Non-linear hybrid surface-defect states in defective Bi$_2$Se$_3$

TL;DR

This study reveals that non-magnetic Se vacancies in Bi$_2$Se$_3$ cause significant non-linear hybrid surface-defect states, challenging the assumption of surface-state robustness in topological insulators.

Contribution

It demonstrates through first-principles calculations that Se vacancies induce non-linear band features and hybridization effects, advancing understanding of defect-related phenomena in topological insulators.

Findings

Se vacancies cause non-linear band pinning near Fermi level

Dirac cone shifts deeper into valence band due to defects

Surface-defect hybridization depends on vacancy position and symmetry

Abstract

Surface-states of topological insulators are assumed to be robust against non-magnetic defects in the crystal. However, recent theoretical models and experiments indicate that even non-magnetic defects can perturb these states. Our first-principles calculations demonstrate that the presence of Se vacancies in Bi$_2$Se$_3$, has a greater impact than a mere n-doping of the structure, which would just shift the Fermi level relative to the Dirac point. We observe the emergence of a non-linear band pinned near the Fermi level, while the Dirac cone shifts deeper into the valence band. We attribute these features in the bandstructure to the interaction between the surface and defect states, with the resulting hybridization between these states itself depending on the position and symmetry of the Se vacancy relative to the surfaces. Our results bring us a step closer to understanding the exotic physics emerging from defects in Bi$_2$Se$_3$ that remained unexplored in prior studies.