(a) Electronic band structure of blue phosphorus oxide (BPO) without strain. A and B label the non-degenerate state at the conduction band bottom and the doubly-degenerate state at the valence band top, respectively. (b) Energies of A and B states versus strain. The crossing of the two corresponds to the quantum phase transition between a semiconductor phase and a symmetry protected semimetal (SSM) phase. (c) Energy dispersion around the band crossing point at Fermi level for critical strain +0.6 % (upper panel) and (lower panel) +4 % strain. The low-energy quasiparticles around the crossing point are described by 2D pseudospin-1 fermions and 2D double Weyl fermions.^[92] Band structures of the (d) BiF and (e) SbF monolayers with (red) and without (gray) SOC. The bands around the Fermi level consist of the p_x and p_y orbitals. The size of the symbols is proportional to the population of the p_x and p_y orbitals.^[96]