Topological insulator materials

Topological insulators possess conductive surface states while the bulk states are insulating. Due to the strong spin-orbit coupling in topological insulators, the surface states are helical, i.e., the spin and momentum of a conduction electron are coupled to each other. Therefore, topological insulators are an interesting system for investigating spin transport.

For the investigation of the magnetic interaction of the helical surface states of topological insulators with ferromagnets, nanosheets of Bi₂Te₃ have been deposited by hot wall epitaxy on Fe-based layers that were initially prepared by molecular beam epitaxy on GaAs substrates. However, Fe-Te alloys were formed during the deposition on pure Fe due to intermixing. In contrast, the generation of the Fe-Te alloys is prevented, if the deposition is carried out on Fe₃Si or Co₂FeSi layers. Based on this experience, the next steps are to realize helical surface states of a topological insulator at the interface with a ferromagnet and to explore spin transport in these hybrid structures.

(111)-oriented InP provides nearly lattice-matched substrates for the growth of topological-insulator layers of Bi₂S₃ and Bi₂Te₃. For the growth of Bi₂Te₃ layers, a rare growth mode of axiotaxy has been identified to occur at high deposition temperatures. While axiotaxy itself is an interesting subject to investigate, this discovery improves the ability to control the epitaxial orientation of topological-insulator layers. It enables us to investigate the transport through helical states along surfaces of different orientations of a single crystal.

When Bi₂S₃ layers are deposited on InP and InAs substrates, β-In₂S₃ layers form as a consequence of material mixing. A similar material substitution is also employed for synthesizing (Bi,Sb)-(Cu,Ag,Ni)-(S,Se,Te) alloys introduced above as a component of  interfacial-resistance-switching devices. Although the crystal structure of β-In₂S₃ is tetragonal, the β-In₂S₃ layers grow semicoherently on the cubic InP and InAs substrates due to a purely accidental lattice matching. This observation allows to combine photovoltaics based on In₂S₃ with optoelectronics based on (In,Ga)As/InP heterostructures in a single-crystal system.

In addition to the interaction of topological insulators with ferromagnets, their coupling with superconductors also produces an exotic phenomenon. When a superconductor is attached to a normal conductor, the electrons in the normal conductor experience Andreev reflection from the interface with the superconductor. An electron-like excitation is reflected from the interface as a hole-like excitation due to the creation of a Cooper pair in the superconductor. The Andreev reflection produces Majorana quasiparticles in the presence of a strong spin-orbit interaction in the normal conductor as for example in a topological insulator. Currently, the non-Abelian braiding statistics of Majorana zero modes is intensively investigated in the field of quantum computing. The aim is to define qubits for topological quantum computation.

Author: Y. Takagaki
Title: Effects of disorder on magnetotransport oscillations in a two-dimensional electron gas terminated by superconductors
Source: J. Appl. Phys. , 128 , 024304 ( 2020 )
DOI: 10.1063/5.0005384

Author: Y. Takagaki , B. Jenichen , O. Brandt
Title: Semicoherent growth of single-crystal beta-In2S3 layers on InP(111) and InAs(111)
Source: CrystEngComm , 21 , 5818 ( 2019 )
DOI: 10.1039/c9ce01135h