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arXiv 1901.01289cond-mat.mtrl-sciphysics.comp-ph
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Rashba或拓扑系统的具有原子分辨的合金理论

Alloy theory with atomic resolution for Rashba or topological systems

Zhi Wang, Jun-Wei Luo, Alex Zunger

摘要:

替代无序合金的兴趣最近重新出现,重点是在合金中的拓扑绝缘和Rashba效应的灵敏性能。替代随机合金体现了局部环境的分布,创造了多形态网络。虽然宏观平均(单晶)结构可能具有组成化合物的原始高对称性,但许多可观察到的物理性质对局部对称性敏感,因此是$<P(S_i)>$而不是$P(S_0)$ = $P(<S_i>)$。多形$<P(S_i)>$和单形$P(S_0)$之间的根本差异导致了经常出现分歧的结果,并失去了识别与原子相关的物理所需的原子尺度分辨率。捕捉多态性方面的一种自然方法是超胞模型,然而,它遭受带折叠的困难(“意大利面条带”),使得拓扑和Rashba物理学中所需的E与k色散,以及实验中所见的,实际上无法实现。一个保留多态性质但恢复E与k关系的解决方案是展开超原胞带。这产生合金有效能带结构(EBS),提供由具有相干和非相干特征的E-和k-依赖的光谱权重组成的光谱密度的3D图片,所有这些都是由许多局部环境的多形态分布自然产生的。我们说明了这种EBS方法的碲化镉,碲化铅,硒化铅和PbS-PbTe合金。我们发现了对于例如拓扑相变和Rashba分裂至关重要但在常规单态方法中完全不存在的性质,包括(1)共存的、依赖于波矢和能量的相干带分裂和非相干带展宽,(2)沿不同k空间方向的相干-非相干跃迁,以及(3)具有相干和非相干特征的Rashba样带分裂。

英文摘要:

Interest in substitutional disordered alloys has recently reemerged with focus on the symmetry-sensitive properties in the alloy such as topological insulation and Rashba effect. A substitutional random alloy manifests a distribution of local environments, creating a polymorphous network. While the macroscopic average (monomorphous) structure may have the original high symmetry of the constituent compounds, many observable physical properties are sensitive to local symmetry, and are hence $<P(S_i)>$ rather than $P(S_0)$=$P(<S_i>)$. The fundamental difference between polymorphous $<P(S_i)>$ and monomorphous $P(S_0)$ led to the often-diverging results and the missing the atomic-scale resolution needed to discern symmetry-related physics. A natural approach capturing the polymorphous aspect is supercell model, which however suffers the difficulty of band folding ('spaghetti bands'), rendering the E vs k dispersion needed in topology and Rashba physics and seen in experiments, practically inaccessible. A solution that retains the polymorphous nature but restores the E vs k relation is to unfold the supercell bands. This yields alloy Effective Band Structure (EBS), providing a 3D picture of spectral density consisting of E- and k-dependent spectral weight with coherent and incoherent features, all created naturally by the polymorphous distribution of many local environments. We illustrate this EBS approach for CdTe-HgTe, PbSe-SnSe and PbS-PbTe alloys. We found properties that are critical for e.g. topological phase transition and Rashba splitting but totally absent in conventional monomorphous approaches, including (1) co-existing, wavevector- and energy-dependent coherent band splitting and incoherent band broadening, (2) coherent-incoherent transition along different k space directions, and (3) Rashba-like band splitting having both coherent and incoherent features.

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