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Alloy theory with atomic resolution for Rashba or topological systems

2019-05-16

Authors: Wang, Z; Luo, JW; Zunger, A
PHYSICAL REVIEW MATERIALS
Volume: 3 Issue: 4 Published: APR 26 2019 Language: English Document type: Article
DOI: 10.1103/PhysRevMaterials.3.044605
Abstract:
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 (AX)(x)(BX)(1-x) of components AX and BX generally manifests a distribution of local environments, whereby each X site, for example, can be locally decorated by different substitutional arrangements of {A, B, X} atoms, thus creating an inherently polymorphous network. Electrons will then respond to the existence of different local environments and site symmetries, creating local charge transfer and atomic displacements patterns observed in experiments. While the macroscopic average structure S-0, as seen by probes with long coherence length, may have the original high symmetry of the constituent compounds, many observable physical properties are sensitive to local symmetry, and are hence the average < P(S-i)> of the properties {P(S-i); i = 1, ... , N} of the individual microscopic configurations {S-i; i = 1, N} rather than the property P(< S-i >) = P(S-0) of the macroscopically averaged high-symmetry (monomorphous) configuration S-0. The fundamental difference between the polymorphous representation < P(S-i)> versus the monomorphous P(S-0) in modeling substitutionally disordered alloys led to the often diverging results between methods that "see" atomic details and those that see only the high symmetry of the constituents, while missing the atomic-scale resolution needed in many cases to discern local symmetry-related physics. A natural approach that captures the polymorphous aspect of random alloys is the well-known supercell approach where lattice sites are occupied by the alloyed elements with a particular form of disorder and solved via periodic electronic structure methods for sufficiently large supercells. However, such approaches tend to produce complex E versus k dispersion relations ("spaghetti bands"), rendering the wave-vector k information needed in theories of topology and Rashba physics and seen in angular resolved experiments, practically inaccessible. The results of such calculations have consequently been displayed as density of states. A solution that retains the polymorphous nature of the random alloy but reinstates the E versus k relation in the base Brillouin zone is to unfold the supercell bands. This yields alloy "effective band structure" (EBS), providing a three-dimensional picture of the distribution of spectral density in the whole Brillouin zone. It consists of E- and k-dependent spectral weight with coherent and incoherent features, all created naturally by the specific nature of the chemical bonding underlying the polymorphous distribution of many local environments. We illustrate this EBS approach for CdTe-HgTe, PbSe-SnSe, and PbS-PbTe alloys, showing atomic-scale effects such as formation of a distribution of A-X and B-X bond lengths, local charge transfer, and the creation and destruction of valley degeneracies. In CdTe-HgTe, the disorder effect is so weak that the incoherent term is negligible, and the monomorphous approaches are still feasible in this alloy. In PbSe-SnSe, the stronger disorder effect introduces significant (similar to 150 meV) band splitting of the topological band inversion, forming a sequential inversion of multiple bands which is important for the topological transition but absent in monomorphous methods. In PbS-PbTe, there is a strong disorder effect, revealing the emergence of ferroelectricity from the polymorphous network in this alloy.
全文链接:https://arxiv.org/abs/1901.01289



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