Blue Phosphorene Oxide: Strain-tunable Quantum Phase Transitions and Novel 2D Emergent Fermions

TL;DR

This paper introduces blue phosphorene oxide as a stable 2D material that undergoes strain-induced quantum phase transitions, hosting novel 2D pseudospin-1 and double Weyl fermions with potential for advanced nanoscale devices.

Contribution

The study predicts a new 2D material, BPO, exhibiting tunable quantum phase transitions and hosting novel emergent fermions, expanding the understanding of quantum materials.

Findings

Strain induces a semiconductor-to-semimetal transition in BPO.

At the critical point, BPO hosts 2D pseudospin-1 fermions.

Beyond the transition, BPO exhibits symmetry-protected double Weyl fermions.

Abstract

Tunable quantum phase transitions and novel emergent fermions in solid state materials are fascinating subjects of research. Here, we propose a new stable two-dimensional (2D) material, the blue phosphorene oxide (BPO), which exhibits both. Based on first-principles calculations, we show that its equilibrium state is a narrow-bandgap semiconductor with three bands at low energy. Remarkably, a moderate strain can drive a semiconductor-to-semimetal quantum phase transition in BPO. At the critical transition point, the three bands cross at a single point at Fermi level, around which the quasiparticles are a novel type of 2D pseudospin-1 fermions. Going beyond the transition, the system becomes a symmetry-protected semimetal, for which the conduction and valence bands touch quadratically at a single Fermi point that is protected by symmetry, and the low-energy quasiparticles become another novel type of 2D double Weyl fermions. We construct effective models characterizing the phase transition and these novel emergent fermions, and we point out several exotic effects, including super Klein tunneling, supercollimation, and universal optical absorbance. Our result reveals BPO as an intriguing platform for the exploration of fundamental properties of quantum phase transitions and novel emergent fermions, and also suggests its great potential in nanoscale device applications.