Twin Domain Structure in Magnetically Doped Bi2Se3 Topological Insulator

Twin domains are naturally present in the topological insulator Bi2Se3 and strongly affect its properties. While studies of their behavior in an otherwise ideal Bi2Se3 structure exist, little is known about their possible interaction with other defects. Extra information is needed, especially for the case of an artificial perturbation of topological insulator states by magnetic doping, which has attracted a lot of attention recently. Employing ab initio calculations based on a layered Green’s function formalism, we study the interaction between twin planes in Bi2Se3. We show the influence of various magnetic and nonmagnetic chemical defects on the twin plane formation energy and discuss the related modification of their distribution. Furthermore, we examine the change of the dopants’ magnetic properties at sites in the vicinity of a twin plane, and the dopants’ preference to occupy such sites. Our results suggest that twin planes repel each other at least over a vertical distance of 3–4 nm. However, in the presence of magnetic Mn or Fe defects, a close twin plane placement is preferred. Furthermore, calculated twin plane formation energies indicate that in this situation their formation becomes suppressed. Finally, we discuss the influence of twin planes on the surface band gap.

Twinning in SnO2-based ceramics doped with CoO and Nb2O5: morphology of multiple twins revealed by electron backscatter diffraction

Electron backscatter diffraction (EBSD) was used for the analysis of multiple cyclic twins in cassiterite (SnO2), which form during sintering of SnO2 with small additions of CoO and Nb2O5. Grain misorientation analysis has shown that about one third of all grains contain {101} twin boundaries (TBs). The majority of these grains are contact twins, whereas a small fraction of grains are multiple, mainly cyclic twins. A procedure was developed in MTEX [Bachmann, Hielscher & Schaeben (2010). Solid State Phenom. 160, 63–88] for automated identification of crystallographically different types of cyclic twins and found two main types: coplanar twins composed of three or four domains with a common [010] axis and alternating twins composed of three to seven domains oriented along the [111] axis. Both types of cyclic twins have a characteristic common origin (nucleus) of all TBs, which is positioned eccentric relative to the grain section and the cycle is closed with a shorter non-crystallographic contact between the first and the last twin domain. The morphology of cyclic twins suggests that they form by nucleation in the initial stages of grain growth. The average size of twinned grains increases with the number of twin domains indicating the influence of TBs formation on the growth of composite grains.

Twin domain imaging in topological insulator Bi2Te3 and Bi2Se3 epitaxial thin films by scanning X-ray nanobeam microscopy and electron backscatter diffraction

The twin distribution in topological insulators Bi2Te3 and Bi2Se3 was imaged by electron backscatter diffraction (EBSD) and scanning X-ray diffraction microscopy (SXRM). The crystal orientation at the surface, determined by EBSD, is correlated with the surface topography, which shows triangular pyramidal features with edges oriented in two different orientations rotated in the surface plane by 60°. The bulk crystal orientation is mapped out using SXRM by measuring the diffracted X-ray intensity of an asymmetric Bragg peak using a nano-focused X-ray beam scanned over the sample. By comparing bulk- and surface-sensitive measurements of the same area, buried twin domains not visible on the surface are identified. The lateral twin domain size is found to increase with the film thickness.

Structure cristalline du composé Hg3-xSbx(S+Se)2+xI2-x(x≃ 0.1)

Single crystals of the mercury chalcohalide Hg3-xSbx(S+Se)2+xI2-x(x≃ 0.1) (mercury antimony sulfide selenide iodide), were grown by a chemical transport reaction. The structure contains three independentA(Hg/Sb) atoms; each atom is strongly covalently bonded with twoX(Se/S) atoms to form approximately linearX–A–Xunits. TheX–A–Xunits link to formA4X4rings, which are combined into infinite crankshaft-type bands running along the [100] direction. Four equatorialE(I/X= Se,S) atoms at relatively long distances complete the distorted octahedral coordination ofA(Hg/Sb). The crystal under investigation was twinned by non-merohedry with a refined twin domain fraction of 0.814 (6):0.186 (6). The structure is isotypic with Hg3Se2I2[Beck & Hedderich (2000).J. Solid State Chem.151, 73–76], but the current determination reveals a coupled substitution, with partial replacement of Hg+2by Sb+3, balanced by the equivalent substitution of I−1by S−2and Se−2. Bond-valence calculations are consistent with this relative substitution model.

Crystal structure oftrans-1-{2-[4-(dimethylamino)phenyl]ethyl}-4-[2-(pyren-1-yl)ethyl]cyclohexane

The title compound, C34H37N, is a pyrene derivative in which the pyrene ring system is linked to an ethylcyclohexane unit which, in turn, carries a [4-(dimethylamino)phenyl]ethyl substituent in theparaposition. The central cyclohexane ring has a chair conformation, with the exocyclic C—C bonds in equatorial orientations. The benzene ring is inclined to the mean plane of the pyrene ring system [maximum deviation = 0.038 (4) Å] by 14.84 (15)°. In the crystal, molecules are linked by C—H...π interactions, forming chains propagating along [010]. The crystal was refined as a non-merohedral twin [domain ratio = 0.9989 (4):0.0011 (4)].