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  • 1.
    Castleton, C W M
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Mirbt, S
    Finite-size scaling as a cure for supercell approximation errors in calculations of neutral native defects in InP2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 19, p. 195202-Article in journal (Refereed)
    Abstract [en]

    The relaxed and unrelaxed formation energies of neutral antisites and interstitial defects in InP are calculated using ab initio density functional theory and simple cubic supercells of up to 512 atoms. The finite-size errors in the formation energies of all the neutral defects arising from the supercell approximation are examined and corrected for using finite-size scaling methods, which are shown to be a very promising approach to the problem. Elastic errors scale linearly, while the errors arising from charge multipole interactions between the defect and its images in the periodic boundary conditions have a linear plus a higher order term, for which a cubic provides the best fit. These latter errors are shown to be significant even for neutral defects. Instances are also presented where even the 512 atom supercell is not sufficiently converged. Instead, physically relevant results can be obtained only by finite-size scaling the results of calculations in several supercells, up to and including the 512 atom cell and in extreme cases possibly even including the 1000 atom supercell.

  • 2.
    Castleton, Christopher W. M.
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Höglund, A.
    Mirbt, S.
    Managing the supercell approximation for charged defects in semiconductors: Finite-size scaling, charge correction factors, the band-gap problem, and the ab initio dielectric constant2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 3, p. 035215-Article in journal (Refereed)
    Abstract [en]

    The errors arising in ab initio density functional theory studies of semiconductor point defects using the supercell approximation are analyzed. It is demonstrated that (a) the leading finite size errors are inverse linear and inverse cubic in the supercell size and (b) finite size scaling over a series of supercells gives reliable isolated charged defect formation energies to around +/- 0.05 eV. The scaled results are used to test three correction methods. The Makov-Payne method is insufficient, but combined with the scaling parameters yields an ab initio dielectric constant of 11.6 +/- 4.1 for InP. Gamma point corrections for defect level dispersion are completely incorrect, even for shallow levels, but realigning the total potential in real-space between defect and bulk cells actually corrects the electrostatic defect-defect interaction errors as well. Isolated defect energies to +/- 0.1 eV are then obtained using a 64 atom supercell, though this does not improve for larger cells. Finally, finite size scaling of known dopant levels shows how to treat the band gap problem: in <= 200 atom supercells with no corrections, continuing to consider levels into the theoretical conduction band (extended gap) comes closest to experiment. However, for larger cells or when supercell approximation errors are removed, a scissors scheme stretching the theoretical band gap onto the experimental one is in fact correct.

  • 3. Höglund, A
    et al.
    Castleton, Christopher W. M.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Mirbt, S
    Relative concentration and structure of native defects in GaP2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 19, p. 195213-Article in journal (Refereed)
    Abstract [en]

    The native defects in the compound semiconductor GaP have been studied using a pseudopotential density functional theory method in order to determine their relative concentrations and the most stable charge states. The electronic and atomic structures are presented and the defect concentrations are estimated using calculated formation energies. Relaxation effects are taken into account fully and produce negative-U charge transfer levels for V-P and P-Ga. The concentration of V-Ga is in good agreement with the results of positron annihilation experiments. The charge transfer levels presented compare qualitatively well with experiments where available. The effect of stoichiometry on the defect concentrations is also described and is shown to be considerable. The lowest formation energies are found for P-Ga(+2) in p-type and V-Ga(-3) in n-type GaP under P-rich conditions, and for Ga-P(-2) in n-type GaP under Ga-rich conditions. Finally, the finite size errors arising from the use of supercells with periodic boundary conditions are examined.

  • 4. Wilkins, S. B.
    et al.
    Stojic, N.
    Beale, T. A. W.
    Binggeli, N.
    Castleton, Christopher
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Bencok, P.
    Prabhakaran, D.
    Boothroyd, A. T.
    Hatton, P. D.
    Altarelli, M.
    Resonant soft x-ray scattering investigation of orbital and magnetic ordering in La0.5Sr1.5MnO42005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 24, p. 245102-Article in journal (Refereed)
    Abstract [en]

    We report resonant x-ray scattering data of the orbital and magnetic ordering at low temperatures at the Mn L-2,L-3 edges in La0.5Sr1.5MnO4. The orderings display complex energy features close to the Mn absorption edges. Systematic modeling with atomic multiplet crystal field calculations was used to extract meaningful information regarding the interplay of spin, orbital, and Jahn-Teller order. These calculations provide a good general agreement with the observed energy dependence of the scattered intensity for a dominant orbital ordering of the d(x)(2)-z(2)/d(y)(2)-z(2) type. In addition, the origins of various spectral features are identified. The temperature dependence of the orbital and magnetic ordering was measured and suggests a strong interplay between the magnetic and orbital order parameters.

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