Bulk metals with helical surface states

TL;DR

This paper theoretically investigates bulk metals near topological insulator states, revealing that they support robust helical surface states that are pushed away from bulk states, forming finite-lifetime surface resonances.

Contribution

It introduces a theoretical model showing how helical surface states in bulk metals are preserved and behave in the presence of hybridization and disorder.

Findings

Surface states are pushed away from bulk states in energy and momentum.

Hybridization can lead to multiple surface state bands.

Surface states form finite-lifetime resonances rather than being absorbed.

Abstract

In the flurry of experiments looking for topological insulator materials, it has been recently discovered that some bulk metals very close to topological insulator electronic states, support the same topological surface states that are the defining characteristic of the topological insulator. First observed in spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the helical surface states in the metallic systems appear to be robust to at least mild disorder. We present here a theoretical investigation of the nature of these "helical metals" - bulk metals with helical surface states. We explore how the surface and bulk states can mix, in both clean and disordered systems. Using the Fano model, we discover that in a clean system, the helical surface states are \emph{not} simply absorbed by hybridization with a non-topological parasitic metallic band. Instead, they are pushed away from overlapping in momentum and energy with the bulk states, leaving behind a finite-lifetime surface resonance in the bulk energy band. Furthermore, the hybridization may lead in some cases to multiplied surface state bands, in all cases retaining the helical characteristic. Weak disorder leads to very similar effects - surface states are pushed away from the energy bandwidth of the bulk, leaving behind a finite-lifetime surface resonance in place of the original surface states.