Unoccupied Topological States on Bismuth Chalcogenides

TL;DR

This study investigates unoccupied topological surface states in bismuth chalcogenides using experimental and theoretical methods, revealing spin-polarized Dirac states above the Fermi level that are robust across different compositions and structural variations.

Contribution

It provides the first detailed analysis of unoccupied topological surface states in Bi$_2$Te$_{x}$Se$_{3-x}$, combining ARPES, DFT, and circular dichroism to confirm their topological nature.

Findings

Unoccupied Dirac surface states found at ~1.3 eV above Fermi level.

Surface states are spin-polarized and topologically protected.

States are robust against compositional and structural changes.

Abstract

The unoccupied part of the band structure of topological insulators Bi$_2$Te$_{x}$Se$_{3-x}$ ($x=0,2,3$) is studied by angle-resolved two-photon photoemission and density functional theory. For all surfaces linearly-dispersing surface states are found at the center of the surface Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical analysis shows that this feature appears in a spin-orbit-interaction induced and inverted local energy gap. This inversion is insensitive to variation of electronic and structural parameters in Bi$_2$Se$_3$ and Bi$_2$Te$_2$Se. In Bi$_2$Te$_3$ small structural variations can change the character of the local energy gap depending on which an unoccupied Dirac state does or does not exist. Circular dichroism measurements confirm the expected spin texture. From these findings we assign the observed state to an unoccupied topological surface state.