Two-Dimensional Dirac Fermions Protected by Space-Time Inversion Symmetry in Black Phosphorus

TL;DR

This paper demonstrates the existence of symmetry-protected Dirac fermions in black phosphorus, a 2D semiconductor, revealing their stability and potential for exploring topological phases and Weyl semimetals.

Contribution

It introduces black phosphorus as a new 2D system hosting protected Dirac points, with tunable band inversion and direct experimental evidence of Dirac point dynamics.

Findings

Dirac points created and moved along symmetry axes

Dirac point stability due to spacetime inversion symmetry

Black phosphorus as a model for 2D topological semimetals

Abstract

We report the realization of novel symmetry-protected Dirac fermions in a surface-doped two-dimensional (2D) semiconductor, black phosphorus. The widely tunable band gap of black phosphorus by the surface Stark effect is employed to achieve a surprisingly large band inversion up to ~0.6 eV. High-resolution angle-resolved photoemission spectra directly reveal the pair creation of Dirac points and their moving along the axis of the glide-mirror symmetry. Unlike graphene, the Dirac point of black phosphorus is stable, as protected by spacetime inversion symmetry, even in the presence of spin-orbit coupling. Our results establish black phosphorus in the inverted regime as a simple model system of 2D symmetry-protected (topological) Dirac semimetals, offering an unprecedented opportunity for the discovery of 2D Weyl semimetals.