engleski

Topological insulators and their crystal structures

Topological insulator (TI) is a novel class of materials which can be described as an insulator that always has a metallic boundary when placed next to the vacuum or an “ordinary” insulator. These metallic boundaries originate from topological invariants which cannot change as long as material remains insulating. 3D TI’s are narrow gap semiconductors with inverted energy gap caused by strong spin-orbit coupling. Surface states behaves like massless Dirac fermions with linear dispersion (in contrast to kvadratic dispersion in normal metallic states), and show a characteristic Dirac cone with a crossing point at/near the Fermi level. Electron transport on the surface is spin dependent and dissipationless. It is interesting that crystal structures for discovered TI’s seem to be not too complex and belongs to just few crystal types. For instance, 3D TI’s known today as Bi2Se3, Bi2Te3, Sb2Te3, Bi2Te2Se ... all share the same crystal symmetry – rhomboedral R3m (#166) – Tetradymite type. Some other, like recently discovered Kondo insulator SmB6 as a TI – belongs to the cubic F43m (#216) – Zincblende type. If you want to make a material to be “topologicaly protected” where to looking for, and what are the basic elements to pay attention? The role of the crystal symmetry and atomic type in looking for TI’s materials, and how the density functional theory (DFT) can help in preliminary selection, will be shown in the talk.

Topological insulators; Crystal structure; Kondo insulator

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