96 research outputs found
Quasiparticle band structure based on a generalized Kohn-Sham scheme
We present a comparative full-potential study of generalized Kohn-Sham
schemes (gKS) with explicit focus on their suitability as starting point for
the solution of the quasiparticle equation. We compare quasiparticle
band structures calculated upon LDA, sX, HSE03, PBE0, and HF functionals for
exchange and correlation (XC) for Si, InN and ZnO. Furthermore, the HSE03
functional is studied and compared to the GGA for 15 non-metallic materials for
its use as a starting point in the calculation of quasiparticle excitation
energies. For this case, also the effects of selfconsistency in the
self-energy are analysed. It is shown that the use of a gKS scheme as a
starting point for a perturbative QP correction can improve upon the
deficiencies found for LDA or GGA staring points for compounds with shallow
bands. For these solids, the order of the valence and conduction bands is often
inverted using local or semi-local approximations for XC, which makes
perturbative calculations unreliable. The use of a gKS starting point
allows for the calculation of fairly accurate band gaps even in these difficult
cases, and generally single-shot calculations following calculations
using the HSE03 functional are very close to experiment
GW band structure of InAs and GaAs in the wurtzite phase
We report the first quasiparticle calculations of the newly observed wurtzite
polymorph of InAs and GaAs. The calculations are performed in the GW
approximation using plane waves and pseudopotentials. For comparison we also
report the study of the zinc-blende phase within the same approximations. In
the InAs compound the In 4d electrons play a very important role: whether they
are frozen in the core or not, leads either to a correct or a wrong band
ordering (negative gap) within the Local Density Appproximation (LDA). We have
calculated the GW band structure in both cases. In the first approach, we have
estimated the correction to the pd repulsion calculated within the LDA and
included this effect in the calculation of the GW corrections to the LDA
spectrum. In the second case, we circumvent the negative gap problem by first
using the screened exchange approximation and then calculating the GW
corrections starting from the so obtained eigenvalues and eigenfunctions. This
approach leads to a more realistic band-structure and was also used for GaAs.
For both InAs and GaAs in the wurtzite phase we predict an increase of the
quasiparticle gap with respect to the zinc-blende polytype.Comment: 9 pages, 6 figures, 3 table
Universality of electron accumulation at wurtzite c- and a-plane and zinc-blende InN surfaces
Electron accumulation is found to occur at the surface of wurtzite (112¯0), (0001), and (0001¯) and zinc-blende (001) InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low Г-point conduction band minimum lying
significantly below the charge neutrality level
Novel Reconstruction Mechanism for Dangling-Bond Minimization: Combined Method Surface Structure Determination of SiC(111)-(3×3)
The SiC(111)−(3×3) phase was analyzed by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) holography, density functional theory (DFT), and conventional LEED. A single adatom per unit cell found in STM acts as a beam splitter for the holographic inversion of discrete LEED spot intensities. The resulting 3D image guides the detailed analyses by LEED and DFT which find a Si tetramer on a twisted Si adlayer with cloverlike rings. This twist model with one dangling bond left per unit cell represents a novel (n×n)-reconstruction mechanism of group-IV (111) surfaces
Electronic structure of the (111) and (-1-1-1) surfaces of cubic BN: A local-density-functional ab initio study
We present ab initio local-density-functional electronic structure
calculations for the (111) and (-1-1-1) surfaces of cubic BN. The energetically
stable reconstructions, namely the N adatom, N3 triangle models on the (111),
the (2x1), boron and nitrogen triangle patterns on the (-1-1-1) surface are
investigated. Band structure and properties of the surface states are discussed
in detail.Comment: 8 pages, 12 figure
Electronic structure and the minimum conductance of a graphene layer on SiO2 from density-functional methods.
The effect of the SiO substrate on a graphene film is investigated using
realistic but computationally convenient energy-optimized models of the
substrate supporting a layer of graphene. The electronic bands are calculated
using density-functional methods for several model substrates. This provides an
estimate of the substrate-charge effects on the behaviour of the bands near
, as well as a variation of the equilibrium distance of the graphene
sheet. A model of a wavy graphene layer is examined as a possible candidate for
understanding the nature of the minimally conducting states in graphene.Comment: 6 pages, 5 figure
Accurate calculation of polarization-related quantities in semiconductors
We demonstrate that polarization-related quantities in semiconductors can be
predicted accurately from first-principles calculations using the appropriate
approach to the problem, the Berry-phase polarization theory. For III-V
nitrides, our test case, we find polarizations, polarization differences
between nitride pairs, and piezoelectric constants quite close to their
previously established values. Refined data are nevertheless provided for all
the relevant quantities.Comment: RevTeX 4 pages, no figure
All electron and pseudopotential study of the spin polarization of the V (001) surface: LDA versus GGA
The spin-polarization at the V(001) surface has been studied by using
different local (LSDA) and semilocal (GGA) approximations to the
exchange-correlation potential of DFT within two ab initio methods: the
all-electron TB-LMTO-ASA and the pseudopotential LCAO code SIESTA (Spanish
Initiative for Electronic Simulations with Thousands of Atoms). A comparative
analysis is performed first for the bulk and then for a N-layer V(001) film (7
< N < 15). The LSDA approximation leads to a non magnetic V(001) surface with
both theoretical models in agreement (disagreement) with magneto-optical Kerr
(electron-capture spectroscopy) experiments. The GGA within the pseudopotential
method needs thicker slabs than the LSDA to yield zero moment at the central
layer, giving a high surface magnetization (1.70 Bohr magnetons), in contrast
with the non magnetic solution obtained by means of the all-electron code.Comment: 12 pages, 1 figure. Latex gzipped tar fil
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