New topological surface state in layered topological insulators: unoccupied Dirac cone

TL;DR

This paper predicts a new unoccupied topological surface state with a Dirac cone in layered topological insulators, expanding understanding of their electronic surface properties using density functional theory.

Contribution

It identifies a previously unreported unoccupied Dirac cone surface state in layered topological insulators through theoretical calculations.

Findings

Surface state with linear dispersion appears in the conduction band gap.

Presence of local gaps and parity change are necessary for the Dirac cone.

The unoccupied Dirac cone is linked to the inverted conduction band energy gap.

Abstract

The unoccupied states in topological insulators Bi_2Se_3, PbSb_2Te_4, and Pb_2Bi_2Te_2S_3 are studied by the density functional theory methods. It is shown that a surface state with linear dispersion emerges in the inverted conduction band energy gap at the center of the surface Brillouin zone on the (0001) surface of these insulators. The alternative expression of Z_2 invariant allowed us to show that a necessary condition for the existence of the second Gamma Dirac cone is the presence of local gaps at the time reversal invariant momentum points of the bulk spectrum and change of parity in one of these points.