Inclusion Of Many-body Effects Research Articles

Electronic and Optical Properties of Alkaline Earth Metal Fluoride Crystals with the Inclusion of Many-Body Effects: A Comparative Study on Rutile MgF2 and Cubic SrF2

We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF2 and cubic SrF2. For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF2 has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF2 and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF2, cubic SrF2, and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems.

Electronic reconstruction and charge transfer in strained Sr2CoIrO6 double perovskite

The electronic, magnetic and optical properties of the double perovskite Sr$_2$CoIrO$_6$ (SCIO) under biaxial strain are explored in the framework of density functional theory (DFT) including a Hubbard $U$ term and spin-orbit coupling (SOC) in combination with absorption spectroscopy measurements on epitaxial thin films. While the end member SrIrO$_3$ is a semimetal with a quenched spin and orbital moment and bulk SrCoO$_3$ is a ferromagnetic (FM) metal with spin and orbital moment of 2.50 and 0.13 $\mu_{B}$, respectively, the double perovskite SCIO emerges as an antiferromagnetic Mott insulator with antiparallel alignment of Co, Ir planes along the [110]-direction. Co exhibits a spin and enhanced orbital moment of $\sim 2.35-2.45$ and $0.31-$0.45 $\mu_{B}$, respectively. Most remarkably, Ir acquires a significant spin and orbital moment of 1.21-1.25 and 0.13 $\mu_{B}$, respectively. Analysis of the orbital occupation indicates an electronic reconstruction due to a substantial charge transfer from minority to majority spin states in Ir and from Ir to Co, signaling an Ir$^{4+\delta}$, Co$^{4-\delta}$ configuration. Biaxial strain, varied from -1.02% ($a_{\rm NdGaO_3}$) through 0% ($a_{\rm SrTiO_3}$) to 1.53% ($a_{\rm GdScO_3}$), influences in partcular the orbital polarization of the $t_{2g}$ states and leads to a nonmonotonic change of the band gap between 163 and 235 meV. The absorption coefficient reveals a two plateau fearure due to transitions from the valence to the lower lying narrow $t_{2g}$ and the higher lying broader $e_{g}$ bands. Inclusion of many body effects, in particular, excitonic effects by solving the Bethe-Salpeter equation (BSE), increases the band gap by $\sim0.2$ and improves the agreement with the measured spectrum concerning the position of the second peak at $\sim 2.6$ eV.

Development of an analytical simulation framework for angle-resolved photoemission spectra

Angle-resolved photoemission spectroscopy (ARPES) is probably the most important experimental technique for the investigation of the electronic structure of solids. With respect to the interpretation of measured complex photoemission spectra, extensive theoretical efforts are often involved, as one needs a calculation of the band structure, a mapping of the band structure to the observed complex photoemission spectrum, and the inclusion of many-body effects. Typical methods that include the mentioned necessary steps for the interpretation are usually theoretically costly (such as time-dependent density functional theory). In contrast to purely numerical methods, analytical physical theories grant an easier approach in the implementation and, in general, allow for a more direct insight into the involved physical processes. Here, we show that combining a Keldysh Green's function formulation of the photoelectric current, together with a tight-binding parametrization of band structures, gives a powerful tool set for the simulation of ARPES spectra. Our approach is fast, capable of including many-body effects, and easy to be implemented and, therefore, from the viewpoint of an experimentalist, a valuable tool for the interpretation of photoemission spectra.

Role of the exchange-correlation functional on the structural, electronic, and optical properties of cubic and tetragonal SrTiO3 including many-body effects

${\mathrm{SrTiO}}_{3}$ is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the ${\mathrm{TiO}}_{6}$ octahedra. Here we study systematically the effect of different exchange-correlation functionals on the structural, electronic, and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06). SCAN is found to significantly improve the description of the structural properties, in particular the rotational angle of the tetragonal phase, comparable to HSE06 at a computational cost similar to GGA. The addition of a Hubbard $U$ term ($\mathrm{SCAN}+U$, $U=7.45$ eV) allows us to achieve the experimental band gap of 3.25 eV with a moderate increase in the lattice constant, whereas within $\mathrm{GGA}+U$ the gap is underestimated even for high $U$ values. The effect of the exchange-correlation functional on the optical properties is progressively reduced from 1.5 eV variance in the onset of the spectrum in the independent particle picture to 0.3 eV upon inclusion of many-body effects within the framework of the $GW$ approximation (single-shot ${G}_{0}{W}_{0}$) and excitonic corrections by solving the Bethe-Salpeter equation (BSE). Moreover, a model BSE approach is shown to reproduce the main features of the optical spectrum at a lower cost compared to ${G}_{0}{W}_{0}+\text{BSE}$. Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. Last but not least, the effect of the tetragonal distortion on the optical spectrum is discussed and compared to available experimental data.

Dispersion Interactions in Water Clusters.

The importance of dispersion forces in water clusters is examined using the effective fragment potential (EFP) method. Since the original EFP1 water potential does not include dispersion, a dispersion correction to the EFP1 potential (EFP1-D) was derived and implemented. The addition of dispersion to the EFP1 potential yields improved geometries for water clusters that contain 2-6 molecules. The importance of the odd E7 contribution to the dispersion energy is investigated. The E7 dispersion term is repulsive for all of the water clusters studied here and can have a magnitude that is as large as half of the E6 value. The E7 term therefore contributes to larger intermolecular distances for the optimized geometries. Inclusion of many-body effects and/or higher order terms may be necessary to further improve dispersion energies and optimized geometries.

Atomic many-body effects and Lamb shifts in alkali metals

We present a detailed study of the Flambaum-Ginges radiative potential method which enables the accurate inclusion of quantum electrodynamics (QED) radiative corrections in a simple manner in atoms, ions, and molecules over the range 10<=Z<=120, where Z is the nuclear charge. Calculations are performed for binding energy shifts to the lowest valence s, p, and d waves over the series of alkali atoms Na to E119. The high accuracy of the radiative potential method is demonstrated by comparison with rigorous QED calculations in frozen atomic potentials, with deviations on the level of 1%. The many-body effects of core relaxation and second- and higher-order perturbation theory on the interaction of the valence electron with the core are calculated. The inclusion of many-body effects tends to increase the size of the shifts, with the enhancement particularly significant for d waves; for K to E119, the self-energy shifts for d waves are only an order of magnitude smaller than the s-wave shifts. It is shown that account of many-body effects is essential for an accurate description of the Lamb shift.

Significant effect of stacking on the electronic and optical properties of few-layer black phosphorus

The effect of the number of stacking layers and the type of stacking on the electronic and optical properties of bilayer and trilayer black phosphorus are investigated by using first principles calcula- tions within the framework of density functional theory. We find that inclusion of many body effects (i.e., electron-electron and electron-hole interactions) modifies strongly both the electronic and opti- cal properties of black phosphorus. While trilayer black phosphorus with a particular stacking type is found to be a metal by using semilocal functionals, it is predicted to have an electronic band gap of 0.82 eV when many-body effects are taken into account within the G0W0 scheme. Though different stacking types result in similar energetics, the size of the band gap and the optical response of bilayer and trilayer phosphorene is very sensitive to the number of layers and the stacking type. Regardless of the number of layers and the type of stacking, bilayer and trilayer black phosphorus are direct band gap semiconductors whose band gaps vary within a range of 0.3 eV. Stacking arrangments different from the ground state structure in both bilayer and trilayer black phosphorus significantly modify valence bands along the zigzag direction and results in larger hole effective masses. The optical gap of bilayer (trilayer) black phosphorus varies by 0.4 (0.6) eV when changing the stacking type. Due to strong interlayer interaction, some stackings obstruct the observation of the optical excitation of bound excitons within the quasi-particle band gap. In other stackings, the binding energy of bound excitons hardly changes with the type of stacking and is found to be 0.44 (0.30) eV for bilayer (trilayer) phosphorous.

Tuning of the electronic and optical properties of single-layer black phosphorus by strain

Using first principles calculations we showed that the electronic and optical properties of single-layer black phosphorus (BP) depend strongly on the applied strain. Due to the strong anisotropic atomic structure of BP, its electronic conductivity and optical response are sensitive to the magnitude and the orientation of the applied strain. We found that the inclusion of many body effects is essential for the correct description of the electronic properties of monolayer BP; for example, while the electronic gap of strainless BP is found to be 0.90 eV by using semilocal functionals, it becomes 2.31 eV when many-body effects are taken into account within the ${G}_{0}{W}_{0}$ scheme. Applied tensile strain was shown to significantly enhance electron transport along zigzag direction of BP. Furthermore, biaxial strain is able to tune the optical band gap of monolayer BP from 0.38 eV (at $\ensuremath{-}8%$ strain) to 2.07 eV (at 5.5%). The exciton binding energy is also sensitive to the magnitude of the applied strain. It is found to be 0.40 eV for compressive biaxial strain of $\ensuremath{-}8%$, and it becomes 0.83 eV for tensile strain of 4%. Our calculations demonstrate that the optical response of BP can be significantly tuned using strain engineering which appears as a promising way to design novel photovoltaic devices that capture a broad range of solar spectrum.

Optical Properties and Quasiparticle Band Gaps of Transition-Metal Atoms Encapsulated by Silicon Cages

Semiconductors assembled upon nanotemplates consisting of metal-encapsulating Si cage clusters (M@Sin) have been proposed as prospective materials for nanodevices. To make an accurate and systematic prediction of the optical properties of such M@Sin clusters, which represent a new type of metal–silicon hybrid material for components in nanoelectronics, we have performed first-principles calculations of the electronic properties and quasiparticle band gaps for a variety of M@Si12 (M = Ti, Cr, Zr, Mo, Ru, Pd, Hf, and Os) and M@Si16 (M = Ti, Zr, and Hf) clusters. At first stage, the electronic structure calculations have been performed within plane-wave density functional theory in order to predict equilibrium geometries, polarizabilities, and optical absorption spectra of these endohedral cagelike clusters. The quasiparticle calculations were performed within the GW approximation, which predict that all of these systems are semiconductors exhibiting large band gaps. The present results have demonstrated that the independent-particle absorption spectra of M@Sin, calculated within the local density or generalized gradient approximations to density functional theory, are dramatically influenced by many-body effects. On average, the quasiparticle band gaps were significantly increased, in comparison with the independent-particle gaps, giving values in the 2.45–5.64 eV range. Consequently, the inclusion of many-body effects in the electron–electron interaction, and going beyond the mean-field approximation of independent particles, might be essential to realistically describe the optical spectra of isolated M@Sin clusters, as well as their cluster-assembled materials.

Effects of electronic correlation on x-ray absorption and dichroic spectra atL2, 3 edge

We present a new theoretical approach to describe x-ray absorption and magneticcircular dichroism spectra in the presence of electron–electron correlation. Ourapproach provides an unified picture to include correlations in both charged andneutral excitations, namely in direct/inversion photoemission where electrons areremoved/added, and photoabsorption where electrons are promoted from corelevels to empty states. We apply this approach to the prototypical case of theL2, 3 edge of 3d transition metals and we show that the inclusion of many-body effects in thecore level excitations is essential to reproduce, together with satellite structures in corelevel photoemission, the observed asymmetric lineshapes in x-ray absorption and dichroicspectra.

Quantum Monte Carlo calculations of the energy-level alignment at hybrid interfaces: Role of many-body effects

An approach is presented for obtaining a highly accurate description of the energy-level alignment at hybrid interfaces, using quantum Monte Carlo calculations to include many-body effects as a correction to the standard single-particle picture. For a representative interface between an organic molecule and an inorganic slab, we illustrate the crucial role of many-body effects for correctly describing the energy-level alignment, leading to qualitatively different optoelectronic properties from the prediction within the single-particle description. Further, the heterojunction behavior as a function of quantum confinement in the slab is predicted to be qualitatively different upon inclusion of many-body effects.

Reduced quantum confinement effect and electron-hole separation in SiGe nanowires

Using first-principles methods, we investigate the structural and electronic properties of SiGe nanowires-based heterostructures, whose lattice contains the same number of Si and Ge atoms but arranged in a different manner. Our results demonstrate that the wires with a clear interface between Si and Ge regions not only form the most stable structures but show a strongly reduced quantum confinement effect. Moreover, we, with the inclusion of many-body effects, prove that these nanowires---under optical excitation---display a clear electron-hole separation property which can have relevant technological applications.

Ab initio study of electronic and optical properties of Be-chalcogenides in GW approximation

An ab initio study of the structural electronic and optical absorption properties of beryllium chalcogenides (BeS, BeSe and BeTe) has been made after using pseudopotential method in generalized gradient approximation (GGA). The many-body effects have been considered in GW approximation. The results for the lattice parameters, bulk modulii and cohesive energies calculated in GGA are in excellent agreement with the experimental data. The lattice parameter increases in the order, BeS, BeSe and BeTe because of an increase in the size of the anion. The bulk modulus and cohesive energy decrease in the order, BeS, BeSe and BeTe because of the enhanced volume of the unit cell. We find that our calculated B's are in close agreement with the experimental values within 9%. For BeTe, the calculated electronic band structure along the Γ–X direction after the inclusion of many body effects is in good agreement with the experimental data. The calculated direct and indirect energy gaps in GW approximation for BeTe are also in close agreement with the experimental data. The optical absorption appears mostly in the ultraviolet region. The no-phonon optical absorption is extended towards the low-energy side monotonically with the size of the anion.

Many-Body Effects on the Electronic and Optical Properties of Bulk GaP

We present an ab-initio study of GaP bulk as a test case to show the importance of going beyond the single particle Density Functional Theory (DFT) scheme in the calculation of excited state properties. The inclusion of many-body effects within the GW approach for the determination of electronic gaps, and of excitonic effects in the calculations of the dielectric function, is discussed. Comparison with one-particle DFT results and with experimental data are presented.

Theoretical Aspects of the Optical Response of Semiconductor Surfaces

We discuss the progress in realistic calculations of the optical response of semiconductor surfaces. It concerns numerical developments as well as the better inclusion of many-body effects. We show that the starting point of such calculations, the atomic structure and the surface equilibrium phase, can be now derived with high accuracy. The different steps are described in detail for the computation of the reflectance anisotropy and the exploration of the In-rich InP(001)2×4 surfaces.

Heavy-hole and light-hole quantum beats in the polarization state of coherent emission from quantum wells

Dual-channel spectral interferometry is used to measure the vectorial dynamics of the the four-wave-mixing signal from a GaAs-AlGaAs multiple quantum well when both heavy-and light-hole excitons are excited. The ellipticity, the orientation, and the sense of rotation of the polarization ellipse associated with this emission are observed to oscillate dramatically at the heavy-hole-light-hole quantum beat frequency. The qualitative nature of the beating can be described by a simple model based on the density matrix equations for two independent three-level systems without the inclusion of many body effects; however, these beats are superposed on a polarization that is dominated by many-body effects.

Non-Markovian Boltzmann Equation

A quantum kinetic equation for strongly interacting particles (generalized binary collision approximation, ladder or T-matrix approximation) is derived in the framework of the density operator technique. In contrast to conventional kinetic theory, which is valid on large time scales as compared to the collision (correlation) time only, our approach retains the full time dependencies, especially also on short time scales. This means retardation and memory effects resulting from the dynamics of binary correlations and initial correlations are included. Furthermore, the resulting kinetic equation conserves total energy (the sum of kinetic and potential energy). The second aspect of generalization is the inclusion of many-body effects, such as self-energy, i.e., renormalization of single-particle energies and damping. To this end we introduce an improved closure relation to the Bogolyubov–Born–Green–Kirkwood–Yvon hierarchy. Furthermore, in order to express the collision integrals in terms of familiar scattering quantities (Møller operator, T-matrix), we generalize the methods of quantum scattering theory by the inclusion of medium effects. To illustrate the effects of memory and damping, the results of numerical simulations are presented.

Simulation of electronic properties and capacitance of quantum dots

The chemical potential and the capacitance of a 2D circular model quantum dot have been investigated for GaAs, InSb, and Si material parameters, covering a range from a few nanometers to micrometer dimensions. The Schrödinger equation has been solved self-consistently, with the inclusion of many-body effects, using a local density approximation as well as the optimized Krieger-Li-Iafrate exchange potential. Gate structures are included by use of the method of images. We have focused on quantum deviations from classical electrostatic capacitive behavior and found such deviations to be significant even for the material parameters of silicon for feature sizes smaller than 30 nm. The most striking features of quantum dot capacitance are signatures of the dot symmetry analogous to the orbital grouping in atoms: we find structure in the dot capacitance arising from quantum effects in correspondence with the filling of each group of energy-degenerate orbitals. We also cover the influence of a magnetic field perpendicular to the dot plane and we report some results for the chemical potential vs magnetic field and electron number, assuming an effective g-factor corresponding to the one of bulk gallium arsenide.

Stability of MX2−3 ions in the gas phase and when do ionic molecules have large ionization potentials

The present work aims at theoretical consideration of the geometrical and electronic structures of a homologous series of free MXmq−-type alkali halides (M=Li, Na, K; X=F, Cl; m=1,2,3; q=0,1,2) in order to get insight into their kinetic and electronic stability. At the ab initio Hartree–Fock self-consistent field (HF-SCF) level of theory, the lowest energy fragmentation channel leading to the decomposition of the dianions MX32− into MX2− and X− has been investigated. The potential energy surface was found to exhibit a broad, but flat energy barrier to fragmentation. These findings have been confirmed using results from configuration interaction calculations and the molecular dianions are predicted to be long-lived species formally existing in a metastable state. The stability of the gas-phase MX32− dianions and of the MX2− fragmentation products with respect to autodetachment of an extra electron has been investigated using ab initio HF-SCF and Green’s function methods. The inclusion of many-body effects by the latter was found to decrease the vertical binding energy of the extra electrons with respect to the Koopmans’ theorem HF-SCF result, but the extra electrons remain bound. At the ab initio level, the compounds studied in this work are predicted to be extremely ionic species. The variation of the properties through a series of MXmq− species has been investigated. Motivated by the overall large ionic character—in particular of the MX32− molecules—the properties of the systems have been studied using a previously derived theoretical approach, referred to as ionic model, based on (classical) electrostatic terms. The versatility of the ionic model scheme has been extended here to obtain information on the tendency of an ionic molecule to accomodate an additional electron.

Valence electron momentum distributions of the hydrogen halides. II. Comparison of EMS measurements and calculations using near Hartree-Fock limit and configuration interaction wavefunctions for hydrogen chloride

The valence orbital electron momentum profiles of hydrogen chloride have been obtained by symmetric non-coplanar electron momentum spectroscopy at an impact energy of 1200 eV. The measurements are significantly different from earlier reported EMS profiles (J. Electron Spectry. Relat. Phenom. 21 (1980) 71) for the (5σ) −1 and (4σ) −1 processes. The measured momentum profiles are compared with calculated cross-sections using a range of wavefunctions from minimal basis to essentially Hartree-Fock limit in quality. The experimental momentum resolution is incorporated into the calculated momentum profiles using the recently developed GW-PG procedure (Chem. Phys. 159 (1992) 347). The effects of correlation and relaxation on the calculated momentum profiles are investigated by MRSD-CI calculations of the full ion-neutral overlap distributions. For all three valence profiles, the inclusion of many-body effects is found to provide little further improvement in the already quite good fit of theory to experimen at the near Hartree-Fock limit level, as has been found previously for other third row hydrides. The effects of final state correlation on the valence binding energy spectrum are investigated using MRSD-CI calculations and the results are compared with other calculations and with experiment.