152 research outputs found
The Electronic Structures and Energies of the Lowest Excited States of the Ns0, Ns+, Ns− and Ns-H Defects in Diamond
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (grant agreement No. 101052200—EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. AP also acknowledges, with thanks, the financial support provided by “Strengthening of the capacity of doctoral studies at the University of Latvia within the framework of the new doctoral model”, No. 8.2.2.0/20/I/006, and Scientific Research Project for Students and Young Researchers, Nr. SJZ/2021/5 implemented at the Institute of Solid State Physics, University of Latvia.This paper reports the energies and charge and spin distributions of the mono-substituted N defects, N0s, N+s, N−s and Ns-H in diamonds from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP function. These predict that (i) Ns0, Ns+ and Ns− all absorb in the region of the strong optical absorption at 270 nm (4.59 eV) reported by Khan et al., with the individual contributions dependent on the experimental conditions; (ii) Ns-H, or some other impurity, is responsible for the weak optical peak at 360 nm (3.44 eV); and that Ns+ is the source of the 520 nm (2.38 eV) absorption. All excitations below the absorption edge of the diamond host are predicted to be excitonic, with substantial re-distributions of charge and spin. The present calculations support the suggestion by Jones et al. that Ns+ contributes to, and in the absence of Ns0 is responsible for, the 4.59 eV optical absorption in N-doped diamonds. The semi-conductivity of the N-doped diamond is predicted to rise from a spin-flip thermal excitation of a CN hybrid orbital of the donor band resulting from multiple in-elastic phonon scattering. Calculations of the self-trapped exciton in the vicinity of Ns0 indicate that it is essentially a local defect consisting of an N and four nn C atoms, and that beyond these the host lattice is essential a pristine diamond as predicted by Ferrari et al. from the calculated EPR hyperfine constants. © 2023 by the authors.--//-- This is an open access article Platonenko A., Mackrodt W.C., Dovesi R.; The Electronic Structures and Energies of the Lowest Excited States of the Ns0, Ns+, Ns− and Ns-H Defects in Diamond; (2023) Materials, 16 (5), art. no. 1979; DOI: 10.3390/ma16051979; https://www.scopus.com/inward/record.uri?eid=2-s2.0-85149871852&doi=10.3390%2fma16051979&partnerID=40&md5=b11fbcbf91ce1013d1a0e817573fd2fe published under the CC BY 4.0 licence.EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (grant agreement No. 101052200—EUROfusion); Latvijas Universitate 8.2.2.0/20/I/006, SJZ/2021/5; Scientific Research Project for Students and Young Researchers, Nr. SJZ/2021/5 implemented at the Institute of Solid State Physics, University of Latvia; the Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01–2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2
The Effect of the Environment on alpha-Al_2O_3 (0001) Surface Structures
We report that calculating the Gibbs free energy of the alpha-Al_2O_3 (0001)
surfaces in equilibrium with a realistic environment containing both oxygen and
hydrogen species is essential for obtaining theoretical predictions consistent
with experimental observations. Using density-functional theory we find that
even under conditions of high oxygen partial pressure, the metal terminated
surface is surprisingly stable. An oxygen terminated alpha-Al_2O_3 (0001)
surface becomes stable only if hydrogen is present on the surface. In addition,
including hydrogen on the surface resolves discrepancies between previous
theoretical work and experimental results with respect to the magnitude and
direction of surface relaxations.Comment: 4 pages including 2 figures. Submitted to Phys. Rev. Lett. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Water Chemisorption and Reconstruction of the MgO Surface
The observed reactivity of MgO with water is in apparent conflict with
theoretical calculations which show that molecular dissociation does not occur
on a perfect (001) surface. We have performed ab-initio total energy
calculations which show that a chemisorption reaction involving a
reconstruction to form a (111) hydroxyl surface is strongly preferred with
Delta E = -90.2kJ/mol. We conclude that protonation stabilizes the otherwise
unstable (111) surface and that this, not the bare (001), is the most stable
surface of MgO under ambient conditions.Comment: RevTeX, 4 pages, 1 Encapsulated Postscript Figur
Transition metal materials: a first principles approach to the electronic structure of the insulating phase
Recent progress in the application of first principles theory to the electronic structure of transition metal materials is reviewed with particular emphasis on the use of the exact exchange interaction. The success of this approach is exemplified by calculations on a range of materials: simple monoxides, chromium cyanides and perovskite structure copper fluorides. The reliability of computed properties is established for lattice structures, spin-couplings, spin-lattice interactions, orbital ordering effects and the changes in the ground state induced by hole doping.</p
Effects of pressure on diffusion and vacancy formation in MgO from non-empirical free-energy integrations
The free energies of vacancy pair formation and migration in MgO were
computed via molecular dynamics using free-energy integrations and a
non-empirical ionic model with no adjustable parameters. The intrinsic
diffusion constant for MgO was obtained at pressures from 0 to 140 GPa and
temperatures from 1000 to 5000 K. Excellent agreement was found with the zero
pressure diffusion data within experimental error. The homologous temperature
model which relates diffusion to the melting curve describes well our high
pressure results within our theoretical framework.Comment: 4 pages, latex, 1 figure, revtex, submitted to PR
Exchange coupling in CaMnO and LaMnO: configuration interaction and the coupling mechanism
The equilibrium structure and exchange constants of CaMnO and LaMnO
have been investigated using total energy unrestricted Hartree-Fock (UHF) and
localised orbital configuration interaction (CI) calculations on the bulk
compounds and MnO and MnO clusters. The
predicted structure and exchange constants for CaMnO are in reasonable
agreement with estimates based on its N\'eel temperature. A series of
calculations on LaMnO in the cubic perovskite structure shows that a
Hamiltonian with independent orbital ordering and exchange terms accounts for
the total energies of cubic LaMnO with various spin and orbital orderings.
Computed exchange constants depend on orbital ordering. UHF calculations tend
to underestimate exchange constants in LaMnO, but have the correct sign
when compared with values obtained by neutron scattering; exchange constants
obtained from CI calculations are in good agreement with neutron scattering
data provided the Madelung potential of the cluster is appropriate. Cluster CI
calculations reveal a strong dependence of exchange constants on Mn d e
orbital populations in both compounds. CI wave functions are analysed in order
to determine which exchange processes are important in exchange coupling in
CaMnO and LaMnO.Comment: 25 pages and 9 postscript figure
Current diagnosis and treatment algorithms for anal incontinence
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75666/1/j.1464-410X.2006.06307.x.pd
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