MnBi2Se4-Based Magnetic Modulated Heterostructures

Thin films of magnetic topological insulators (TIs) are expected to exhibit a quantized anomalous Hall effect when the magnetizations on the top and bottom surfaces are parallel and a quantized topological magnetoelectric effect when the magnetizations have opposite orientations. Progress in the observation of these quantum effects was achieved earlier in the films with modulated magnetic doping. On the other hand, the molecular-beam-epitaxy technique allowing the growth of stoichiometric magnetic van der Waals blocks in combination with blocks of topological insulator was developed. This approach should allow the construction of modulated heterostructures with the desired architecture. In the present paper, based on the first-principles calculations, we study the electronic structure of symmetric thin film heterostructures composed of magnetic MnBi2Se4 blocks (septuple layers, SLs) and blocks of Bi2Se3 TI (quintuple layers, QLs) in dependence on the depth of the magnetic SLs relative to the film surface and the TI spacer between them. Among considered heterostructures we have revealed those characterized by nontrivial band topology.

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.