21 research outputs found
High spin states of cation vacancies in GaP, GaN, AlN, BN, ZnO and BeO: A first principles study
High spin states of cation vacancies in GaP, GaN, AlN, BN, ZnO and BeO were
analyzed by first principles calculations. The spin-polarized vacancy-induced
level is located in the band gap in GaP, ZnO and BeO. In the nitrides, the
stronger exchange coupling forces the vacancy states to be resonant with
valence bands, forbids formation of positively charged vacancies in GaN and BN,
and allows Al vacancy in p-AlN to assume the highest possible S=2 spin state.
The shape of the spin density, isotropic in the zinc blende structure, has a
pronounced directional character in the wurtzite structure. Stability of spin
polarization of the vacancy states is determined by spin polarization energies
of anions, as well as by interatomic distances between the vacancy neighbors,
and thus is given by both the lattice constant of the host and the atomic
relaxations around the vacancy. Implications for experiment are discussed.Comment: 9 pages, 5 figure
Ag and N acceptors in ZnO: ab initio study of acceptor pairing, doping efficiency, and the role of hydrogen
Efficiency of ZnO doping with Ag and N shallow acceptors, which substitute
respectively cations and anions, was investigated. First principles
calculations indicate a strong tendency towards formation of nearest neighbor
Ag-N pairs and N-Ag-N triangles. Binding of acceptors stems from the formation
of quasi-molecular bonds between dopants, and has a universal character in
semiconductors. The pairing increases energy levels of impurities, and thus
lowers doping efficiency. In the presence of donors, pairing is weaker or even
forbidden. However, hydrogen has a tendency to form clusters with Ag and N,
which favors the Ag-N aggregation and lowers the acceptor levels of such
complexes.Comment: 10 pages, 4 figure
Effect of non-magnetic impurities on the magnetic states of anatase TiO
The electronic and magnetic properties of TiO, TiO,
TiON, and TiOF compounds have been studied
by using \emph{ab initio} electronic structure calculations. TiO is found
to evolve from a wide-band-gap semiconductor to a narrow-band-gap semiconductor
to a half-metallic state and finally to a metallic state with oxygen vacancy,
N-doping and F-doping, respectively. Present work clearly shows the robust
magnetic ground state for N- and F-doped TiO. The N-doping gives rise to
magnetic moment of 0.4 at N-site and 0.1 each at
two neighboring O-sites, whereas F-doping creates a magnetic moment of
0.3 at the nearest Ti atom. Here we also discuss the possible
cause of the observed magnetic states in terms of the spatial electronic charge
distribution of Ti, N and F atoms responsible for bond formation.Comment: 11 pages, 4 figures To appear J. Phys.: Condens. Matte
Ab initio probing of the electronic band structure and Fermi surface of fluorine-doped WO3 as a novel low-TC superconductor
First-principles calculations were performed to investigate the electronic
structure and the Fermi surface of the newly discovered low-temperature
superconductor: fluorine-doped WO3. We find that F doping provides the
transition of the insulating tungsten trioxide into a metallic-like phase
WO3-xFx, where the near-Fermi states are formed mainly from W 5d with admixture
of O 2p orbitals. The cooperative effect of fluorine additives in WO3 consists
in change of electronic concentration as well as the lattice constant. At
probing their influence on the near-Fermi states separately, the dominant role
of the electronic factor for the transition of tungsten oxyfluoride into
superconducting state was established. The volume of the Fermi surface
gradually increases with the increase of the doping. In the sequence WO3
\rightarrow WO2.5F0.5 the effective atomic charges of W and O ions decrease,
but much less, than it is predicted within the idealized ionic model - owing to
presence of the covalent interactions W-O and W-F.Comment: 8 pages, 4 figure