arXiv subjects

Abdallah Qteish

Publications and source records attributed to Abdallah Qteish.

6 recordsLinked to original sources

arXiv2022

A first-principles investigation of the linear thermal expansion coefficients of BeF$_2$: Giant thermal expansion

We present the results of a theoretical investigation of the linear thermal expansion coefficients (TECs) of BeF$_2$, within a direct Gruneisen formalism where symmetry-preserving deformations are employed. The required physical quantities such as the optimized crystal structures, elastic constants, mode Gruneisen parameters, and phonon density of states are calculated from first-principles. BeF$_2$ shows an extensive polymorphism at low pressures, and the lowest energy phases [$α$-cristobalite with space group (SG) P$4_1 2_1 2$ and its similar phase with SG P$4_3 2_1 2$] are considered in addition to the experimentally observed $α$-quartz phase. For benchmarking purposes, similar calculations are performed for the rutile phase of ZnF$_2$, where the volumetric TEC ($α_v$), derived from the calculated linear TECs along the $a$ ($α_a$) and $c$ ($α_c$) directions, is in very good agreement with experimental data and previous theoretical results. For the considered phases of BeF$_2$, we do not find any negative thermal expansion (NTE). However we observe diverse thermal properties for the distinct phases. The linear TECs are very large, especially $α_c$ of the $α$-cristobalite phase and its similar phase, leading to giant $α_v$ ($\sim 175 \times 10^{-6} {\rm K}^{-1}$ at 300 K). The giant $α_v$ arises from large Gruneisen parameters of low-frequency phonon modes, and the C13 elastic constant that is negatively signed and large in magnitude for the $α$-cristobalite phase. The elastic constants, high-frequency dielectric constants, Born effective charge tensors, and thermal properties of the above phases of BeF$_2$ are reported for the first time and hence serve as predictions.

cond-mat.mtrl-sci↗

arXiv2022

Performance of the modified Becke-Johnson potential employing the pseudopotential plane-wave approach for band structure calculations

The modified Becke-Johnson exchange potential combined with local-density approximation correlation (mBJLDA) has recently attracted interest because it provides highly improved band gaps at a very low computational cost. In this work we performed an extensive investigation of the performance of the mBJLDA potential employing a norm-conserving pseudopotential plane-waves approach (mBJLDA@PP), as implemented in the abinit code, using a test set of 83 solids representing a wide range of semiconductors and insulators. Our results confirm the conclusion of our previous study that the number of electrons treated as valence in the pseudopotentials of the cations can have a significant impact on the calculated mBJLDA@PP band gaps. More specifically, while the use of typical pseudopotentials leads to accurate band gaps of certain systems, it yields significantly underestimated band gaps for other systems compared to experiment and to those of the all-electron mBJLDA (mBJLDA@AE) approach. The classes of the latter systems are identified, and this problem is resolved by including some outer core states as valence. The resulting mean absolute error in the calculated band gaps (compared to experiment) is of 0.46 eV, which is comparable to that of the mBJLDA@AE band gaps, reflecting the accuracy and reliability of the mBJLDA@PP approach for the band gap calculations.

cond-mat.mtrl-sci↗

arXiv2008

Consistent set of band parameters for the group-III nitrides AlN, GaN, and InN

We have derived consistent sets of band parameters (band gaps, crystal field-splittings, band gap deformation potentials, effective masses, Luttinger and EP parameters) for AlN, GaN, and InN in the zinc-blende and wurtzite phases employing many-body perturbation theory in the G0W0 approximation. The G0W0 method has been combined with density-functional theory (DFT) calculations in the exact-exchange optimized effective potential approach (OEPx) to overcome the limitations of local-density or gradient-corrected DFT functionals (LDA and GGA). The band structures in the vicinity of the Gamma-point have been used to directly parameterize a 4x4 k.p Hamiltonian to capture non-parabolicities in the conduction bands and the more complex valence-band structure of the wurtzite phases. We demonstrate that the band parameters derived in this fashion are in very good agreement with the available experimental data and provide reliable predictions for all parameters which have not been determined experimentally so far.

cond-mat.mtrl-sci↗

arXiv2006

Exact-exchange based quasiparticle energy calculations for the band gap, effective masses and deformation potentials of ScN

The band gaps, longitudinal and transverse effective masses, and deformation potentials of ScN in the rock-salt structure have been calculated employing G0W0-quasiparticle calculations using exact-exchange Kohn-Sham density functional theory one-particle wavefunctions and energies as input. Our quasiparticle gaps support recent experimental observations that ScN has a much lower indirect band gap than previously thought. The results are analyzed in terms of the influence of different approximations for exchange and correlation taken in the computational approach on the electronic structure of ScN.

cond-mat.mtrl-sci↗

arXiv2006

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.

cond-mat.mtrl-sci↗

arXiv2005

Combining GW calculations with exact-exchange density-functional theory: An analysis of valence-band photoemission for compound semiconductors

We report quasiparticle-energy calculations of the electronic bandstructure as measured by valence-band photoemission for selected II-VI compounds and group-III-nitrides. By applying GW as perturbation to the ground state of the fictitious, non-interacting Kohn-Sham electrons of density-functional theory (DFT) we systematically study the electronic structure of zinc-blende GaN, ZnO, ZnS and CdS. Special emphasis is put on analysing the role played by the cation semicore d-electrons that are explicitly included as valence electrons in our pseudopotential approach. Unlike in the majority of previous GW studies, which are almost exlusively based on ground state calculations in the local-density approximation (LDA), we combine GW with exact-exchange DFT calculations in the optimised-effective potential approach (OEPx). This is a much more elaborate and computationally expensive approach. However, we show that applying the OEPx approach leads to an improved description of the d-electron hybridisation compared to the LDA. Moreover we find that it is essential to use OEPx pseudopotentials in order to treat core-valence exchange consistently. Our OEPx based quasiparticle valence bandstructures are in good agreement with available photoemission data in contrast to the ones based on the LDA. We therefore conclude that for these materials OEPx constitutes the better starting point for subsequent GW calculations.

cond-mat.mtrl-sci↗