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Blue Phosphorene Oxide: Strain-tunable Quantum Phase Transitions and Novel 2D Emergent Fermions
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.Comment: 23 pages, 5 figure
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