Manabu Usuda

All-electron GW calculation based on the LAPW method: Application to wurtzite ZnO

We present a new, all-electron implementation of the GW approximation and apply it to wurtzite ZnO. Eigenfunctions computed in the local-density approximation (LDA) by the full-potential linearized augmented-plane-wave (LAPW) or the linearized muffin-tin-orbital (LMTO) method supply the input for generating the Green function G and the screened Coulomb interaction W. A mixed basis is used for the expansion of W, consisting of plane waves in the interstitial region and augmented-wavefunction products in the augmentation-sphere regions. The frequency-dependence of the dielectric function is computed within the random-phase approximation (RPA), without a plasmon-pole approximation. The Zn 3d orbitals are treated as valence states within the LDA; both core and valence states are included in the self-energy calculation. The calculated bandgap is smaller than experiment by about 1eV, in contrast to previously reported GW results. Self-energy corrections are orbital-dependent, and push down the deep O 2s and Zn 3d levels by about 1eV relative to the LDA. The d level shifts closer to experiment but the size of shift is underestimated, suggesting that the RPA overscreens localized states.

Band Structures of Wurtzite InN and Ga1-xInxN by All-Electron

The experimentally reported bandgap of wurtzite-type InN dramatically decreased from 1.9 eV to 0.7–0.8 eV very recently. In this paper we report first-principles electronic band-structure calculations of InN by using the all-electron full-potential linearized augmented-plane-wave (FLAPW) method in both local-density approximation (LDA) and GW approximation (GWA), and provide the reliable theoretical bandgap of InN. Our calculation suggests that InN is a narrow-gap semiconductor and strongly supports the recently reported smaller bandgaps. Moreover, we reproduce the bandgap change of Ga1-xInxN ternary alloys as a function of In content x. The present work supports the possibility of bandgap control in the entire range of visible light, using nitrides alone.

GW

Electronic-structure calculations have been carried out for hexagonal NiS with the use of the full-potential linearized augmented plane-wave (FLAPW) method in the local-spin-density approximation + U (LSDA+ U ) approach. The U parameter is treated as an empirical parameter to improve the band structure near the Fermi energy. It is found that this approach succeeds in obtaining the antiferromagnetic state which is the ground state of NiS, and the insulating state with an energy gap of ∼0.1 eV. The photoemission spectra obtained from the density of states of the LSDA+ U , however, fail to explain experiment. We conclude that further advanced treatment of electron-electron interaction is needed for describing the single-electron excited state of hexagonal NiS.

Calculation

We investigate the magnetic resonant x-ray scattering spectra around the K edge of Ni in antiferromagnetic NiO using an ab initio band-structure calculation based on the density-functional theory. By taking account of orbital polarization through the spin-orbit interaction, we reproduce well the spectra obtained experimentally, thus demonstrating the usefulness of the ab initio calculation. It is shown that the main-edge peak, which mainly comes from the dipolar (Is→4p) transition, is a direct reflection of the orbital polarization of the 4p states. It is clarified that the 4p orbital polarization is mainly induced from the spin polarization on the 4p states by the spin-orbit interaction. The 3d orbital polarization at neighboring Ni sites gives rise to only a minor contribution to the 4p orbital polarization through a p-d mixing. It is also shown that the pre-edge peak, which mainly originates from the quadrupolar (1s→3d) transition, is a direct reflection of the orbital polarization of the unoccupied 3d states. It shows a Fano-type antiresonant dip due to interference with the nonresonant contribution, in agreement with the experimental result.