Absence of helical surface states in bulk semimetals with broken inversion symmetry

TL;DR

This paper investigates how breaking inversion symmetry affects the existence and properties of topological surface states in mercury chalcogenide semimetals, revealing their instability and tunability.

Contribution

It demonstrates that even slight inversion symmetry breaking destabilizes helical semimetal states and influences surface Dirac cone characteristics in these materials.

Findings

Inversion symmetry breaking destabilizes helical semimetal states.

Small BIS causes anisotropic renormalization of surface Dirac cones.

Biaxial stress can flip the handedness of Dirac cones.

Abstract

Whereas the concept of topological band-structures was developed originally for insulators with a bulk bandgap, it has become increasingly clear that the prime consequences of a non-trivial topology -- spin-momentum locking of surface states -- can also be encountered in gapless systems. Concentrating on the paradigmatic example of mercury chalcogenides HgX (X = Te, Se, S), we show that the existence of helical semimetals, i.e. semimetals with topological surface states, critically depends on the presence of crystal inversion symmetry. An infinitesimally small broken inversion symmetry (BIS) renders the helical semimetallic state unstable. The BIS is also very important in the fully gapped regime, renormalizing the surface Dirac cones in an anisotropic manner. As a consequence the handedness of the Dirac cones can be flipped by a biaxial stress field.