Quantum-confinement and Structural Anisotropy result in Electrically-Tunable Dirac Cone in Few-layer Black Phosphorous

TL;DR

This study demonstrates that quantum confinement and structural anisotropy in few-layer black phosphorus enable an electric-field-tunable Dirac cone, with potential for electronic applications due to controllable topological phase transitions.

Contribution

The paper reveals that applying an electric field to few-layer black phosphorus induces a Dirac cone, a phenomenon tunable by film thickness and electric field strength, which is a novel finding in 2D materials.

Findings

Electric field reduces band gap leading to insulator-metal transition.

Dirac cone can be induced and tuned by electric field in thin black phosphorus.

Spin-orbit coupling affects the Dirac cone, enabling topological phase transitions.

Abstract

2D materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, E_ext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, E_c. Increasing E_ext beyond E_c can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semi-metal transition.