【作者】 李平；

【导师】 罗卫东；

【作者基本信息】 上海交通大学 ， 物理学， 2017， 博士

【摘要】 本论文用第一性原理计算方法,在二维（2D）材料的设计与改性,拓扑绝缘体（TI）的电子态调控,确定不明界面结构,判别和矫正目前被广泛误用的晶体结构,预测电子结构,以及半导体的杂质性质等方面进行了多项研究。此处介绍其中的六项主要研究及其成果。1)石墨烯、硅烯、锗烯、黑磷等是近年来被广泛关注的元素2D材料。这些材料均具有honeycomb或变形的honeycomb结构,由六原子环组成。我们预测了一种由八原子环组成的V族元素的2D新结构,称为八角砖（OT）结构。声子谱和第一性原理分子动力学模拟表明,该结构具有良好的动力学和热力学稳定性。这些OT材料具有0.3到2.0 eV不等的带隙,因此在近红外和中红外光电器件等方面有潜在应用。OT-Bi是大带隙2D TI,带隙值为0.33 eV,并且该带隙可以通过应变进一步增加。2)TI表面态的全局磁交换带隙是很多新奇量子现象实现的必要条件。理论预测在拓扑绝缘体/磁性绝缘体（TI/MI）异质结中可以实现上述带隙,但由于能带弯曲,实际的TI/MI系统总是呈n型导电性。我们提出了一种电荷补偿方法来解决上述问题。用P或As原子替换Bi₂Se₃/MnSe的界面Se原子,可以补偿界面电荷,矫正能带弯曲,在狄拉克表面态打开全局带隙。上述电荷补偿方法被应用于多个TI/MI异质结,获得了19到48 meV不等的全局带隙。论文详细研究了带隙打开后狄拉克表面态的自旋结构,并引入模型哈密顿量帮助理解其中的磁交换耦合作用。电荷补偿方法为实验上TI/MI异质结全局带隙的实现提供了参考方案。3)2D TIs的实际应用要求它们具有大的带隙,且在实验衬底上能够保持拓扑非平凡属性。理论提出了很多2D TIs,但只有HgTe量子阱得到了实验确认,一个重要的原因可能是衬底破坏了拓扑相。第一性原理计算发现,氢化和卤化可以将本征性质为金属的Bi（110）双原子层变为带隙从0.45到0.93 eV的大带隙2D TIs。p_z-σ成键-反键机制和电荷转移机制可以分别解释氢化和卤化情况下Bi（110）的相变。上述材料表现出良好的动力学和热力学稳定性,且其拓扑属性可以在较大应变下保持不变。在MoSe₂和黑磷衬底上,氢化的Bi层是近freestanding状态的大带隙2D TI。4)理论预测s波超导体（SC）和拓扑绝缘体表面态的近邻效应会在超导涡旋中产生Majorana费米子。Wang等[Science 336,52（2012）]在实验上成功制备出了高质量的NbSe₂/Bi₂Se₃异质结,并在其中同时观测到了超导带隙和拓扑表面态。NbSe₂和Bi₂Se₃的面内晶格失配达到20%,为了理解高质量NbSe₂/Bi₂Se₃的生长过程,我们与Wang等合作研究了该SC/TI异质结的界面结构。第一性原理计算结合STM形貌研究表明,在NbSe₂衬底上同时淀积Bi原子和Se原子时,会先形成BiSe双原子层界面结构。BiSe双原子层具有类NaCl结构,在衬底上形成chain-like图样以降低能量。BiSe界面与Bi₂Se₃在一个晶格方向上具有良好的晶格匹配,有利于高质量NbSe₂/Bi₂Se₃的制备。5)γ-PtBi₂因为具有线性巨磁阻等性质引起了人们的广泛关注。之前对该材料的理论和实验研究均基于一种具有Pˉ3空间群对称性的γ-PtBi₂结构,此处称为γ₁-PtBi₂。结构优化及声子谱计算都表明γ₁-PtBi₂相是不稳定的。相反,另一种具有P31m空间群对称性的γ₂-PtBi₂具有良好的稳定性。最近实验上测得的γ-PtBi₂的STM形貌和AR PES能谱都表明该材料具有两个不同的解理面,与γ₂-PtBi₂的情况一致,并且ARPES能谱与第一性原理计算的γ₂-PtBi₂的能带很好地符合。因此,γ₂-PtBi₂很可能是实验上γ-PtBi₂的正确结构。计算发现,γ₂-PtBi₂具有沿方位角（θ,?）≈（30^?,12^?）方向的倾斜柱状费米面,可据此预言该方向的强de Haas-van Alphen震荡。6)C掺杂ZnO的室温铁磁以及C浓度越高其平均饱和磁矩M_s越小的规律一直缺少相关理解。杂化泛函计算为上述现象提供了定性解释。在Zn-rich条件和n型ZnO中,C_O替位占支配地位,且根据费米能级的位置不同,每个C_O替位可携带1或2μ_B的磁矩。在低C浓度下,C_O替位距离较远时呈现铁磁耦合,因此M_s较大;C浓度增加时,C_O替位会与近邻其它C原子聚合为非磁性的C₂分子缺陷,从而M_s变小。研究表明,高温退火可以淬灭C掺杂ZnO的磁性。此外,该研究不支持C作为ZnO的p型掺杂剂。更多还原

【Abstract】 Using first-principles computational methods,several studies have been carried out in this thesis,including the design of two-dimensional（2D）materials,manipulation of the topolog-ical surface states of topological insulators（TIs）,understanding interfacial atomic structures,determination of bulk crystal structures,prediction of electronic structures,and the impurity properties of semiconductors.Among these,six studies are summarized in the following.1)The elemental 2D materials such as graphene,silicene,germanene,and black phospho-rus have attracted considerable attention due to their fascinating physical properties.Structurally they possess the honeycomb or distorted honeycomb lattices,which are composed of six-atom rings.Here we find a new structure of 2D allotropes of group V elements composed of eight-atom rings,which we name as the octagonal tiling（OT）structure.First-principles calculations show that these allotropes are dynamically stable and are also thermally stable at temperatures up to 600 K.These allotropes are semiconductors with band gaps ranging from 0.3 to 2.0 eV,thus they are potentially useful in near-and mid-infrared optoelectronic devices.Furthermore,OT-Bi is a 2D TI with a band gap of 0.33 eV,which is the largest among the reported elemental2D TIs,and this gap can be increased further by applying compressive strains.2)A magnetic exchange energy gap on the topological surface states of 3D topological insu-lators is essential for observing many exotic phenomena.It was theoretically predicted to occur in a topological insulator/magnetic insulator（TI/MI）heterostructure,however real samples are often n-type due to the interfacial band bending.In this study a charge compensation method is proposed to solve this problem.By substituting the interfacial Se atoms in Bi₂Se₃/MnSe het-erostructures with P or As atoms,a global energy gap opens at the Dirac surface states.The interfacial charges are compensated by the interfacial substitutions,and band bending is mostly corrected.This compensation mechanism also applies to other TI/MI heterostructures,and the full gaps in our studied systems range from 19 to 48 meV.The spin textures of the resulting massive Dirac fermion states are studied in details,and a model Hamiltonian is introduced to help understand the magnetic exchange coupling.The charge compensation approach in this work may facilitate the realization of full energy gaps in experimental TI/MI systems.3)To realize device applications of 2D TIs,the materials need to possess large energy gaps,and to retain their topological properties when fabricated on substrates.Although many2D TIs have been theoretically proposed,only the HgTe/CdTe quantum well is experimentally confirmed.One important reason may be that the substrate effects destroy the topological phase.Here based on first-principles calculations we find that the Bi（110）bilayer,which is metallic in its pristine form,can transform into 2D TIs through hydrogenation and halogenation.Their energy gaps are among the largest for 2D TIs,ranging from 0.45 to 0.93 eV.A p_z-σbonding-antibonding splitting mechanism and a charge-transfer mechanism are proposed to understand them.These topologically insulating films exhibit excellent dynamical and thermal stability,and could retain their nontrivial topological properties against large strains and substrates in-teraction.On MoSe₂and black phosphorus substrates,the functionalized Bi films form nearly freestanding 2D TIs and show large global energy gaps,suitable for room temperature applica-tions.4)In theory the proximity effect between an s-wave superconductor（SC）and the surface states of a TI creates Majorana fermions in the vortex.Wang et al[Science 336,52（2012）]have fabricated the high-quality NbSe₂/Bi₂Se₃heterostructure,where the superconducting gap as well as the topological surface states coexist.However,the in-plane lattice mismatching of these two materials is as large as 20%.To understand the growth process of the high-quality NbSe₂/Bi₂Se₃,their interfacial structure has been investigated by experimentally collaborating with Wang et al.Studies based on first-principles calculations and STM images show that a BiSe bilayer forms as the interfacial layer when Bi and Se atoms are codeposited on the NbSe₂substrate.BiSe bilayer has a NaCl-like structure,forming a chain-like pattern on the substrate to lower energy.This interfacial BiSe provides good lattice matching to Bi₂Se₃along one lattice vector,helpful to the obtaining of high-quality NbSe₂/Bi₂Se₃.5)γ-PtBi₂has attracted much attention due to its gaint linear magneto-resistance.Both theoretical and experimental studies on this material are based on theγ-PtBi₂structure with a Pˉ3 group symmetry,which is calledγ₁-PtBi₂here.Structural relaxation and phonon spectrum all show the instability of this phase.On the contrary,aγ₂-PtBi₂phase with P31m group symmetry is found to be very stable.Both the STM topography and ARPES images indicateγ-PtBi₂has two different cleavage surfaces,in agreement with the case inγ₂-PtBi₂,and ARPES bands also agree well with the first-pinciples bands ofγ₂-PtBi₂.Therefore,γ₂-PtBi₂must be the correct structure of the experimentalγ-PtBi₂.Calculations also find that a cylinder-shaped Fermi surface exists inγ-PtBi₂along（θ,?）≈（30^?,12^?）direction,implying strong De Haas-Van Alphen oscillation in this direction.6)The mechanism of room temperature（RT）ferromagnetism in C-doped ZnO and the trend that the higher the C concentration,the smaller the saturation magnetic moment（M_s）,remain controversial and puzzling for a long time.Using density-functional theory calculations with hybrid functional,we qualitatively understand these experimental phenomena.The sub-stitutional C_Odefects dominate in Zn-rich conditions and n-type materials.They carry 1 or 2μ_B/C,depending on the position of the Fermi level.In low C concentrations and at RT,the C_Odefects are kept isolated and prefer to couple ferromagnetically,thus the M_sis larger.As the C concentration increases,nonmagnetic C₂dimers form through binding of two C_Odefects or binding of one C_Odefect with one interstitial C,thus decreasing the M_s.Our results also suggest that the ferromagnetism from C_Odefects can be quenched by higher annealing temper-atures.Furthermore,the p-type conductivity from carbon dopant is not supported in the present study.更多还原