Heterostructures of Graphene and Topological Insulators Bi2Se3, Bi2Te3, and Sb2Te3

Klaus Zollner,Jaroslav Fabian

doi:10.1002/pssb.202000081

Klaus Zollner, Jaroslav Fabian

Open Access

https://doi.org/10.1002/pssb.202000081

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Abstract

Prototypical 3D topological insulators of the Bi2Se3 family provide a beautiful example of the appearance of the surface states inside the bulk bandgap caused by spin‐orbit coupling‐induced topology. The surface states are protected against backscattering by time reversal symmetry, and exhibit spin‐momentum locking whereby the electron spin is polarized perpendicular to the momentum, typically in the plane of the surface. In contrast, graphene is a prototypical 2D material, with negligible spin‐orbit coupling. When graphene is placed on the surface of a topological insulator, giant spin‐orbit coupling is induced by the proximity effect, enabling interesting novel electronic properties of its Dirac electrons. A detailed theoretical study of the proximity effects of monolayer graphene and topological insulators Bi2Se3, Bi2Te3, and Sb2Te3 is presented, and the appearance of the qualitatively new spin‐orbit splittings, well described by a phenomenological Hamiltonian, is elucidated by analyzing the orbital decomposition of the involved band structures. This should be useful for building microscopic models of the proximity effects between the surfaces of the topological insulators and graphene.

Highlights

The 3D topological insulators[1] Bi2Se3, Bi2Te3, and Sb2Te3, are prototypical bulk materials demonstrating topological surface states, with a potential for practical applications, such as photodetectors and transistors.[2]
We compare the energy splitting ΔE, extracted from the calculated band structures, with the estimated potential difference ΔV, as function of the applied electric field. Both depend linearly on the applied field, as expected, but the energy splitting ΔE is smaller than the estimated potential difference ΔV for all field values. This can be attributed to the fact that the surface states are localized within top and bottom quintuple layers (QLs) and their spatial separation is not exactly equal to the thickness d, as we use in the estimation for ΔV
We find that the Dirac point of graphene, as well as the band edge originating from the topological insulator is located at the Fermi level

Summary

Introduction

The 3D topological insulators[1] Bi2Se3, Bi2Te3, and Sb2Te3, are prototypical bulk materials demonstrating topological surface states, with a potential for practical applications, such as photodetectors and transistors.[2]. Our first-principles results, considering eight layers of Bi2Te3 where the Dirac surface states have already formed, are consistent with literature We consider graphene/topological insulator bilayers where we are interested in the proximity-induced SOC in graphene. The proximity-induced SOC in graphene is similar, but with variations in magnitude (0.1–1 meV), for the considered topological insulators Bi2Se3, Bi2Te2Se, and Sb2Te3. Motivated by the recent spin-charge conversion experiments in graphene on ðBi0.15Sb0.85Þ2Te3,[40] we extensively discuss the case of graphene/Sb2Te3, including spin-orbit fields, and the gate tunability of proximity-induced SOC and the doping level. We find a giant electric field tunability of Rashba and intrinsic SOC, in magnitude and sign, which is important to interpret the aforementioned experimental data

Topological Band Structure of Bi2Te3

Proximitized Graphene

Sb2Te3 Substrate

Conclusion