Two-dimensional Dirac Fermions on Oxidized Black Phosphorous

We explore the oxidation of a single layer of black phosphorus using ab initio density functional theory calculation. We search for the equilibrium structures of phosphorene oxides, POx with various oxygen concentrations x (0≤x≤1). By evaluating the formation energies with diverse configurations and their vibrational properties for each of various x values, we identify a series of stable oxidized structures with x and confirm that the oxidation occurs naturally. We also find that oxidation makes some modes from the P-O bonds P-P bonds IR-active implying that the infrared spectra can be used to determine the degree of oxidation of phosphorene. Our electronic structure calculation reveals that the fully oxidized phosphorene (PO) has a direct bandgap of 0.83 eV similar to the pristine phosphorene. Intriguingly, the PO possesses two nonsymmorphic symmetries with the inversion symmetry broken, guaranteeing a symmetry-protected band structure including the band degeneracy and four-fold degenerate Dirac points. Our results provide an important guide in searching for the rare example of Dirac semimetal with a higher level of degeneracy, giving significant insight into the relations between the symmetry of lattice and band topology of electrons.