508 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
Coarsened Lattice Spatial Disorder in the Thermodynamic Limit
In this Rapid Research Note the application of recently introduced [Physica A
277 (2000) 157] entropic measure S_Delta of spatial disorder for systems of
finite-sized objects is presented. In the thermodynamic limit the critical
behaviour of coarsened lattice model of random two-phase systems is illustrated
for certain grain size distributions (GSDs) and chosen parameters. Also the
changes of spatial disorder, quantified by S_Delta, between the limit GSDs
clearly show that the topological equivalence of the two phases is broken.Comment: 3 pages, 1 figur
Anomalous Angular Dependence of the Dynamic Structure Factor near Bragg Reflections: Graphite
The electron energy-loss function of graphite is studied for momentum
transfers q beyond the first Brillouin zone. We find that near Bragg
reflections the spectra can change drastically for very small variations in q.
The effect is investigated by means of first principle calculations in the
random phase approximation and confirmed by inelastic x-ray scattering
measurements of the dynamic structure factor S(q,\omega). We demonstrate that
this effect is governed by crystal local field effects and the stacking of
graphite. It is traced back to a strong coupling between excitations at small
and large momentum transfers
ZnSe/GaAs(001) heterostructures with defected interfaces: structural, thermodynamic and electronic properties
We have performed accurate \emph{ab--initio} pseudopotential calculations for
the structural and electronic properties of ZnSe/GaAs(001) heterostructures
with interface configurations accounting for charge neutrality prescriptions.
Beside the simplest configurations with atomic interdiffusion we consider also
some configurations characterized by As depletion and cation vacancies,
motivated by the recent successfull growth of ZnSe/GaAs pseudomorphic
structures with minimum stacking fault density characterized by the presence of
a defected (Zn,Ga)Se alloy in the interface region. We find that--under
particular thermodynamic conditions--some defected configurations are favoured
with respect to undefected ones with simple anion or cation mixing, and that
the calculated band offsets for some defected structures are compatible with
those measured. Although it is not possible to extract indications about the
precise interface composition and vacancy concentration, our results support
the experimental indication of (Zn,Ga)Se defected compounds in high-quality
ZnSe/GaAs(001) heterojunctions with low native stacking fault density. The
range of measured band offset suggests that different atoms at interfaces
rearrange, with possible presence of vacancies, in such a way that not only
local charges but also ionic dipoles are vanishing.Comment: 26 pages. 5 figures, revised version, in press (Physical Review B
Structural Examination of Au/Ge(001) by Surface X-Ray Diffraction and Scanning Tunneling Microscopy
The one-dimensional reconstruction of Au/Ge(001) was investigated by means of
autocorrelation functions from surface x-ray diffraction (SXRD) and scanning
tunneling microscopy (STM). Interatomic distances found in the SXRD-Patterson
map are substantiated by results from STM. The Au coverage, recently determined
to be 3/4 of a monolayer of gold, together with SXRD leads to three
non-equivalent positions for Au within the c(8x2) unit cell. Combined with
structural information from STM topography and line profiling, two building
blocks are identified: Au-Ge hetero-dimers within the top wire architecture and
Au homo-dimers within the trenches. The incorporation of both components is
discussed using density functional theory and model based Patterson maps by
substituting Germanium atoms of the reconstructed Ge(001) surface.Comment: 5 pages, 3 figure
Statistical Model Applied To Ax By C1-x-y D Quaternary Alloys: Bond Lengths And Energy Gaps Of Alx Gay In1-x-y X (x=as, P, Or N) Systems
We extend the generalized quasichemical approach (GQCA) to describe the Ax By C1-x-y D quaternary alloys in the zinc-blende structure. Combining this model with ab initio ultrasoft pseudopotential calculations within density functional theory, the structural and electronic properties of Alx Gay In1-x-y X (X=As, P, or N) quaternary alloys are obtained, taking into account the disorder and composition effects. Results for the bond lengths show that the variation with the compositions is approximately linear and also does not deviate very much from the value of the corresponding binary compounds. The maximum variation observed amounts to 3.6% for the In-N bond length. For the variation of band gap, we obtain a bowing parameter b=0.26 eV for the (Ga0.47 In0.53 As)z (Al0.48 In0.52 As)1-z quaternary alloy lattice matched to InP, in very good agreement with experimental data. In the case of AlGaInN, we compare our results for the band gap to data for the wurtzite phase. We also obtained a good agreement despite all evidences for cluster formation in this alloy. Finally, a bowing parameter of 0.22 eV is obtained for zinc-blende AlGaInN lattice matched with GaN. © 2006 The American Physical Society.7323Li, J., Nam, K.B., Kim, K.H., Lin, J.Y., Jiang, H.X., (2001) Appl. Phys. Lett., 78, p. 61. , APPLAB 0003-6951 10.1063/1.1331087Adivarahan, V., Chitnis, A., Zhang, J.P., Shatalov, M., Yang, J.W., Simin, G., Asif Khan, M., Shur, M.S., (2001) Appl. Phys. Lett., 79, p. 4240. , APPLAB 0003-6951 10.1063/1.1425453Yasan, A., McClintock, R., Mayes, K., Darvish, S.R., Kung, P., Razegui, M., (2002) Appl. Phys. Lett., 81, p. 801. , APPLAB 0003-6951 10.1063/1.1497709Nagahama, S., Yanamoto, T., Sano, M., Mukai, T., (2001) Jpn. J. Appl. Phys., Part 1, 40, p. 788. , JAPNDE 0021-4922 10.1143/JJAP.40.L788Fujii, T., Nakata, Y., Sigiyama, Y., Hiyiamizu, S., (1986) Jpn. J. Appl. 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Ab-initio calculation of optical absorption in semiconductors: A density-matrix description
We show how to describe Coulomb renormalization effects and dielectric
screening in semiconductors and semiconductor nanostructures within a
first-principles density-matrix description. Those dynamic variables and
approximation schemes which are required for a proper description of dielectric
screening are identified. It is shown that within the random-phase
approximation the direct Coulomb interactions become screened, with static
screening being a good approximation, whereas the electron-hole exchange
interactions remain unscreened. Differences and similarities of our results
with those obtained from a corresponding GW approximation and Bethe-Salpeter
equation Green's function analysis are discussed.Comment: 10 pages, to be published in Physical Review
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