1,710 research outputs found
Band gap and band parameters of InN and GaN from quasiparticle energy calculations based on exact-exchange density-functional theory
We have studied the electronic structure of InN and GaN employing G0W0
calculations based on exact-exchange density-functional theory. For InN our
approach predicts a gap of 0.7 eV. Taking the Burnstein-Moss effect into
account, the increase of the apparent quasiparticle gap with increasing
electron concentration is in good agreement with the observed blue shift of the
experimental optical absorption edge. Moreover, the concentration dependence of
the effective mass, which results from the non-parabolicity of the conduction
band, agrees well with recent experimental findings. Based on the quasiparticle
band structure the parameter set for a 4x4 kp Hamiltonian has been derived.Comment: 3 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Beyond the Random Phase Approximation for the Electron Correlation Energy: The Importance of Single Excitations
The random phase approximation (RPA) for the electron correlation energy,
combined with the exact-exchange energy, represents the state-of-the-art
exchange-correlation functional within density-functional theory (DFT).
However, the standard RPA practice -- evaluating both the exact-exchange and
the RPA correlation energy using local or semilocal Kohn-Sham (KS) orbitals --
leads to a systematic underbinding of molecules and solids. Here we demonstrate
that this behavior is largely corrected by adding a "single excitation" (SE)
contribution, so far not included in the standard RPA scheme. A similar
improvement can also be achieved by replacing the non-self-consistent
exact-exchange total energy by the corresponding self-consistent Hartree-Fock
total energy, while retaining the RPA correlation energy evaluated using
Kohn-Sham orbitals. Both schemes achieve chemical accuracy for a standard
benchmark set of non-covalent intermolecular interactions.Comment: 5 pages, 4 figures, and an additional supplementary materia
GaN/AlN Quantum Dots for Single Qubit Emitters
We study theoretically the electronic properties of -plane GaN/AlN quantum
dots (QDs) with focus on their potential as sources of single polarized photons
for future quantum communication systems. Within the framework of eight-band
k.p theory we calculate the optical interband transitions of the QDs and their
polarization properties. We show that an anisotropy of the QD confinement
potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to
a pronounced linear polarization of the ground state and excited state
transitions. An externally applied uniaxial stress can be used to either induce
a linear polarization of the ground-state transition for emission of single
polarized photons or even to compensate the polarization induced by the
structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte
Exciting prospects for solids: Exact-exchange based functionals meet quasiparticle energy calculations
Focussing on spectroscopic aspects of semiconductors and insulators we will illustrate how quasiparticle energy calculations in the G0W0 approximation can be successfully combined with density-functional theory calculations in the exact-exchange optimised e ective potential approach (OEPx) to achieve a first principles description of the electronic structure that overcomes the limitations of local or gradiant corrected DFT functionals (LDA and GGA)
Benchmarking the accuracy of the separable resolution of the identity approach for correlated methods in the numeric atom-centered orbitals framework
Four-center two-electron Coulomb integrals routinely appear in electronic
structure algorithms. The resolution-of-the-identity (RI) is a popular
technique to reduce the computational cost for the numerical evaluation of
these integrals in localized basis-sets codes. Recently, Duchemin and Blase
proposed a separable RI scheme [J. Chem. Phys. 150, 174120 (2019)], which
preserves the accuracy of the standard global RI method with the Coulomb metric
(RI-V) and permits the formulation of cubic-scaling random phase approximation
(RPA) and approaches. Here, we present the implementation of a separable
RI scheme within an all-electron numeric atom-centered orbital framework. We
present comprehensive benchmark results using the Thiel and the GW100 test set.
Our benchmarks include atomization energies from Hartree-Fock, second-order
M{\o}ller-Plesset (MP2), coupled-cluster singles and doubles, RPA and
renormalized second-order perturbation theory as well as quasiparticle energies
from . We found that the separable RI approach reproduces RI-free HF
calculations within 9 meV and MP2 calculations within 1 meV. We have confirmed
that the separable RI error is independent of the system size by including
disordered carbon clusters up to 116 atoms in our benchmarksComment: 16 pages, 8 figure
Effects of strain on the band structure of group-III nitrides
We present a systematic study of strain effects on the electronic band structure of the group-III-nitrides (AlN, GaN and InN) in the wurtzite phase. The calculations are based on density functional theory (DFT) with band-gap-corrected approaches including hybrid functional (HSE) and quasiparticle G0W0 methods. We study strain effects under realistic strain conditions, hydrostatic pressure and biaxial stress. The strain-induced modification of the band structures is found to be nonlinear; transition energies and crystal-field splittings show a strong nonlinear behavior under biaxial stress. For the linear regime around the experimental lattice parameters, we present a complete set of deformation potentials (acz, act, D1, D2, D3, D4, D5, D6) that allows us to predict the band positions of group-III nitrides and their alloys (InGaN and AlGaN) under realistic strain conditions. The benchmarking G0W0 results for GaN agree well with the HSE data and indicate that HSE provides an appropriate description for the band structures of nitrides. We present a systematic study of strain effects on the electronic band structure of the group-III-nitrides (AlN, GaN and InN). We quantify the nonlinearity of strain effects by introducing a set of bowing parameters. We apply the calculated deformation potentials to the prediction of strain effects on transition energies and valence-band structures of InGaN alloys and quantum wells grown on GaN, in various orientations (including c-plane, m-plane, and semipolar). The calculated band gap bowing parameters including the strain effect for c-plane InGaN agrees well with the results obtained by hybrid functional alloy calculations. For semipolar InGaN QWs grown in (20\overline 2 1), (30\overline 3 1), and (30\overline 3 \overline 1) orientations, our calculated deformation potentials have provided results for polarization ratios in good agreement with the experimental observations, providing further confidence in the accuracy of our values
Social media, protest cultures and political subjectivities of the Arab spring
This article draws on phenomenological perspectives to present a case against resisting the objectification of cultures of protest and dissent. The generative, self-organizing properties of protest cultures, especially as mobilized through social media, are frequently argued to elude both authoritarian political structures and academic discourse, leading to new political subjectivities or âimaginariesâ. Stemming from a normative commitment not to over-determine such nascent subjectivities, this view has taken on a heightened resonance in relation to the recent popular uprisings in the Middle East and North Africa. The article argues that this view is based on an invalid assumption that authentic political subjectivities and cultures naturally emerge from an absence of constraint, whether political, journalistic or academic. The valorisation of amorphousness in protest cultures and social media enables affective and political projection, but overlooks politics in its institutional, professional and procedural forms
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