Quasiparticle Band Structure Research Articles

Electronic excitations of bulk LiCl from many-body perturbation theory

We present the quasiparticle band structure and the optical excitation spectrum of bulk LiCl, using many-body perturbation theory. Density-functional theory is used to calculate the ground-state geometry of the system. The quasiparticle band structure is calculated within the GW approximation. Taking the electron-hole interaction into consideration, electron-hole pair states and optical excitations are obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function. The calculated band gap is 9.5 eV, which is in good agreement with the experimental result of 9.4 eV. And the calculated optical absorption spectrum, which contains an exciton peak at 8.8 eV and a resonant-exciton peak at 9.8 eV, is also in good agreement with experimental data.

Field-Driven Dynamics of Correlated Electrons in LiH andNaBH4Revealed by Femtosecond X-Ray Diffraction

We study the quasi-instantaneous change of electron density in the unit cells of LiH and NaBH4 in response to a nonresonant strong optical field. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The light-induced charge relocation in NaBH4 exhibits an electron transfer from the anion (BH(4)(-)) to the Na(+) cation. In contrast, LiH displays the opposite behavior, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This behavior originates from strong electron correlations in LiH, as is evident from a comparison with quasiparticle band structures calculated within the Coulomb-hole-plus-screened-exchange formalism.

Quasiparticle band structure and optical spectrum of LiBr

We report the quasiparticle band structure and optical absorption spectrum of bulk LiBr calculated from first-principles approaches. The quasiparticle band structure is calculated within the GW approximation. Taking the electron-hole interaction into consideration, the optical excitation is investigated by solving the Bethe-Salpeter equation for the electron-hole two-particle Green’s function. The obtained results for the band gap and optical absorption spectrum are in good agreement with experimental measurements.

Quasiparticle band structure and optical properties of theα12Si-Ge superstructure from first principles

The quasi-particle band structure and dielectric function for the so-called magic sequence SiGe$_2$Si$_2$Ge$_2$SiGe$_{12}$ (or $\alpha_{12}$) structure [PRL 108, 027401 (2012)] are calculated by many-body perturbation theory (MBPT) within an ab-initio framework. On top of density functional calculations within the local density approximation (LDA) leading to a fundamental band gap of 0.23 eV, we have computed the quasi-particle band structure within the G$_0$W$_0$ approach opening the gap to 0.61 eV. Moreover, we have calculated the optical properties by solving the Bethe-Salpeter equation (BSE) for the electron-hole two-particle correlation function. When comparing the imaginary part of the dielectric function obtained at various levels of approximation, \emph{i.e.} the independent particle approximation (or random phase approximation) based on (i) the LDA or (ii) GW band structures, and (iii) the BSE including local field effects and electron-hole correlations, we observe that the important first transition is better explained with taking into account excitonic effects. Moreover, the onset transition originating from the direct transition of the magic sequence structure is also investigated.

Quasiparticle electronic structure of barium-silicon oxynitrides for white-LED application

Ba${}_{3}$Si${}_{6}$O${}_{12}$N${}_{2}$:Eu${}^{2+}$ and Ba${}_{3}$Si${}_{6}$O${}_{9}$N${}_{4}$:Eu${}^{2+}$ have strikingly similar atomistic structures, but the former is an efficient green phosphor at working temperature while the latter is a bluish-green phosphor whose luminescence decreases quite fast with temperature. Aiming at the understanding of such different behavior, we compute the quasiparticle electronic band structure of the two hosts, Ba${}_{3}$Si${}_{6}$O${}_{12}$N${}_{2}$ and Ba${}_{3}$Si${}_{6}$O${}_{9}$N${}_{4}$, thanks to many-body perturbation theory in the ${G}_{0}{W}_{0}$ approximation. The gap differs by about 0.43 eV. We analyze the eigenfunctions at the top of the valence band, at the bottom of the conduction band, and also the chemical shifts for the Ba site in the two hosts. The valence bands, directly impacted by the different stoichiometric ratio, are not thought to play a large role in the luminescence. Deceivingly, the dispersive bottom of the conduction band, directly related to luminescent properties, is similar in both compounds. The spatial topology of the probability density of the bottom of the conduction bands differs, as well as the location of the 5d peak, with a much higher energy than the bottom of the conduction band in Ba${}_{3}$Si${}_{6}$O${}_{12}$N${}_{2}$ than in Ba${}_{3}$Si${}_{6}$O${}_{9}$N${}_{4}$. Electromagnetic absorption spectra are also computed for both compounds.

Quasiparticle states in the antiferromagnetic phase of double exchange systems

We present a theory for the quasiparticle band structure in the antiferromagnetic phase of a Kondo-lattice Hamiltonian with ferromagnetic Hund’s rule exchange between conduction electrons and localized spins and antiferromagnetic superexchange between localized spins. The quantum mechanical character of the localized spins is kept and no semiclassical approximation is made. Comparison of the obtained quasiparticle spectrum with exact diagonalization of finite clusters shows good agreement. Ferromagnetic polarization is energetically favorable only for very weak superexchange and small conduction electrons density. We discuss localized states where a conduction electron is trapped near a misalligned localized spin as well as superlattices of such misalligned spins.

Van der Waals bonding and the quasiparticle band structure of SnO from first principles

In this work we have investigated the structural and electronic properties of SnO, which is built up from layers kept together by van der Waals (vdW) forces. The combination of a vdW functional within density functional theory (DFT) and quasiparticle band structure calculations within the $GW$ approximation provides accurate values for the lattice parameters, atomic positions, and the electronic band structure including the fundamental (indirect) and the optical (direct) band gap without the need of experimental or empirical input. A systematic comparison is made between different levels of self-consistency within the $GW$ approach ${$following the scheme of Shishkin et al. [Phys. Rev. B 75, 235102 (2007)]$}$ and the results are compared with DFT and hybrid functional results. Furthermore, the effect of the vdW-corrected functional as a starting point for the $GW$ calculation of the band gap has been investigated. Finally, we studied the effect of the vdW functional on the electron charge density.

Quasiparticle band structures and interface physics of SnS and GeS

We perform first-principles, density-functional-theory calculations of the electronic structure for the layered bulk materials SnS and GeS which are of interest for photovoltaic applications. Band gap corrections are computed within the GW approximation to the electron self-energy. The resulting quasiparticle gaps in both SnS and GeS are in excellent agreement with the majority of existing experimental measurements. In order to better understand the possible use of GeS layers as a carrier-confining barrier within a SnS-based photovoltaic device, we compute the band offsets for different orientations of a SnS/GeS heterojunction. We find the valence band offsets to be almost independent of interfacial direction while the conduction band offsets show a strong anisotropy as a result of the variation in the band gap caused by epitaxial strain along the different directions.

GWquasiparticle band structures of stibnite, antimonselite, bismuthinite, and guanajuatite

We present first-principles calculations of the quasiparticle band structures of four isostructural semiconducting metal chalcogenides A$_2$B$_3$ (with A = Sb, Bi and B = S, Se) of the stibnite family within the G$_0$W$_0$ approach. We perform extensive convergence tests and identify a sensitivity of the quasiparticle corrections to the structural parameters and to the semicore $d$ electrons. Our calculations indicate that all four chalcogenides exhibit direct band gaps, if we exclude some indirect transitions marginally below the direct gap. Relativistic spin-orbit effects are evaluated for the Kohn-Sham band structures, and included as scissor corrections in the quasiparticle band gaps. Our calculated band gaps are 1.5 eV (Sb$_2$S$_3$), 1.3 eV (Sb$_2$Se$_3$), 1.4 eV (Bi$_2$S$_3$) and 0.9 eV (Bi$_2$Se$_3$). By comparing our calculated gaps with the ideal Shockley-Queisser value we find that all four chalcogenides are promising as light sensitizers for nanostructured photovoltaics.

A DFT study on structural, vibrational properties, and quasiparticle band structure of solid nitromethane

We report a detailed theoretical study of the structural and vibrational properties of solid nitromethane using first principles density functional calculations. The ground state properties were calculated using a plane wave pseudopotential code with either the local density approximation, the generalized gradient approximation, or with a correction to include van der Waals interactions. Our calculated equilibrium lattice parameters and volume using a dispersion correction are found to be in reasonable agreement with the experimental results. Also, our calculations reproduce the experimental trends in the structural properties at high pressure. We found a discontinuity in the bond length, bond angles, and also a weakening of hydrogen bond strength in the pressure range from 10 to 12 GPa, picturing the structural transition from phase I to phase II. Moreover, we predict the elastic constants of solid nitromethane and find that the corresponding bulk modulus is in good agreement with experiments. The calculated elastic constants show an order of C11> C22 > C33, indicating that the material is more compressible along the c-axis. We also calculated the zone center vibrational frequencies and discuss the internal and external modes of this material under pressure. From this, we found the softening of lattice modes around 8-11 GPa. We have also attempted the quasiparticle band structure of solid nitromethane with the G0W0 approximation and found that nitromethane is an indirect band gap insulator with a value of the band gap of about 7.8 eV with G0W0 approximation. Finally, the optical properties of this material, namely the absorptive and dispersive part of the dielectric function, and the refractive index and absorption spectra are calculated and the contribution of different transition peaks of the absorption spectra are analyzed. The static dielectric constant and refractive indices along the three inequivalent crystallographic directions indicate that this material has a considerable optical anisotropy.

Comparative study on the performance of exchange and correlation in wide-gap semiconductors: the case of BeS, BeSe, and BeTe

The structural and elastic properties of Be-chalcogenides belonging to zinc-blende group are computed using various flavors of exchange–correlation (XC) functionals. The present results are compared with respect to those of the common generalized-gradient approximation GGA–PBE. Based on AM05 functional, we find that the optimized lattice parameters corroborate well the experimental data. Using the self-consistent GW approximation, the quasiparticle band structures are calculated within the AM05 optimized structural parameters. In this respect, the results of the electronic excitations rely on an initial electronic structure determined through the hybrid XC functional. The values of band gap energies are adequately estimated. For comparison, the emerged hybrid functional HSE06 in conjunction with the self-consistent GW calculations is presented and this drives to amply adequate values for the band gaps. Moreover, it has been exhibited that the new MBJLDA approximation provides an excellent description for the energy gaps along Γ-X points with regards to GW results and experimental realizations.

Quasiparticle band structures and optical properties of strained monolayer MoS2and WS2

The quasiparticle (QP) band structures of both strainless and strained monolayer MoS$_{2}$ are investigated using more accurate many body perturbation \emph{GW} theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent \emph{GW$_{0}$} (sc\emph{GW$_{0}$}) calculation, the predicted optical gap magnitude is in a good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The sc\emph{GW$_{0}$} and BSE calculations are also performed on monolayer WS$_{2}$, similar characteristics are predicted and WS$_{2}$ possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calculation results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.

First-principles and experimental characterization of the electronic and optical properties of CaS and CaO

Doped alkaline-earth chalcogenides are interesting photoluminescent materials for opto-electronic applications. It is crucial to have an extended knowledge about the undoped bulk CaS and CaO since all the excited state properties of the doped material heavily depend on it. In this work we investigate the structural parameters, electronic band structures, macroscopic dielectric constants and absorption spectra for CaS and CaO compounds. Their quasi-particle band structure in the GW approximation yields a value of 4.28eV and 6.02eV for the indirect theoretical particle gap of CaS and CaO, respectively. The imaginary part of the macroscopic dielectric function ε(ω) is computed including excitonic effects through the Bethe–Salpeter equation. The onset of absorption is within 0.1eV of the experimental one and the calculated spectrum shows a qualitative agreement with experiment. Our computed exciton binding energies are 0.27eV and 0.40eV for CaS and CaO, respectively.

Effects of momentum-dependent self-energy in the electronic structure of correlated materials

We study how the $\mathbf{k}$ dependence in the self-energy affects the quasiparticle band structure and one-particle spectral functions. It is known that, in electron-gas-like materials, the self-energy depends significantly on $\mathbf{k}$ and there is a strong cancellation between the $\mathbf{k}$ dependence and the energy dependence of the self-energy. Analysis of the $GW$ self-energy reveals that, even in correlated materials with narrow bands, such as ${\mathrm{SrVO}}_{3}$, the self-energy significantly depends on $\mathbf{k}$. When the nonlocal effect is neglected, the quasiparticle band structure is over-renormalized, yielding too large mass enhancement compared to the case of k-dependent self-energy. The present result suggests that partial cancellation between the frequency dependence and the k dependence in the self-energy is important when discussing the quasiparticle band structure of correlated materials.

Quasiparticle semiconductor band structures including spin–orbit interactions

We present first-principles calculations of the quasiparticle band structure of the group IV materials Si and Ge and the group III–V compound semiconductors AlP, AlAs, AlSb, InP, InAs, InSb, GaP, GaAs and GaSb. Calculations are performed using the plane wave pseudopotential method and the ‘one-shot’ GW method, i.e. G0W0. Quasiparticle band structures, augmented with the effects of spin–orbit, are obtained via a Wannier interpolation of the obtained quasiparticle energies and calculated spin–orbit matrix. Our calculations explicitly treat the shallow semicore states of In and Ga, which are known to be important in the description of the electronic properties, as valence states in the quasiparticle calculation. Our calculated quasiparticle energies, combining both the ab initio evaluation of the electron self-energy and the vector part of the pseudopotential representing the spin–orbit effects, are in generally very good agreement with experimental values. These calculations illustrate the predictive power of the methodology as applied to group IV and III–V semiconductors.

Optical and energy-loss spectra of the antiferromagnetic transition metal oxides MnO, FeO, CoO, and NiO including quasiparticle and excitonic effects

We calculate the frequency-dependent dielectric function for the series of antiferromagnetic transition metal oxides (TMOs) from MnO to NiO using many-body perturbation theory. Quasiparticle, excitonic, and local-field effects are taken into account by solving the Bethe-Salpeter equation in the framework of collinear spin polarization. The optical spectra are based on electronic structures which have been obtained using density-functional theory with a hybrid functional containing screened exchange (HSE03) and a subsequent quasiparticle calculation in the $GW$ approximation to describe exchange and correlation effects adequately. These sophisticated quasiparticle band structures are mapped to electronic structures resulting from the computationally less expensive $\mathrm{GGA}+U+\ensuremath{\Delta}$ scheme that includes an on-site interaction $U$ and a scissors shift $\ensuremath{\Delta}$ and allows us to calculate the large number of electronic states that is necessary to construct the Bethe-Salpeter Hamiltonian. For an accurate description of the optical spectra, an appropriate treatment of the strong electron-hole attraction is mandatory to obtain agreement with the experimentally observed absorption-peak positions. The itinerant $s$ and $p$ states as well as the localized transition metal $3d$ states have to be considered on an equal footing. We find that a purely atomic picture is not suitable to understand the optical absorption spectra of the TMOs. Reflectivity spectra, absorption coefficients, and loss functions at vanishing momentum transfer are computed in a wide spectral range and discussed in light of the available experimental data.

Quasiparticle band structure of rocksalt-CdO determined using maximally localized Wannier functions

CdO in the rocksalt structure is an indirect band gap semiconductor. Thus, in order to determine its band gap one needs to calculate the complete band structure. However, in practice, the exact evaluation of the quasiparticle band structure for the large number of k-points which constitute the different symmetry lines in the Brillouin zone can be an extremely demanding task compared to the standard density functional theory (DFT) calculation. In this paper we report the full quasiparticle band structure of CdO using a plane-wave pseudopotential approach. In order to reduce the computational effort and time, we make use of maximally localized Wannier functions (MLWFs). The MLWFs offer a highly accurate method for interpolation of the DFT or GW band structure from a coarse k-point mesh in the irreducible Brillouin zone, resulting in a much reduced computational effort. The present paper discusses the technical details of the scheme along with the results obtained for the quasiparticle band gap and the electron effective mass.

Combined GW and dynamical mean-field theory: Dynamical screening effects in transition metal oxides

We present the first dynamical implementation of the combined GW and dynamical mean-field scheme (“GW + DMFT”) for first-principles calculations of the electronic properties of correlated materials. The application to the ternary transition metal oxide SrVO3 demonstrates that this scheme inherits the virtues of its two parent theories: a good description of the local low-energy correlation physics encoded in a renormalized quasi-particle band structure, spectral weight transfer to Hubbard bands, and the physics of screening driven by long-range Coulomb interactions. Our data is in good agreement with available photoemission and inverse photoemission spectra; our analysis leads to a reinterpretation of the commonly accepted “three-peak structure” as originating from orbital effects rather than from the electron addition peak within the t2g manifold.

Model GW study of the late transition metal monoxides

The model GW method [F. Gygi and A. Baldereschi, Phys. Rev. Lett. 62, 2160 (1989)] is an efficient simplification to the standard GW approximation which uses model dielectric function to describe the long range Coulomb interactions in semiconductors. In this work, the model GW method is used to calculate the quasiparticle band structures of MnO, FeO, CoO, and NiO. All four late transition metal monoxides are predicted to be insulators. The band gaps, magnetic moments, and quasiparticle spectra are in good agreement with the experiments, except for the satellite structures which are missing in the density of states because the model GW self-energy is static. The high accuracy of model GW is due to the usage of the accurate dielectric constants in the construction of the model dielectric functions which ensures the correct asymptotic behavior of the long range Coulomb interactions. Besides, we find that the transition metal 4s states are irrelevant to the formation of the band gaps, which supports the local approaches and the experimental interpretations of the band gaps by photoemission and electron energy loss spectroscopy, while contradicts the recent calculations by hybrid functionals, exact exchange, and one shot GW approximations.

Band structure of gold from many-body perturbation theory

The bandstructure of gold is calculated using many-body perturbation theory (MBPT). Different approximations within the GW approach are considered. Standard single shot G0W0 corrections shift the unoccupied bands up by ~0.2 eV and the first sp-like occupied band down by ~0.4 eV, while leaving unchanged the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the wavefunctions lowers the occupied 5d bands by 0.35 eV. Globally, many-body effects achieve an opening of the interband gap (5d-6sp gap) of 0.35 to 0.75 eV approaching the experimental results. Finally, the quasiparticle bandstructure is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and Ernzerhof) hybrid functional.