dimensional materials is one of the key points in the study of the maximum increase of topological non mediocre bandgap and superconducting gap. The practical application of the corresponding electronic states provides material selection. At the same time, it is of great significance to explore the generation and regulation mechanism of the two-dimensional topological electronic state and superconducting properties. It is of great significance to improve the topological band gap and superconducting gap, or to produce novel quantum phenomena, thus providing theoretical guidance for the design, and then the preparation of two dimensional materials with more excellent properties. In this paper, a series of new two-dimensional materials are taken as the research object. Using the first principle calculation method based on the density functional theory, the theoretical prediction and regulation of the superconductivity caused by the topological non mediocre and electron phonon coupling caused by the spin orbit coupling are theoretically predicted and regulated. The representative achievements are as follows: 1 The regulation of the electronic band structure in Graphitic Carbon Nitrides is shown, and the possibility of the existence of the quantum abnormal Holzer state in this light element material is proved successfully. On the basis of the regulation of doping and tensile strain on the electron spin polarization and magnetic coupling mode, two two-dimensional graphite like carbon nitride materials with spin polarized zero band gap semiconductor (Spin-Gapless Semiconductor) properties are predicted. The two materials of g-C10N9 and g-C14N12. have stable ferromagnetic coupling ground state, and the stronger self The spin orbit coupling effect opens the topological non ordinary band gap of Chen Shu (Chern number) at Fermi level, which is three orders of magnitude larger than the topological band gap of graphene, corresponding to the higher working temperature. Therefore, the quantum anomalous Holzer effect is expected to be realized in this non metallic light element material and opens the application of the carbon nitride material. A new field of.2, a two-dimensional metal organic material (Metal-Organic Frameworks, MOF), consisting of metal atoms and organic ligands, has a rich crystal structure and novel physical properties. In this paper, a two-dimensional organic topological insulator with Kagome lattice, HTT-Pt, is used to explore the electronic charge doping to this two-dimensional material. The regulation of spin polarization and topological properties has been predicted theoretically that the change of the electron doping concentration can lead to the transformation of HTT-Pt from ordinary insulators to quantum spin Holzer insulators and to quantum abnormal Holzer insulators. At the same time, the heavy metal element platinum (Pt) has a strong spin orbit coupling effect on the HTT-Pt, and the topology is not flat. The mean band gap can reach to 42.5 meV., so HTT-Pt is expected to achieve quantum spin and even quantum abnormal Holzer effect at higher temperatures by proper electron doping. Thanks to the improvement of the experimental nanotechnology in recent years, the study of the superconductivity of two dimensional materials or atomic layer thickness materials has been established and obtained. Great progress. Based on an experimentally synthesized metal organic material with excellent conductivity, Cu-BHT, we have theoretically predicted the possibility of the existence of superconductivity in MOF for the first time. The superconductivity transition temperature (T.) of a two-dimensional single layer Cu-BHT is about 4.43K, higher than the transition temperature of the corresponding three-dimensional bulk material (Tc 1.58K). For a single Cu-BHT, the low-frequency vibration modes of copper and sulfur atoms are strongly coupled with the appropriate electronic states near the Fermi level for a single layer of copper. When a single layer of Cu-BHT is stacked into a three-dimensional bulk material, the strong interlayer interaction destroys the above electronic state. It weakens the electron phonon coupling strength, which is different from the mechanism of improving the superconducting transition temperature of two-dimensional materials, such as the interface effect, and is of great significance to the further enhancement of the superconducting gap of the two-dimensional superconducting material. Based on the experimental synthesis of the C36 fullerene film, a kind of quasi sp2-sp3 orbital hybrid carbon atoms has been successfully prefaced. The two-dimensional Dirac material: ph-graphene. is similar to graphene. The PZ orbit of the carbon atom forms a spin degenerate Dirac cone at Fermi level. The Fermi velocity can reach 2.8 * 105m/s. hydrogen adsorption, which can effectively regulate its electrical properties, including the generation of electron spin polarization and opening of the band gap. At the same time, the ph-graphene has "softness". The In-plane Stiffness is less than 1/10 of graphene, so it is expected to be applied in the field of flexible electronics.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469

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