Intrinsic and extrinsic perturbations on the topological insulator Bi2Se3 surface states

TL;DR

This study investigates how intrinsic factors like atomic relaxations and extrinsic factors such as dielectric films influence the surface states of Bi2Se3 topological insulators, revealing effects on the Dirac cone and surface state confinement.

Contribution

It provides a detailed analysis of how various perturbations affect the topological surface states of Bi2Se3, including the role of dielectric substrates and film thickness.

Findings

Atomic relaxations do not affect Dirac cone degeneracy.

Decreasing film thickness opens a gap in surface states.

Quartz substrate's surface termination impacts Dirac cone preservation.

Abstract

Using a density functional based electronic structure method, we study the effect of perturbations on the surface state Dirac cone of a strong topological insulator Bi$_2$Se$_3$ from both the intrinsic and extrinsic sources. We consider atomic relaxations, and film thickness as intrinsic and interfacial thin dielectric films as an extrinsic source of perturbation to the surface states. We find that atomic relaxations has no effect on the degeneracy of the Dirac cone whereas film thickness has considerable effect on the surface states inducing a gap which increases monotonically with decrease in film thickness. We consider two insulating substrates BN and quartz as dielectric films and show that surface terminations of quartz with or without passivation plays critical role in preserving Dirac cone degeneracy whereas BN is more inert to the TI surface states. The relative orbital contribution with respect to bulk is mapped out using a simple algorithm, and with the help of it we demonstrate the bulk band inversion when spin-orbit coupling is switched on. The layer projected charge density distributions of the surface states shows that these states are not strictly confined to the surface. The spatial confinement of these states extends up to two to three quintuple layers, a quintuple layer consists of five atomic layers of Bi and Se