Electronic structure of ScN and YN： density-functional theory LDA and GW approximation calculations

The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, the local-density approximation(LDA) and GW approximation i.e. combination of the Green function G and the screened Coulomb interaction W, have been performed. This paper investigates the bulk electronic and physical properties of early transition metal mononitrides, ScN and YN in the rocksalt structure. In this paper, the semicore electrons are regarded as valance electrons. ScN appears to be a semimetal, and YN is semiconductor with band gap of 0.142eV within the LDA, but are in fact semiconductors with indirect band gaps of 1.244 and 0.544\,eV respectively, as revealed by calculations performed using GW approximation.     

BaSe的准粒子能带结构

运用标准的准粒子GW方法重新考察了BaSe的准粒子能带结构.为便于比较，同时计算了局域密度近似(LDA)和广义梯度近似(GGA)下的能带.结果表明，LDA和GGA方法都不能准确描述这个材料的带隙.与实验测量值对比，其误差分别达到39.9%和32.6%.GW准粒子能带的结果则可以对其带隙作出大幅度的修正，得到与实验测量相当符合的理论结果.与已有的计算结果不同，B1结构BaSe准粒子能带具有Γ点直接带隙特性，表明在Ba价电子组态中考虑4d电子的作用至关重要. 关键词： BaSe GW 能带结构 带隙     

Electronic structure calculations of lead chalcogenides PbS, PbSe, PbTe

In this work, we have extended our study of the mechanical properties and the electronic structure of PbTe to include other Pb chalcogenide compounds (PbSe, PbS). The calculations were performed self-consistently using the scalar-relativistic full-potential linearized augmented plane wave method. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) to density-functional theory were applied.The equilibrium lattice constants and the bulk modulus of a number of structures (NaCl, CsCl, ZnS) were calculated as well as the elastic constants for the structures (NaCl, CsCl). The NaCl structure is found to be the most stable one among all the three phases considered. We have found that the GGA predicts the elastic constants in good agreement with experimental data.Both the LDA and GGA were successful in predicting the location of the band gap at the L point of the Brillouin zone but they are inconclusive regarding the value of the band-gap width. To resolve the issue of the gap, we performed Slater-Koster (SK) tight-binding calculations, including the spin-orbit coupling in the SK Hamiltonian. The SK results that are based on our GGA calculations give the best agreement with experiment.Results are reported for the pressure dependence of the energy gap of these compounds in the NaCl structure. The pressure variation of the energy gap indicates a transition to a metallic phase at high pressure. Band structure calculations in the CsCl structure show a metallic state for all compounds. The electronic band structure in the ZnS phase shows an indirect band gap at the W and X point of the Brillouin zone.     

Quasiparticle band structures and optical properties of magnesium fluoride

The quasiparticle and optical properties of magnesium fluoride (MgF(2)) are computed within the GW approximation based on many-body perturbation theory (MBPT). The many-body effects appearing in self-energy and electron-hole interactions have an important influence on the electronic and optical properties. The DFT-LDA calculation shows a 6.78 eV band gap. Two methods are employed to evaluate the self-energy within the GW approximation in the present work. The generalized plasmon pole model (GPP) provides a band gap of 12.17 eV, which agrees well with the experimental value of 12.4 eV (Thomas et al 1973 Phys. Status Solidi b 56 163). Another band gap value of 11.30 eV is obtained by using a full frequency-dependent self-energy, which is also not far from the experimental value and is much better than the result from the LDA calculation. The calculated optical spectrum within DFT is significantly different from the experiment. Although the calculated optical absorption threshold within the GW method is close to the experiment, the overall shape of the spectrum is still similar to the case of DFT. However, the overall shape of the spectrum via the Bethe-Salpeter equation (BSE) method agrees well with the experiment.     

Electron correlation effects on magnetic Compton profiles of nickel in the GW approximation

The magnetic Compton profiles (MCPs) of 3d transition metal nickel are calculated by the GW approximation with FLAPW basis sets on the LSDA. The 3d valence band width narrowing, which is ascribed to the dynamical screening effects, contributes to eliminate an anomalous peak at 0.7 a.u. in the 〈110〉 direction, and also to diminish the excessive estimation of the Umklapp effects, in the LSDA MCPs. These are in good agreement with experimental observation.     

Ab initio prediction of conduction band spin splitting in zinc blende semiconductors

We use a recently developed self-consistent GW approximation to present systematic ab initio calculations of the conduction band spin splitting in III-V and II-VI zinc blende semiconductors. The spin-orbit interaction is taken into account as a perturbation to the scalar relativistic Hamiltonian. These are the first calculations of conduction band spin splittings based on a quasiparticle approach; and because the self-consistent GW scheme accurately reproduces the relevant band parameters, it is expected to be a reliable predictor of spin splittings. The results are compared to the few available experimental data and a previous calculation based on a model one-particle potential. We also briefly address the widely used k x p parametrization in the context of these results.     

Ag Raman modes of RBCO (R = Gd, Pr) by density functional theory approach

Ab initio total energy calculations have been performed for superconducting GdBa₂Cu₃O₇ and insulating PrBa₂Cu₃O₇ using the full-potential linear augmented plane-wave method in the local density approximation (LDA) and generalized gradient approximation (GGA). The comparison of the calculated unit cell volume and lattice parameters with the experimental data indicates the improvement of these parameters in the GGA relative to LDA. LDA and GGA give the equilibrium unit cell volume about 6% smaller and 1.25% larger than the experimental data, respectively for both systems. Thus frozen phonon calculations have been performed to determine the eigenvalues and eigenvectors of the k=0 A_g modes of the two systems in equilibrium structure have been obtained in GGA. The calculated frequencies in the GGA are in good agreement with the other LDA calculations for similar systems. Comparison of computational data with experimental data indicates that calculations determine the frequencies about ten percent below the experimental data. Even by improving LDA to GGA in these calculations, the calculated phonon frequencies have remained almost ten percent below the experimental data, even though the calculated unit cell volumes are nearly equal to the experimental data. So, applying GGA has not considerably decreased the difference between the computational and experimental data. The effect of Pr doping on the eigenvalues and eigenvectors have also been investigated.     

First-principle calculations of structural, electronic and optical properties of BaTiO3 and BaZrO3 under hydrostatic pressure

A theoretical study of structural, electronic and optical properties of cubic BaTiO₃ and BaZrO₃ perovskites is presented, using the full-potential linear augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus, its pressure derivative, band structure, density of states, pressure coefficients of energy gaps and refractive indices. The results are compared with previous calculations and experimental data.     

Electronic structure of strongly correlated systems emerging from combining path-integral renormalization group with the density-functional approach

A new scheme of first-principles computation for strongly correlated electron systems is proposed. This scheme starts from the local-density approximation (LDA) at high-energy band structure, while the low-energy effective Hamiltonian is constructed by a downfolding procedure using combinations of the constrained-LDA and the GW method. The obtained low-energy Hamiltonian is solved by the path-integral renormalization-group method, where spatial and dynamical fluctuations are fully considered. An application to Sr2VO4 shows that the scheme is powerful in agreement with experimental results. It further predicts a nontrivial orbital-stripe order.     

Valence band x-ray emission spectra of compressed germanium

We report measurements of the valence band width in compressed Ge determined from x-ray emission spectra below the Ge K edge. The width of the valence band does not show any pressure dependence in the semiconducting diamond-type structure of Ge below 10 GPa. On the other hand, in the metallic beta-Sn phase above 10 GPa the valence band width increases under compression. Density-functional calculations show an increasing valence band width under compression both in the semiconducting phase (contrary to experiment) and in the metallic beta-Sn phase of Ge (in agreement with observed pressure-induced broadening). The pressure-independent valence band width in the semiconducting phase of Ge appears to require theoretical advances beyond the density-functional theory or the GW approximation.     

Optoelectronic and thermoelectric properties of Zintl YLi3X2(X=Sb,Bi) compounds through modified Becke—Johnson potential

In the present work, we investigate the structural, optoelectronic and thermoelectric properties of the YLi₃X₂(X=Sb, Bi) compounds using the full potential augmented plane wave plus local orbital (FP-APW+lo) method. The exchange-correlation potential is treated with the generalized gradient approximation/local density approximation (GGA/LDA) and with the modified Becke-Johnson potential (TB-mBJ) in order to improve the electronic band structure calculations. In addition, the estimated ground state properties such as the lattice constants, external parameters, and bulk moduli agree well with the available experimental data. Our band structure calculations with GGA and LDA predict that both compounds have semimetallic behaviors. However, the band structure calculations with the GGA/TB-mBJ approximation indicate that the ground state of the YLi₃Sb₂ compound is semiconducting and has an estimated indirect band gap (Γ-L) of about 0.036 eV while the ground state of YLi₃Bi₂ compound is semimetallic. Conversely the LDA/TB-mBJ calculations indicate that both compounds exhibit semiconducting characters and have an indirect band gap (Γ-L) of about 0.15 eV and 0.081 eV for YLi₃Sb and YLi₃Bi₂ respectively. Additionally, the optical properties reveal strong responses of the herein materials in the energy range between the IR and extreme UV regions. Thermoelectric properties such as thermal conductivity, electrical conductivity, Seebeck coefficient, and thermo power factors are also calculated.     

Valence electron photoemission spectrum of semiconductors: ab initio description of multiple satellites

The experimental valence band photoemission spectrum of semiconductors exhibits multiple satellites that cannot be described by the GW approximation for the self-energy in the framework of many-body perturbation theory. Taking silicon as a prototypical example, we compare experimental high energy photoemission spectra with GW calculations and analyze the origin of the GW failure. We then propose an approximation to the functional differential equation that determines the exact one-body Green's function, whose solution has an exponential form. This yields a calculated spectrum, including cross sections, secondary electrons, and an estimate for extrinsic and interference effects, in excellent agreement with experiment. Our result can be recast as a dynamical vertex correction beyond GW, giving hints for further developments.     

Trap-recharging waves versus damped,forced charge-density oscillations in hexagonal silicon carbide

The structural parameters and hydrostatic pressure coefficients of CdS_xTe_1-x in the two phases, namely zinc-blende and NaCl as well as the transition pressures from zinc-blende to NaCl structures at various S concentrations are presented. The calculations are performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the local density approximation (LDA), and two developed refinements, namely the generalized gradient approximation (GGA) of Perdew et al. for the structural properties and Engel-Vosko for the band structure calculations. Detailed comparisons are made with published experimental and theoretical data and show generally good agreement. The present results regarding the studied quantities for compositions x in the 0–1 range (0 < x < 1) and for the NaCl phase are predictions and may serve as a reference for experimental work.     

High pressure phase transition and band structures of different phases in CeO2

We report a detailed theoretical calculation of the electronic band structure of CeO₂ in cubic and orthorhombic phases under pressure using a tight-binding linear muffin-tin orbital method (TB-LMTO) within local density approximation (LDA). The compressibility behavior of this compound was discussed in the light of the changes occurring in the electronic structure. Apart from the electronic band structure and structural stability calculations, the density of states (DOS) and Fermi energies (E_f) at various pressures are calculated. The calculated lattice parameter, transition pressure, bulk modulus and the pressure-volume relation are found out to be in good agreement with experimental results.     

Electronic structure calculations using dynamical mean field theory

The calculation of the electronic properties of materials is an important task of solid-state theory, albeit particularly difficult if electronic correlations are strong, e.g., in transition metals, their oxides and in f-electron systems. The standard approach to material calculations, the density functional theory in its local density approximation (LDA), incorporates electronic correlations only very rudimentarily and fails if the correlations are strong. Encouraged by the success of dynamical mean field theory (DMFT) in dealing with strongly correlated model Hamiltonians, physicists from the bandstructure and the many-body communities have joined forces and developed a combined LDA + DMFT method recently. Depending on the strength of electronic correlations, this new approach yields a weakly correlated metal as in the LDA, a strongly correlated metal or a Mott insulator. This approach is widely regarded as a breakthrough for electronic structure calculations of strongly correlated materials. We review this LDA + DMFT method and also discuss alternative approaches to employ DMFT in electronic structure calculations, e.g., by replacing the LDA part with the so-called GW approximation. Different methods to solve the DMFT equations are introduced with a focus on those that are suitable for realistic calculations with many orbitals. An overview of the successful application of LDA + DMFT to a wide variety of materials, ranging from Pu and Ce, to Fe and Ni, to numerous transition metal oxides, is given.     

First principles study of structural and electronic properties of different phases of boron nitride

A theoretical study of structural and electronic properties of the four phases of BN (zincblende, wurtzite, hexagonal and rhombohedral) is presented. The calculations are done by full potential (linear) augmented plane wave plus local orbitals (APW+lo) method based on the density functional theory (DFT) as employed in WIEN2k code. Using the local density approximation (LDA) and generalized gradient approximation (GGA-PBE) for the exchange correlation energy functional, we have calculated lattice parameters, bulk modulus, its pressure derivative and cohesive energy. In order to calculate electronic band structure, another form of the generalized gradient approximation proposed by Engel and Vosko (GGA-EV) has been employed along with LDA and GGA-PBE. It is found that all the three approximations exhibit similar band structure qualitatively. However, GGA-EV gives energy band gap values closer to the measured data. Our results for structural and electronic properties are compared with the experimental and other theoretical results wherever these are available.     

Density functional study of optical properties of beryllium chalcogenides compounds in nickel arsenide B8 structure

The structural, electronic and optical properties of beryllium chalcogenides BeS, BeSe and BeTe using the full-potential linear augmented plane wave (FP-LAPW) method are investigated. The exchange-correlation energy within the local density approximation (LDA) and the generalized gradient approximation (GGA) are described. The Engel-Vosko (EVGGA) formalism is applied for electronic and optical properties. The structural parameters of our model and the transition pressure from zinc-blende (B3) to the NiAs (B8) phase are confirmed. It is found that these compounds have indirect band gaps except for BeTe in NiAs (B8) phase. The results of reflectivity, refractive index and optical dielectric functions of Be compounds are investigated. An agreement is found between our results and those of other theoretical calculations and the experimental data.     

Structural and electronic properties of H-passivated graphene

The atomic and electronic structures of graphane (hydrogen-passivated graphene) are theoretically investigated using the local density approximation (LDA) of the density functional theory (DFT) and the pseudopotential method. Our total energy calculations suggest that the chairlike configuration for graphane is more energetically stable than the boatlike and tablelike configurations by approximately 0.129 eV/cell and 0.655 eV/cell, respectively. Our calculations suggest that the LDA band gap of the chairlike structure is approximately 3.9 eV. The equilibrium geometry and the band structure of the chairlike conformer are investigated and compared with the available experimental and theoretical data. We further present total and partial charge density to reveal the orbital nature of the highest occupied and the lowest unoccupied states.     

Theoretical investigation of sulfur defects on structural,electronic,and elastic properties of ZnSe semiconductor

The structural, electronic, and elastic properties of Zn Se1-xSx for the zinc blende structures have been studied by using the density functional theory. The calculations were performed using the plane wave pseudopotential method, as implemented in Quantum ESPRESSO. The exchange-correlation potential is treated with the local density approximation pz-LDA for these properties. Moreover, LDA+U approximation is employed to treat the "d" orbital electrons properly. A comparative study of the band gap calculated within both LDA and LDA+U schemes is presented. The analysis of results show considerable improvement in the calculation of band gap. The inclusion of compositional disorder increases the values of all elastic constants. In this study, it is found that elastic constants C11, C12, and C44 are mainly influenced by the compositional disorder. The obtained results are in good agreement with literature.