Symmetry-protected degeneracies in the electronic band structure in a phosphorene oxide

Phosphorene, a single layer of black phosphorus, has attracted a lot of interest since its isolation from black phosphorus by mechanical exfoliation. Since it is extremely reactive with oxygen owing to the lone pairs of electrons in the phosphorous atoms, oxidation is unavoidable during synthesis and fabrication processes. The oxidation removes inversion symmetry in the system while keeping two nonsymmorphic symmetries allowing rich features in the electronic band structure. We have found two separate double degenerate band lines cause by different symmetry origins as well as a circular line node and 4-fold degenerate Dirac point without an inversion symmetry. Even though the electronic filling of the system is not 4n+2, such energy levels may be reached with reasonable electronic doping. In the same system, we have also observed a topological phase transition controlled by tensile strain due to level inversion based on first-principles calculations and explicit evaluation of Z₂ topological invariant.