Ab Initio Pseudopotentials Research Articles

Dynamical properties of the hexagonalclose-packed metals Sc, Y, and Ru

In this paper we perform a comparative study of the dynamicalresponse of the three hexagonal close-packedmetals Sc, Y, and Ru for small- andmedium-sized wavevectors up to 50 eV. The calculations arebased on ground states which are determined using abinitio pseudopotentials together with a plane-wave expansionof the wavefunctions. We consider many-body as well as crystallocal-field effects which both turn out to be small. In all ofthe three elements we find a strong anisotropy of the responsewith respect to the direction of the wavevector transfer.Furthermore, we obtain almost undamped plasmon excitations forsmall wavevectors in the direction normal to the hexagonalplanes for the response of Sc and Y.

Lead Zirconate at Ambient and High Pressure

A recently reported pressure-induced phase transformation in PbZrO 3 is investigated by simulated annealing, with ab initio molecular dynamics and pseudopotentials. Our results suggest that, as pressure increases, M 3 -like oxygen rotations are frozen into the orthorhombic antiferroelectric structure. The first order phase transformation occurs at a pressure in reasonable agreement with experiments. This high pressure orthorhombic phase ( I ') exhibits small but reproducible o -like (i.e., ferroelectric) distortions. We also re-examine the PbZrO 3 antiferroelectric phase structure at ambient pressure. Strain and metal ion relaxations are found to significant lowering in energy. The low temperature rhombohedral ferroelectric phase and the orthorhombic antiferroelectric phase are shown to be very similar in energy.

Bulk excitonic effects in surface optical spectra.

We calculate the surface optical properties of the passivated Si(110) surface using a real-space multigrid technique and ab initio pseudopotentials. Rather than from the usual eigenvalue representation, the macroscopic polarizability is obtained from the solution of an initial-value problem, which allows inclusion of excitonic and local-field effects in addition to the electronic self-energy in the surface calculations. It is shown that the electron-hole attraction is largely responsible for the peculiar line shape of the surface reflectance anisotropy.

Basis set limit extrapolation of ACPF and CCSD(T) results for the third and fourth lanthanide ionization potentials

Theoretical values for the third and fourth ionization potentials of the lanthanide elements from La to Lu are presented. The results are based on large-scale multi-reference averaged coupled-pair functional calculations starting from a complete active space reference wavefunction and single-reference coupled-cluster calculations. Relativistic effects were accounted for by small-core energy-consistent ab initio pseudopotentials. The results obtained for optimized uncontracted valence basis sets including up to i functions were extrapolated to the basis set limit. The importance of higher angular momentum basis functions in ab initio calculations for energy differences between configurations with different 4f occupation numbers is discussed.

Electronic structure of1×1GaN(0001) andGaN(0001¯)surfaces

The electronic structure of clean and Ga- or N-covered $1\ifmmode\times\else\texttimes\fi{}1$ GaN(0001) and $\mathrm{GaN}(0001\ifmmode\bar\else\textasciimacron\fi{})$ surfaces is studied using the local-density approximation of density-functional theory employing ab initio pseudopotentials together with Gaussian orbital basis sets. We use both standard and self-interaction- and relaxation-corrected pseudopotentials. The latter allow for a most accurate description of the electronic structure of these surfaces. Comparing the formation energies for the clean and adatom-covered $1\ifmmode\times\else\texttimes\fi{}1$ configurations, we determine optimal surface structures for various growth conditions. For the GaN(0001) surface in the Ga-rich case, we find a structural model consisting of Ga adatoms adsorbed in ${T}_{4}$ positions above the substrate surface to be most favorable. In the N-rich case, the clean GaN(0001) surface is the most stable $1\ifmmode\times\else\texttimes\fi{}1$ configuration. For the $\mathrm{GaN}(0001\ifmmode\bar\else\textasciimacron\fi{})$ surface, our results for both Ga- and N-rich growth conditions indicate that a full monolayer of Ga adatoms adsorbed in on top positions is the most stable configuration. Our theoretical results allow for a comparison of full calculations of the surface electronic structure for a number of optimized structural models with most recent angle-resolved photoemission spectroscopy data.

A comparative study of surface phonons on CdTe(1 1 0) and InSb(1 1 0)

We have studied the lattice dynamics of the CdTe(110) and InSb(110) surfaces by using the adiabatic bond-charge model. The atomic geometry and electronic informations for the phonon calculations have been obtained by using the local density approximation of the density functional theory and ab initio pseudopotentials. The calculated phonon dispersion curves for InSb(110) are in agreement with the results obtained in a recent helium atom scattering experiment. The surface phonon modes on the InSb(110) and CdTe(110) surfaces exhibit nearly similar displacement patterns. This is mainly due to the fact that both bulk materials have nearly identical lattice constants, as well as a similar cation and anion mass difference. It is pointed out that, due to the larger ionicity of CdTe, the surface phonon energies are generally lower for CdTe(110) than for InSb(110).

Ab Initio Pseudopotential Calculations of Electronic Structure of Off-Stoichiometric ZnO

We have investigated the electronic structure and valence electron density distribution of ideal-stoichiometric and off-stoichiometric ZnO. The ab initio pseudopotential calculation is carried out in the framework of the density function theory within the local density approximation. In calculating the Zn pseudopotential, the errors, particularly those of elements that have 3d electrons, introduced by the frozen-core approximation and linearizing the exchange-correlation potential, were corrected. The electronic structures of ideal-stoichiometric and off-stoichiometric ZnO shows a trend of increasing conductivity introduced by excess Zn, which is consistent with the experimental results. The calculated valence electron density distribution reveals the strong ionic characteristic Zn–O bond in ideal-stoichiometric and off-stoichiometric ZnO.

Optical second-harmonic spectra of Si(001) with H and Ge adatoms: First-principles theory and experiment

We present calculated second-harmonic-generation (SHG) spectra of the Si(001) surface based on a first-principles description of eigenvalues and eigenvectors using ab initio pseudopotentials. We also present SHG spectra for Ge-covered Si(001). The theoretical results explain all essential features of recent experimental SHG spectra of the $\mathrm{Si}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface with low coverages of hydrogen and/or germanium, which alter the ${E}_{1}$ resonance in contrasting ways. The strong adatom specificity of the spectra results from redistribution of the adatom-related electronic states on the surface.

Ab-initio study of Coulomb-correlated optical properties in conjugated polymers

The spatial extension and binding energy of excitons in semiconducting conjugated polymers are still the subject of a great debate. We address this problem through first-principles calculations (within DFT-LDA, plane-waves and ab-initio pseudopotentials), which allow to include electron-hole correlation effects in a fully three-dimensional approach through the density-matrix formalism. We show results for the correlated optical spectrum and the exciton wavefunctions of single-chain poly(para)phenylene-vinylene (PPV), that support the picture of a strongly bound anisotropic exciton localized over ∼ 4-5 monomers.

CCSD(T) calculation of the ground-state potential curves for the Zn-rare gas van der Waals molecules

The spectroscopic parameters ( R e, D e, ω e) for the ground state of the weakly bound Zn-rare gas (RG) van der Waals molecules have been derived from ab initio calculated potential curves. The potential curve calculations have been carried out using the coupled-cluster with single and double excitations and perturbative contribution of connected triple excitations (CCSD(T)) method for the 20 electrons of Zn and the valence electrons of RG atom, while the Zn 20+ and RG 8+ cores are replaced by ab initio quasirelativistic energy-consistent pseudopotentials. The theoretical potentials are discussed in the context of available experimental data. Very good agreement between theory and experiment has been obtained.

Atomic geometry, electronic structure and image state for the Si(1 1 1)–In(4×1) nanowire

We have performed a detailed theoretical study of the atomic geometry, electronic structure, and dispersion of the most tightly bound ( n=1) image state for the Si(1 1 1)–In(4×1) nanowire system. The calculations were performed using ab initio pseudopotentials, based on the local density approximation and a first-order energy correction for its asymptotic classical-image behaviour. The calculated atomic geometry, within the structural model proposed by Bunk et al. [Phys. Rev. B 59 (1999) 12228], agrees well with their X-ray diffraction studies, and the electronic band structure calculations confirm the quasi-one-dimensional semimetallic behaviour, in agreement with previous photoemission studies. The anisotropic dispersion of the image state measured in a recent inverse photoemission study by Hill and McLean [Phys. Rev. Lett. 82 (1999) 2155] is verified, and an explanation based on the calculated surface corrugation potential is presented.

Ab initiocalculations for structure and temperature effects on the polarizabilities ofNan(n<~20)clusters

We present a rigorous, ab initio theoretical calculation of the dependence of Na cluster polarizability on cluster size, up to 20 atoms, obtained by combining ab initio pseudopotentials with a gradient-corrected density functional. Using molecular dynamics, we find that for clusters as small as nine atoms, a multitude of degenerate isomers exists even at $T=0.$ By calculating the polarizability of these isomers, we reproduce the generally decreasing nature of the measured polarizability curve, as well as its dips at ``magic'' numbers corresponding to closed electronic shells. Moreover, we find that the effect of a finite temperature on the cluster structure suffices to account for most of the quantitative discrepancy between theory and experiment.

Combined pseudopotential and density functional study of Bis-eta6-benzene d and f element complexes.

The electronic and molecular structures of selected zerovalent d and f metal bis-eta6-benzene sandwich complexes M(C6H6)2 (M = Sc, Ti, V, Cr, Y, Zr, Nb, Mo, La, Ce, Gd, Lu, Hf, Ta, Nb, Th) were investigated at the scalar-relativistic level using a combination of energy-consistent ab initio pseudopotentials and gradient-corrected density functionals. The results are compared to previous pseudopotential coupled-cluster and all-electron density functional results based on the zero-order regular approximate Hamiltonian of Chang, Pelissier, and Durand as well as to experimental data.

Gold clusters(AuN,2<~N<~10)and their anions

Atomic and electronic structures of neutral and anionic gold clusters $({\mathrm{Au}}_{N}$ and ${\mathrm{Au}}_{N}^{\ensuremath{-}},$ $2l~Nl~10)$ are investigated using the density-functional theory with scalar-relativistic ab initio pseudopotentials and a generalized gradient correction. The ground state optimal geometries of the neutral and anionic clusters are found to be planar up to $N=7$ and 6, respectively, with the much studied ${\mathrm{Au}}_{6}^{\ensuremath{-}}$ cluster predicted to have a ${D}_{3h}$ triangular structure. The calculated electron detachment energies of the ground-state anions exhibit strong odd-even alternations and structural sensitivity and agree quantitatively with photoelectron measurements. Photofragmentation patterns of the cluster anions and their interplay with the photodetachment process, are investigated and correlated with experiments.

Optical properties of bulk and thin-film SrTiO3 on Si and Pt

We have studied the optical properties (complex dielectric function) of bulk SrTiO3 and thin films on Si and Pt using spectroscopic ellipsometry over a very broad spectral range, starting at 0.03 eV [using Fourier transform infrared (FTIR) ellipsometry] to 8.7 eV. In the bulk crystals, we analyze the interband transitions in the spectra to determine the critical-point parameters. To interpret these transitions, we performed band structure calculations based on ab initio pseudopotentials within the local-density approximation. The dielectric function was also calculated within this framework and compared with our ellipsometry data. In the FTIR ellipsometry data, we notice a strong lattice absorption peak due to oxygen-related vibrations. Two longitudinal optic (LO) phonons were also identified. In SrTiO3 films on Si, the refractive index below the band gap decreases with decreasing thickness because of the increasing influence of the amorphous interfacial layer between the SrTiO3 film and the Si substrate. There is also a decrease in amplitude and an increase in broadening of the critical points with decreasing thickness. In SrTiO3 films on Pt, there is a strong correlation between the crystallinity and texture of the films (mostly aligned with the Pt pseudosubstrate) and the magnitude of the refractive index, the Urbach tail below the bulk band edge, and the critical-point parameters. FTIR reflectance measurements of SrTiO3 on Pt (reflection–absorption spectroscopy) show absorption peaks at the LO phonon energies, a typical manifestation of the Berreman effect for thin insulating films on a metal. The Urbach tail in our ellipsomety data and the broadening of the optical phonons in SrTiO3 on Pt are most likely caused by oxygen vacancy clusters.

Understanding reflectance anisotropy: Surface-state signatures and bulk-related features

Using a real-space multigrid method and ab initio pseudopotentials we calculated the reflectance anisotropy spectra for (001) growth structures of GaP and InP as well as for stepped Si(111):H and hydrogenated Si(110) surfaces. Our results, obtained within DFT-LDA, show good qualitative agreement with recent experiments. Further improvement is achieved by using a numerically efficient GW approach with approximations for local-field effects and dynamical screening. We find the contributions to the anisotropy signal related either directly to surface states or to transitions between surface-perturbed bulk wave functions. Our results demonstrate the high sensitivity of RAS to the surface structure and chemistry and show that the optical transitions causing the anisotropy signal take place mainly in a few uppermost atomic layers of the substrate.

Atomic geometry, electronic states and vibrational properties of the AlAs(110) surface

We have investigated the atomic geometry and electronic structure of the AlAs(110) surface by using the local density approximation of the density functional theory and ab initio pseudopotentials. The structural and electronic results have been used in the application of the adiabatic bond charge model for surface phonon studies. By comparing the results with other III-V(110) surfaces, we have obtained some general trends regarding the surface dynamics.

Effective Pair Interaction from Ab Initio Pseudopotentials - Importance and Applications

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Step-induced optical anisotropy of Si(111):H surfaces

The step-induced optical anisotropy of the hydrogenated Si(111) surface has been calculated using a real-space multigrid method together with ab initio pseudopotentials. The investigated step configurations give rise to relatively similar anisotropy spectra with maxima close to the ${E}_{1}$ and ${E}_{2}$ critical points of the silicon bulk band structure. The spectrum for $〈112\ifmmode\bar\else\textasciimacron\fi{}〉$ steps resembles the reflectance anisotropy for Si(110):H and is in good agreement with measurements for Si(111):H vicinally cut toward $[112\ifmmode\bar\else\textasciimacron\fi{}].$ Our results for $〈1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2〉$ steps, however, are qualitatively different from the corresponding measurements for surfaces miscut toward $[1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2].$ This apparent discrepancy can be resolved if the formation of staircaselike structures of monohydride terminated steps is assumed.

Density functional study of the `titanium effect' at metal-ceramic interfaces

A thin interlayer of an early transition metal, e.g. titanium, can enhance the bonding at a metal-ceramic interface considerably. The role of the electronic structure in this phenomenon (the `titanium effect') is studied quantitatively by means of density functional calculations, employing norm-conserving ab initio pseudopotentials and a mixed basis of localized functions and plane waves. The weakly bonding model interface Ag(100)/MgAl2O4(100) is chosen, in order to minimize the lattice mismatch and to concentrate on electronic interactions. The stepwise addition of Ti atoms at the interface from zero to one monolayer of Ti leads to a pronounced enhancement of the bonding strength (Ag/Ti/MgAl2O4). From a comparison to the more strongly bonding systems Ag/Al/MgAl2O4 with a monatomic Al interlayer and Al/MgAl2O4 it is concluded that this enhancement correlates with the reduction of the electron density in the interface layer. It is proposed that the interlayer accommodates Pauli repulsion between O anions and electron-rich Ag atoms by suitable electron redistribution, which is accompanied by pronounced energy band shifts.