Band gap and partial density of states for ZnO: Under high pressure

The first-principle investigation of SiFe2O4 (SFO) spinel was performed with the help of a plane-wave pseudopotential technique within the generalized gradient approximation (GGA) and local density approximation (LDA) as implemented in Quantum Espresso Simulation package. The Electronic band structure and optical properties of SFO spinel-type material have been investigated and discussed in this paper. The calculated band structure reveals that SFO spinel-type material is a direct bandgap semiconductor. Using GGA + U and LDA + U the band gap value so obtained is 3.52 eV and 2.96 eV respectively. The contribution to valence and conduction bands due to different bands was analyzed on the basis of the total and partial density of state. The Optical properties of SFO spinel-type material have been calculated and discussed in detail. The real, and the imaginary, part of the complex dielectric constants is found to be 6.52 and 5.42 at energies of 3.44 eV and 6.21 eV respectively. The refractive index and the reflectivity index at zero energy value were found to be 1.88 and 10% respectively. We found that SFO spinel-type material has good properties for optical devices.

This contribution concerning the effect of spin–orbit coupling on the magnetic properties of materials is divided into two sections. In the first section we review the method based on the density functional theory (DFT) within the local density approximation (LDA) used to compute the electronic structure, the magnetic anisotropy, the x-ray absorption spectra, and the x-ray magnetic circular dichroism. We give the major approximations used to derive the Kohn–Sham equations with or without the Hubbard interaction for correlated orbitals. We give also a brief introduction to the generalized gradient approximation (GGA). We then provide a solution of the latter equations using the full-potential linear augmented plane wave (FLAPW) basis set and discuss the so-called LDA+U method, where the Hubbard U is included for localized orbitals. We show how the relativistic effects, such as the spin–orbit coupling, can be introduced into band structure calculations and show their effect on magnetism, i.e., magnetic anisotropy energy (MAE), magnetooptical properties, and x-ray magnetic circular dichroism (XMCD). Then we show a brief derivation of the force theorem for the calculation of the magnetic anisotropy as well as a description of its application to the MAE calculations and show the details of the calculation of the XMCD matrix elements in the electric dipole approximation. The second section of this contribution includes some applications of the method to the computation of the electronic, magnetic, and spectroscopic properties of spintronics materials. In particular, we investigate the electronic structure and x-ray magnetic circular dichroism (XMCD) of Sr2FeMoO6 (SFMO for short) and other useful ferromagnetic half-metals with 100% spin polarization, materials useful for spin injection. In particular, we show that the spin–orbit coupling reduces the spin polarization, while the intra-site electronic correlations tend to increase it. For example, SFMO is found to be a half-metallic ferrimagnet with a gap in the spin-up channel. The calculated spin magnetic moments on iron and Mo sites confirm the ferromagnetic ordering and settle the controversy existing between the earlier experimental works. The orbital magnetism at the Fe and Mo sites agrees quite well with the recent experimental XMCD measurements. The computed L2,3 XMCD at the Fe and the Mo sites compares fairly well with the experiment. The XMCD sum rule computed spin and orbital magnetic moments are in good agreement with the values obtained from the direct self-consistent calculations. In the last application, we focus on the GGA+U treatment of the electronic and magnetic structure of Gd and Gd-related compounds, such as GdN and GdFe2. We compare the calculated density of states to the experimental photoemission and inverse photoemission spectra (XPS and BIS) and determine the Fermi surface with and without the Hubbard U and spin–orbit coupling. The GGA+U is found to be the most appropriate for treating the 4f Gd electrons. We have investigated the bulk properties and calculated the XMCD spectra at the L2,3 edges at the Gd site of GdN. The agreement of the calculated spectra with experiment is the indication of the relevance of the XMCD formalism within the one-electron picture. The results also show that the ground-state electronic structure of GdN is that of a half-metal. Finally our computational method is used to determine the magnetic anisotropy aspect of Gd and its compounds GdN and GdFe2. Using force theorem, we have calculated the MAE of Gd, GdN, and GdFe2 for different directions of the magnetization. Indeed, owing to the nil spin–orbit interaction of the 4f half-filled shell, the force theorem is expected to be efficient for Gd and Gd compounds’ MAE calculations. This theorem allows a considerable computational effort gain since the spin–orbit coupling could be calculated only for one self-consistent iteration. Once again, the GGA+U method is found to be the most adequate approach for the force theorem calculations of the Gd MAE. The GGA and GGA-core model treatments of the 4f states have led to a wrong MAE. It turns out that the electronic properties and the magnetic properties of 4f systems are tightly related, and the 4f electrons play a crucial role in the computed magnetic anisotropy. Although the Gd MAE is found to be similar to that of a typical 3d transition metal like hcp Co, the GdN and GdFe2 cubic crystal MAEs are found to be different from that of a pure 3d cubic material like fcc Ni.

This contribution concerning the effect of spin–orbit coupling on the magnetic properties of materials is divided into two chapters. In the first chapter we review the method based on the density functional theory (DFT) within the local density approximation (LDA) used to compute the electronic structure, the magnetic anisotropy, the x-ray absorption spectra, and the x-ray magnetic circular dichroism. We give the major approximations used to derive the Kohn–Sham equations with or without the Hubbard interaction for correlated orbitals. We give also a brief introduction to the generalized gradient approximation (GGA). We then provide a solution of the latter equations using the full-potential linear augmented plane wave (FLAPW) basis set and discuss the so-called LDA+U method, where the Hubbard U is included for localized orbitals. We show how the relativistic effects, such as the spin–orbit coupling, can be introduced into band structure calculations and show their effect on magnetism, i.e., magnetic anisotropy energy (MAE), magneto-optical properties, and x-ray magnetic circular dichroism (XMCD). Then we show a brief derivation of the force theorem for the calculation of the magnetic anisotropy as well as a description of its application to the MAE calculations and show the details of the calculation of the XMCD matrix elements in the electric–dipole approximation. The second chapter of this contribution includes some applications of the method to the computation of the electronic, magnetism, and spectroscopic properties of spintronics materials. In particular , we investigate the electronic structure and x-ray magnetic circular dichroism (XMCD) of Sr2FeMoO6 (SFMO for short) and other useful ferromagnetic half-metals with 100% spin polarization, materials useful for spin injection. In particular, we show that the spin–orbit coupling reduces the spin polarization while the intra-site electronic correlations tend to increase it. For example, SFMO is found to be a half-metallic ferrimagnet with a gap in the spin-up channel. The calculated spin magnetic moments on Fe and Mo sites confirm the ferromagnetic ordering and settle the controversy existing between the earlier experimental works. The orbital magnetism at the Fe and Mo sites agrees quite well with the recent experimental XMCD measurements. The computed L2,3 XMCD at the Fe and the Mo sites compares fairly well with experiment. The XMCD sum rule computed spin and orbital magnetic moments are in good agreement with the values obtained from the direct self-consistent calculations. In the last application, we focus on the GGA+U treatment of the electronic and magnetic structure of Gd and Gd-related compounds, such as GdN and GdFe2. We compare the calculated density of states to the experimental photoemission and inverse photoemission spectra (XPS and BIS) and determine the Fermi surface with and without the Hubbard U and spin–orbit coupling. The GGA+U is found to be the most appropriate for treating the 4f Gd electrons. We have investigated the bulk properties and calculated the XMCD spectra at the L2,3 edges at the Gd site of GdN. The agreement of the calculated spectra with experiment is the indication of the relevance of the XMCD formalism within the one-electron picture. The results also show that the ground-state electronic structure of GdN is that of a half-metal. Finally our computational method is used to determine the magnetic anisotropy aspect of the Gd and its compounds GdN and GdFe2. Using the force theorem, we have calculated the MAE of Gd, GdN, and GdFe2 for different directions of the magnetization. Indeed, owing to the nil spin–orbit interaction of the 4f half-filled shell, the force theorem is expected to be efficient for Gd and Gd compounds MAE calculations. This theorem allows a considerable computational effort gain since the spin–orbit coupling could be calculated only for one self-consistent iteration. Once again, the GGA+U method is found to be the most adequate approach for the force theorem calculations of the Gd MAE. The GGA and GGA-core model treatments of the 4f states have led to a wrong MAE. It turns out that the electronic properties and the magnetic properties of 4f systems are tightly related, and the 4f electrons play a crucial role in the computed magnetic anisotropy. Although the Gd MAE is found to be similar to that of a typical 3d transition metal like hcp Co, the GdN and GdFe2 cubic crystals MAEs are found to be different from that of a pure 3d cubic material like fcc Ni.

Electronic structure calculations of lead chalcogenides PbS, PbSe, PbTe

Study of the structural and electronic properties of FeO at the LDA and GGA level

Band structure and spin–orbit coupling engineering in transition‐metal dichalcogenides

Spin-orbit (SO) coupling effects can play an important role in the accurate theoretical treatment of the quantum dynamics and the spectroscopy of molecular systems. It often goes hand in hand with vibronic coupling and thus calls for a diabatic treatment of all relevant couplings. While the treatment of vibronic coupling is well established, accurate diabatic SO models remain scarce. Therefore, a detailed study of the SO coupling for the relevant electronic states of the nitrate radical (NO3) is presented here. In contrast to most of the previously studied systems, the SO coupling effect is distributed across several atoms and cannot be localized at a single relativistic atom. Based on our previously developed diabatic potential energy model for NO3, a fully geometry-dependent diabatic SO and vibronic coupling model is presented. A link to the atomic nature of SO coupling is established based on an intuitive and simple semi-quantitative model, and the geometry dependence of the SO coupling is analyzed in detail. The necessity of including the geometry dependence is carefully analyzed for the different kinds of nuclear motions. Experimental evidence for SO coupling effects in NO3 has been reported previously, and the present results are in excellent agreement with these experimental findings.

The structural and electronic properties of cubic GaNxAs1−x with N-concentration varying between 0.0 and 1.0 with step of 0.25 were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have used the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange and correlation potential. In addition the Engel-Vosko generalized gradient approximation (EVGGA) was used for the band-structure calculations. The structural properties of the binary and ternary alloys were investigated. The electronic band structure, total and partial density of states as well as the electron charge density were determined for both the binary and their related ternary alloys. The energy gap of the alloys decreases when we move from x=0.0 to 0.25; then it increases by a factor of about 1.8 when we move from 0.25 to 0.5, 0.75 and 1.0 using EVGGA. For both LDA and GGA moving from x=0.0 to 0.25 causes the band gap to close, showing the metallic nature of the GaN0.25As0.75 alloy. When the composition of N moves through x=0.25, 0.5, 0.75 and 1, the band gap increases.

In this work, we have investigated the structural and electronic properties of both bulk and monolayer MoS2 based on the density functional theory (DFT) implemented in the CASTEP of Materials Studio package. The calculations are performed with the local density approximation (LDA) and generalized gradient approximation (GGA) functionals for crystal structure optimization and band structure of MoS2 bulk and monolayer. Our calculations show that the GGA functional calculated excellent band gap for bulk MoS2, while LDA functional is found to perform better for band gap calculations of a monolayer. The influence of composition in the energy bands has been realized by analyzing the partial density of states (PDOS) of each atom and density of states (DOS). By reducing the layer thickness from bulk to monolayer, it is found that band structure has the transitions from indirect band gap in the bulk MoS2 (1.53 eV) to direct band gap in the monolayer (1.82 eV). On the other hand, the charge density difference along z-direction shows that the major charge transfer occurs on the surface of the S atoms and there is a little accumulation around the surface of the Mo atoms. This property highlights the promising of MoS2 in improving the fabrication of optoelectronic devices in the future.

BaTiO3 perovskite for optoelectronics application: A DFT study

First-principles calculations to investigate the elastic, electronic, dynamical, and optical properties of cubic ZrCoAs half-Heusler semiconductor for photovoltaic applications

Fundamental understanding and optimization of the emerging mixed organic-inorganic hybrid perovskites for solar cells require multiscale modeling starting from ab initio quantum mechanics methods. Particularly, it is important to correctly predict the structural and electronic properties such as phase stability, lattice parameters, band gaps, and band structures. Although density functional theory is the method of choice to address these properties and generate the input for subsequent multiscale, high-throughput, and data-driven approaches, standard exchange correlation functionals fail to reproduce the bandgap, particularly if spin-orbit coupling (SOC) is correctly taken into account. While many SOC-included hybrid functionals suffer from low transferability between different molecular ions and are computationally costly, we propose an efficient multistep simulation protocol based on the DFT-1/2 method. We apply this approach to APbI3 with A: FA, MA, Cs, and systems with mixed cations and show how the choice of the A-cation modifies the Pb-I scaffold and the hydrogen bonding and discuss their interplay with structural stability. Furthermore, band gaps, band structures, Rashba band splitting, Born effective charges as well as partial density of states (PDOS) are compared for different cases w/wo the SOC effect and the DFT-1/2 approach.

This study examined the theoretical impact and modeling of photocatalyst, MoS 2 , on organic pollutants and wastewater treatment. The electronic band structures, density of state (total and partial), optical properties, and photocatalytic operation under UV or visible light were investigated by using the first principle method for MoS 2 and W doped by 5%. In order to calculate band gap, generalized gradient approximation (GGA) based on Perdew- Burke- Ernzerhof (PBE) was used. The band gap for MoS 2 was recorded at 1.78 eV which is close to the experimental value of 1.72-1.8 eV. To recognize the character of photocatalyst activities, the optical properties were investigated and calculated. Moreover, the total density of state and partial density of state were estimated for exploring the nature of 5s, 4d for Mo, and 3p for S atom for MoS 2 material. Concurrently, optical properties, absorption, reflection, refractive index, conductivity, dielectric function, and loss function were calculated. Having doped W with MoS 2 , the band gap, optical properties had changed and improved the photocatalytic effect to the hybridization of W. From the value of band gap and optical properties, it is clear that Mo 0.95 W 0.05 S 2 can provide better UV or visible light rather than MoS 2 .

The Mg(BiO2)2 is the orthorhombic crystal system acting as semiconductor in electric devices. To evaluate electronic band structures, the total density of state (TDOS) and the partial density of state (PDOS), Generalized Gradient Approximation (GGA) based on the Perdew–Burke–Ernzerhof (PBE0) was used for Mg(BiO2)2. The band gap was recorded at 0.959 eV, which is supported by a good semiconductor. The density of states and partial density of states were simulated for evaluating the nature of 5s, 4d for Mg, 6s, 4f, 5d, 6p for Bi and 2s, 2p for O atom for Mg(BiO2)2 to explain the transition of the electron due to hybridization. From the PDOS, it was illustrated that the d orbital of Bi atom responses for conducting the electronic holes.

The electronic and geometrical properties of bulk americium and square and hexagonal americium monolayers have been studied with the full-potential linearized augmented plane wave (FP-LAPW) method. The effects of several common approximations are examined: (1) non-spin polarization (NSP) vs. spin polarization (SP); (2) scalar-relativity (no spin-orbit coupling (NSO)) vs. full-relativity (i.e., with spin-orbit (SO) coupling included); (3) local-density approximation (LDA) vs. generalized-gradient approximation (GGA). Our results indicate that both spin polarization and spin orbit coupling play important roles in determining the geometrical and electronic properties of americium bulk and monolayers. A compression of both americium square and hexagonal monolayers compared to the americium bulk is also observed. In general, the LDA is found to underestimate the equilibrium lattice constant and give a larger total energy compared to the GGA calculations. While spin orbit coupling shows a similar effect on both square and hexagonal monolayer calculations regardless of the model, GGA versus LDA, an unusual spin polarization effect on both square and hexagonal monolayers is found in the LDA results as compared with the GGA results. The 5f delocalization transition of americium is employed to explain our observed unusual spin polarization effect. In addition, our results at the LDA level of theory indicate a possible 5f delocalization could happen in the americium surface within the same Am II (fcc crystal structure) phase, unlike the usually reported americium 5f delocalization which is associated with crystal structure change. The similarities and dissimilarities between the properties of an Am monolayer and a Pu monolayer are discussed in detail.