INVITED TALKS
and
ORAL CONTRIBUTIONS

Total energies from self-energy operators

R.W. Godby$^{1}$, P. García-González$^{2}$, K.T. Delaney$%% ^{1}$, P. Rinke$^{1}$, and T. Gould$^{1}$

$^{1}$Department of Physics, University of York, United Kingdom
$^{2}$Departamento de Fisica de la Materia Condensada, Universidad Autonoma de Madrid, Spain

Progress has become slow in constructing ever better approximations to the exact Kohn-Sham energy functional of density-functional theory that are applicable in a wide range of situations, reflecting the extreme complexity of the exact functional, including its ultra-non-local dependence on the electron density in certain situations. These non-analyticities in the exchange-correlation energy functional may be circumvented by reformulating the total energy using Green's-function many-body perturbation theory, as a practical alternative to DFT. A real-space-imaginary-time representation offers an efficient framework for implementing a fully self-consistent GW approximation, which is a conserving approximation. We show results for total energies of homogeneous and inhomogeneous systems within GW [1]. We also discuss prospects for improved density-based models of self-energies. Building on earlier work [2], these would allow inexpensive total-energy calculations within the framework of a generalised Kohn-Sham theory.

[1] P. García-González and R.W. Godby, Phys. Rev. Lett. 88, 056406 (2002).
[2] Paula Sánchez-Friera and R.W. Godby, Phys. Rev. Lett. 85, 5611 (2000).

The application of variational many-body functionals to the total energies of atoms

N.-E. Dahlen and U. von Barth

Solid State Theory Group, Physics Department, Lund University, Sweden

In the present work we have tested variational energy functionals obtained from many-body perturbation theory in a number of atoms. We have tested the functional due to Luttinger and Ward (LW) as well as the recently proposed functional due to Almbladh, von Barth and van Leeuwen (ABL). The functionals are variational in the sense that they can be evaluated at rather crude approximations to their independent variables which are i) the one-electron Green function, and ii) the Green function and the screened interaction respectively for the two functionals mentioned. The quality of the results will depend on the level of sophistication of the underlying perturbation expansions. The functionals were previously applied to the electron gas and shown to be extraordinarily accurate already at the level of the so called $GW$ approximation (GWA). At the same level, they have also been applied to linear Hubbard chains with less encouraging results. We here find that the LW functional, at the $GW$ level, corrects approximately half the error in the $RPA$ correlation energies of atoms. In going to second order in the screened interaction we find it important to make full use of the variational property of the ABL functional with respect to the screened interaction. Evaluation of the latter functional at a non-interaction Green function and a statically screened interaction gives errors in the correlation energies of atoms of the order of 10-20%.
The actual calculations are of a complexity that would allow for these methods to be applied also to molecules and solids. Our applications to molecular binding energies are in progress.

Pair correlation functions from GW calculations

Bengt Holm$^1$ and Ulf von Barth$^2$

$^1$Dept of Physics, Graduate School of Science, University of Tokyo, Japan
$^2$Solid State Theory Group, Physics Department, Lund University, Sweden

The GW method is emerging as one of the leading tools to calculate excited states in a many body system. In the formulation of the theory, a self consistent procedure is prescribed. However, since self consistency is time consuming and makes calculations complicated, the usual implementations of the GW approximation involve what is usually referred to as a one iteration scheme.
In the presentation, some light will be shed on the properties of self consistency. Further, results of the calculation of total energies are presented, and we discuss the properties of pair correlation functions generated from various Green's functions ; the ones from self-consistent, partially self-consistent and one iteration schemes respectively.

Density functionals based on the adiabatic-connection fluctuation-dissipation theorem: application to H$_2$ and Be$_2$

X. Gonze$^{1}$, M. Fuchs$^{1,2}$, Y.-M. Niquet$^{1}$, K. Burke$^{3}$

$^{1}$Universite Catholique de Louvain, Belgium
$^{2}$Fritz-Haber-Institut der Max-Planck-Gesellshaft, Berlin, Germany
$^{3}$Rutgers University, USA

Fully nonlocal exchange-correlation functionals derived from the adiabatic-connection fluctuation-dissipation theorem can go beyond local or gradient corrected functionals and include the van der Waals interaction. We implement three functionals of this class, in a pseudopotential plane-wave framework, (1) using the random-phase approximation (RPA), (2) adding to the RPA short-range correlations (RPA+), and (3) beyond RPA, including an approximate exchange kernel. We work in a non-self-consistent framework, starting from the converged exact-exchange (only for H$_2$) or LDA Kohn-Sham potential. We find the binding energy of the H$_2$ and Be$_2$ molecules described, by all three functionals, within 0.1 eV accuracy. Equilibrium bond lengths and harmonic vibrational frequencies generally improve upon LDA and GGA. We then explore the H$_2$ full dissociation curve. The ACFD approach solves the "DFT symmetry-dilemma" encountered for large H$_2$ internuclear separations, but creates an unphysical potential barrier to dissociation, for distances on the order of 5 bohrs.

Exchange correlation potentials in the adiabatic connexion fluctuation dissipation framework

Y. M. Niquet$^1$, X. Gonze$^1$, and M. Fuchs$^{1,2}$

$^1$Unité PCPM, Université Catholique de Louvain, Belgium
$^2$Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

We investigate the possibility of making self-consistent calculations of the total energy in the adiabatic connexion fluctuation dissipation (ACFD) framework. We provide the ACFD exchange-correlation (xc) potential in the general case, and focus on the random-phase approximation (RPA) as an example. We establish the links between the ACFD-RPA and the many-body perturbation theory in the GW approximation, both at the level of total energy (Luttinger Ward formula) and xc potential (Sham-Schlüter equation). We show that the ACFD-RPA xc potential is likely to diverge far from a finite system, due to the presence of the empty kohn-sham states in the expression of the ACFD-RPA total energy. We discuss the implications of this divergence for self-consistent ACFD calculations, and propose some modified potentials that exhibit correct asymptotic behaviour.

Density-Functional Approach for the Electronic Self-Energy

Arno Schindlmayr

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Within density-functional theory a perturbation series that yields the exact quasiparticle excitation energies in terms of the Kohn-Sham orbitals and eigenvalues in derived. In contrast to a coupling-constant expansion of the self-energy, the present scheme is designed such that the first-order term includes dynamic screening and closely resembles the non-self-consistent $GW$ approximation, which is widely used in electronic-structure calculations. However, it also solves some known deficiencies of the latter. In particular, it is internally consistent, the spectral function integrates to the correct particle number in every order of perturbation theory, and the energy of the highest occupied state always equals the exact chemical potential. The perturbation series provides a systematic procedure for generating higher-order self-energy corrections beyond the $GW$ approximation.

Exchange and Correlation Effects in Small Jellium Clusters

P. Rinke$^1$, P. García-González$^2$, and R. W. Godby$^1$

$^1$Department of Physics, University of York, United Kingdom
$^2$Departamento de Física de la Materia Condensada Universidad Autónoma de Madrid, Spain

We present ab initio electronic structure calculations for small, spherical jellium clusters or quantum dots (QD) in the $GW$ self-energy approach. In particular we analyse the effects of exchange and correlation, which give rise to image effects in these structures.
In recent years, $GW$ ($G$: Greens Function, $W$: Screened Coulomb Potential) has successfully been applied to calculate band structures of bulk materials and surfaces as well as properties of small molecules and clusters. We apply ab initio many-body perturbation theory (MBPT) in the $GW$ approximation to solve the quasiparticle equation, using a space-time approach [1] for the self-energy.
We analyse the structure of the self-energy operator in real-space including its non-locality and frequency dependence. Substantial changes (up to about 1 eV) in the quasiparticle energy spectrum are observed, which arise in part from physical effects inherently absent in other methods such as density-functional theory (DFT), in the usual approximations. The wave functions for unoccupied states near the vacuum level deviate considerably from eigenstates of the DFT system at similar energy, which we attribute mainly to image effects at the surface of the cluster. A comparison with calculations based on a classical electrostatic image potential [2] is made.

[1] H.N. Rojas, R.W. Godby and R.J. Needs, Phys. Rev. Lett 74, 1827 (1995)
[2] Patrick Rinke, MSc Dissertation: Image Effects in Quantum Dots (1999)

Ab-initio calculation of local excited states in strongly correlated systems

Marie-Bernadette Lepetit

Université Paul Sabatier, Toulouse, France

The physics of strongly correlated materials have attracted a lot of attention in the last two decades. These systems are strongly multiconfigurational in nature and the ab-initio methods such as DFT usually fail to decribe their low energy physics and fascinating properties. There physics is usually described by model Hamiltonians with local effective interactions that can be extracted from the low energy local spectroscopy. The present talk will show the key ingredients necessary for the determination of accurate local excitations. We will discuss in particular the importance of the crystal environment, of the multiconfigurational nature of the wave-function of the dynamical polarisation and correlation effects.

Ab initio exchange coupling constant calculations for manganites and cuprates

Charles H. Patterson, G. Zhengg, and M. Nicastro

Department of Physics, Trinity College, Dublin, Ireland

Strongly correlated electron materials are generally studied using model Hamiltonians with parameter values adjusted to give best fits to experiment. We present results of bulk Unrestricted Hartree-Fock (UHF) and cluster configuration interaction (CI) calculations on LaMnO$_{3}$, La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and La$_{2}$CuO$_{4}$, from which exchange coupling constants and low-lying excitation energies are extracted. Cluster CI calculations are performed in a localised orbital basis and configurations in the ground state wave function show which quantum fluctuations are important in determining exchange coupling energies. A localised orbital basis also permits an easy mapping to a model Hamiltonian. Calculations of the exchange coupling constants in LaMnO$_{3}$ show good agreement with experimentally derived values; UHF calculations on La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ show that the ground state contains only d$^{4}$ Mn ions and both O$^{2-}$ and O$^{-}$ ions. Hence a double-exchange Hamiltonian is inappropriate for doped manganites. Cluster CI calculations of the exchange coupling constants in La$_{2}$CuO$_{4}$ show reasonable agreement with experimentally derived values.

Electron-hole excitations in semiconductors and insulators

Brice Arnaud$^{1}$, Sebastien Lebegue$^{2}$, and Mebarek Alouani$%% ^{2}$

$^{1}$GMCM, campus de Beaulieu, Rennes, France
$^{2}$IPCMS, Strasbourg, France

Ab initio calculations of the electronic structure of insulators entail big difficulties involved with the treatment of excitation energies and many-body effects. The most successful first-principles method, the density functional theory (DFT) within the local density approximation (LDA) is designed for ground state properties and can not provide a proper description of the band structures of insulators. However, it is possible to accurately calculate the quasiparticle energies and band gaps from first principles by solving Hedin's set of equations for the full Green's function in the so-called GW approximation.
An approach based on an all-electron method (PAW) has been developed to compute the quasiparticle energies within the GW approximation. Within this approximation, the self-energy is given as a product of the one-particle Green's function G and the dynamically screened interaction W computed within the random phase approximation (RPA). Starting from the calculated local density approximation (LDA) ground state, the LDA eigenvalues are corrected by treating the difference between the self-energy and the exchange-correlation potential as a perturbation. The calculated quasiparticle energies obtained by means of this procedure are, generally, in good agreement with photoemission experiments, and are found to be neither sensitive to the scheme used for decoupling core and valence electrons nor to the different plasmon-pole models used to reproduce the frequency dependence of the dynamically screened interaction W.
The quasiparticle spectra are then used to compute the imaginary part of the macroscopic dielectric function including both local-field and excitonic (electron-hole attraction) effects. The standard procedure for including these effects in the calculation of the dielectric function consists in solving the so called Bethe-Salpeter equation. This approach has been applied to different semiconductors and insulators, and it has been shown that the inclusion of electron-hole attraction is crucial for an adequate comparison of the theoretical and experimental optical spectra.

Surface excitons at insulator surfaces

M. Rohlfing, N.-P. Wang, P. Krüger, and J. Pollmann

Institut für Festkörpertheorie, Universität Münster, Germany

We investigate the properties of excited electronic states at insulator surfaces from first principles. Based on density functional theory for the electronic ground state, single-particle excitations and coupled electron-hole excitations are discussed within many-body perturbation theory (GW approach and Bethe-Salpeter equation). The electron self-energy operator and the corresponding electron-hole interaction kernel are treated within the GW approximation. As one prototype example, we discuss surface excitons at the LiF(001)-(1x1) surface, that lead to a characteristic surface peak in the reflectivity spectrum of the material. Work on clean and adsorbate-covered MgO surfaces is under progress.

Excitons in confined systems: Surfaces and nanocrystals

Friedhelm Bechstedt

Friedrich-Schiller-Universität, Jena, Germany

Ab initio calculations of optical spectra and pair excitation energies are presented for nanostructures such as semiconductor surfaces and nanocrystals including the interaction of electrons and holes. Two different approaches are used. For calculations of spectra usually the polarization function has to be computed by solving the Bethe-Salpeter equation. We replace it by the solution of an initial-value problem for the time-dependent polarization. As examples reflectance-anisotropy spectra of semiconductor surfaces are calculated. In order to calculate pair excitation energies a special $\Delta$SCF method is used. The total energy of the excited system with electron-hole pair is calculated under an occupation constraint for the HOMO state of the ground-state system. The method is applied to Ge, Si, and Ge$%% _{1-x}$Si$_x$ nanocrystals. Pair excitation energies are obtained for singlet and triplet excitons versus diameter and composition. Stokes shifts are discussed.

Dynamical excitons in metals and semiconductors

A. Marini$^1$ and R. del Sole$^2$

$^1$Donostia International Physics Center (DIPC), San Sebastián, Spain
$^2$Istituto Nazionale per la Fisica della Materia - Dipartimento di Fisica dell'Università di Roma Tor Vergata, Italy

It is well known that to reproduce correctly the experimental absorption spectrum of semiconductors both self-energy and excitonic effects are required. However only quasiparticle corrections are usually included while the dynamical renormalization factors $Z_{qp}$ are not. When included, the renormalization of the single-particle spectral functions strongly reduces the spectral intensity worsening considerably the theoretical absorption spectrum if compared with the experimental results. Using a generalized ladder approximation we show that the solution of the Bethe-Salpeter equation in the presence of a frequency dependent screened interaction $W(\omega)$ can be rewritten in terms of a new effective two particle kernel $\Pi(\omega)$. This kernel is itself an expansion in terms of the screened interaction but we show that only terms up to the second order in $W(\omega)$ are important. We show that the presence of a frequency dependent potential $\Pi(\omega)$ renormalizes the non-interacting electron-hole Green's function and the static electron-hole interaction compensating dynamical self-energy effects and reproducing the well-known excitonic effects observed experimentally.
It is also well known that metals do not seem to exhibit any static excitonic-like effect. However in the present scheme dynamical excitonic effects in metals are possible and indeed occur, as we show in the case of copper. The absorption spectrum turns out to be in agreement with experimental results when both self-energy and excitonic effects are included.

Aspects of polarizabilities in real materials: models and symmetry properties

Eric L. Shirley

NIST, US Department of Commerce, Gaithersburg, USA

The community continually makes great progress in ab initio and model polarizabilities for real materials. Polarizabilities are ubiquitous in condensed matter. This talk will address two practical examples: the macroscopic optical properties of wide-gap insulators used in photolithography, and calculations of the screened potential of a core hole in a solid. Optical properties are described by the dielectric tensor relating electric field and displacement. In cubic systems, this tensor is isotropic, except for effects of finite wavelength of light. The effects of finite wavelength might have a significant effect on transmissive optical elements in the deep ultraviolet. Here, related symmetry-breaking effects are calculated. This symmetry breaking gives insight into anisotropies of excitons, including energies and oscillator strengths. In the Bethe-Salpeter calculations, great care was taken to suppress spurious symmetry-breaking effects. Calculating the screening of a core hole or other charged defect in a solid can depend on detailed calculation of the two-point irreducible (RPA) polarizability. Here, we shall present development and results of model calculations that attempt to calculate the polarizability more efficiently. Aspects of sum-rules and scaling properties will be touched on.

Time-dependent Optimized Effective Potential: An Approach to the Vertex

Stefan Kurth$^1$ and Ulf von Barth$^{2}$

$^1$Institute for Theoretical Physics, Free University Berlin, Germany
$^2$Solid State Theory Group, Department of Physics, Lund University, Sweden

The proper description of optical absorption has been a long-standing problem within many-body perturbation theory. It is well known that in the GW approximation, the dressing up of the one-electron Green function results in optical absorption intensities which are much too low. In addition, the essential f-sum rule for the linear density response function is no longer satisfied, unless the particle-hole interactions or vertex corrections are properly accounted for. The main problem is thus to dress up Green functions and simultaneously include particle-hole interactions in a dynamically consistent way. One possibility is to use time-dependent Hartree-Fock (TDHF) theory , but this has so far proven to be too complicated to apply, even for simple systems as the uniform electron gas.
Time-dependent density functional theory (TDDFT) provides an alternative description of optical absorption. While the popular adiabatic local density approximation (ALDA) gives reasonable absorption spectra for atoms, it is inadequate for insulators. Here we study the next level of approximation beyond the ALDA, the optimized effective potential (OEP). In the exact exchange version of static DFT (or exchange-only OEP), the nonlocal Hartree-Fock potential is replaced by the local, exact exchange potential. Similarly, the integral equation for the response function of TDHF is here replaced by an explicit expression for the exact exchange kernel of TDDFT. The scheme is consistent in the sense that it results in a response function which obeys the f-sum rule at the same time as it provides a dynamic vertex with proper analyticity. We present encouraging results for atoms and the uniform electron gas and argue that applications to molecules and solids are within reach.

Long range behavior and frequency dependence of exchange-correlation kernels in solids

Rodolfo Del Sole$^1$, Giovanni Adragna$^{1}$, Valerio Olevano$^2$, and Lucia Reining$^2$

$^1$Istituto Nazionale per la Fisica della Materia - Dipartimento di Fisica dell'Università di Roma Tor Vergata, Italy
$^2$Laboratoire des Solides Irradiés, CNRS-CEA, Ecole Polytechnique, Palaiseau, France

We define an effective exchange-correlation kernel $f_{xc}^{{\rm eff}}$ which allows to obtain correct absorption and energy loss spectra starting from an electronic structure obtained within some given approximation. We consider, in particular, the Kohn Sham electronic structure calculated in the Local-Density Approximation, and the one obtained from a quasiparticle calculation. We show that in both cases, the main feature able to account for the experimental spectra is a sizeable, complex, frequency and material dependent long-range contribution to $f_{xc}^{{\rm eff}}$. We write, in terms of this contribution, an expression for the macroscopic dielectric function which is a generalization of the well-known contact-exciton approximation. For silicon and diamond, accurate absorption and electron energy loss spectra are obtained.
We also outline a method to calculate the xc kernel without solving the Bethe-Salpeter equation. Application to bulk Si yields an excellent optical spectrum at a reduced computational cost. The method looks promising for treating complex systems.

Long-range contribution to the xc-kernel of TDDFT

Valerio Olevano$^1$, Silvana Botti$^1$, Francesco Sottile$^1$, Nathalie Vast$^1$,
Lucia Reining$^1$, Angel Rubio$^2$ and Giovanni Onida$^3$

$^1$Laboratoire des Solides Irradiés, CNRS-CEA, Ecole Polytechnique, Palaiseau, France
$^2$Departamento de Física de Materiales, Facultad de Ciencias Químicas and DIPC, Universidad del Pais Vasco, San Sebastián, Spain
$^3$Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica dell'Università di Milano, Italy

Starting from the many-body Bethe-Salpeter equation we derive [L. Reining et al., Phys. Rev. Lett. 88, 066404 (2002)] an exchange-correlation kernel $f_{xc}$ that reproduces excitonic effects in bulk materials within time-dependent density functional theory. The resulting $f_{xc}$ accounts for both self-energy corrections and the electron-hole interaction. It is static, non-local and has a long-range Coulomb tail. Taking the example of bulk silicon and other semiconductors as well as insulators, we show that the $- \alpha / q^2$ divergency is crucial and can, in the case of continuum excitons, even be sufficient for reproducing the excitonic effects and yielding excellent agreement between the calculated and the experimental absorption spectrum. We also show that, within the range of validity of the method, the parameter $\alpha$ depends linearly on the inverse of the dielectric constant.

Many-body Diagrammatic Expansion for the Exchange-Correlation Kernel in Time Dependent Density Functional Theory

O. Pankratov, I. Tokatly, and R. Stubner

Lst. f. Theor. Festkörperphysik, Universität Erlangen-Nürnberg, Germany

Time Dependent Density Functional Theory (TDDFT) is a promising method for the calculation of excitation energies of many-electron systems. However, the analytical properties of the dynamic exchange-correlation (xc) kernel $f_{xc}$, which plays a key role in TDDFT (similar to XC-potential in DFT), are largely unknown. We developed the diagrammatic rules for a perturbative expansion of $f_{xc}$ using the Kohn-Sham-based many-body diagrammatic technique. In this technique the Kohn-Sham (KS) Green's functions are the basic propagators and the diagrammatic representation of the xc-potential arises from the requirement that the KS density is exact. We find that at KS frequencies $f_{xc}(\omega_{ij})$ is infinitely-ranged (in extended systems), which is closely related to the discontinuity of the xc-potential. We also show that $f_{xc}(\omega_{ij})$ has no singularities at KS transition energies $\omega_{ij}$ in every order of the perturbation theory. However, it may diverge with the system size if the states $\vert i \rangle$ and $\vert j \rangle$ are delocalized. This signifies that any particular perturbative approximation for $f_{xc}$ requires a consistent perturbative treatment of the response function (that is, it should be treated in the same order, but not in any "better" approximation!) to avoid uncontrollable errors in the many-body corrections to the excitations energies.

Current through finite 1D wires: Non-equilibrium Green's functions approach

B. Tobiyaszewska, Ulf von Barth, and C.-O. Almbladh

Solid State Theory Group, Physics Department, Lund University, Sweden

We study the charge transport phenomena in 1D atomic wire in nonlinear regime. To model this system we use finite chain connected to macroscopic leads (which play the role of reservoirs). All parts (chain and leads) are described by tight-binding Hamiltonian. In the chain we include additionally the on-site and inter-site interactions which establish proper
shape of potential along the chain. The formula for current is obtained from Keldysh non-equilibrium Green's functions (NEG) and the nonlinear I-V curves have been calculated. The influence of system parameters (i.e. coupling between keads and chain, chain length and interactions) on transport has been analyzed.

Self-consistent current-carrying steady states from a maximum entropy principle

Peter Bokes$^{1,2}$ and Rex W. Godby$^1$

$^1$Department of Physics, University of York, United Kingdom
$^2$Department of Physics, Slovak Technical University (FEI STU), Bratislava, Slovakia

We suggest that a reasonable approach to describe current-carrying steady states at the ab-initio level in nanostructures could be built on general grounds of the maximum entropy principle for the statistical density matrix [1]. The presence of an imposed electrical current is reflected as a non-equilibrium constraint on the maximization of the information entropy. The requirement of time-independence of the ensemble introduces another Lagrange multiplier, the $\lambda$ operator. While the $\lambda$ operator complicates the calculation significantly for an interacting system, we develop a generalised density-current functional scheme and discuss the simplest nontrivial functional that can be used. We relate our approach to other maximum entropy calculations [2], and compare the method to the widely employed occupation scheme for current-carrying calculations [3].

[1] E.T.Jaynes in The Maximum Entropy Formalism ed. by R.D.Levine, M.Tribus, pp 15-118, Cambridge, MIT Press 1978.
[2] e.g. O.Heinonen and M.D.Johnson, Phys.Rev.Lett. 71, 1447 (1993).
[3] McCann and Brown, Surf.Sci. 194, 44 (1988).

Effects of core-level degeneracies and crystal fields on core-level spectra

C.-O. Almbladh and M. Birgersson

Solid State Theory Group, Department of Physics, Lund University, Sweden

Effects of core-level degeneracies on x-ray photoemission spectra (XPS) are considered theoretically and computationally. Processes within each degenerate sub-level manifold are treated by a leading-order cumulant approximation, and processes which connect spin-orbit or crystal-field split levels to leading order in the core-electron self energy. The cumulant approximation is consistent with earlier asymptotic results and correctly describe the leading moments of the core-electron spectrum. The central quantity in both cases is an exponent function which may be expressed in terms of projected state densities and core-valence Coulomb matrix elements. In the past, only the core-valence exchange interaction has been taken into account, but in the general case also the direct core-valence interaction couples the core sublevels. Numerical results are presented for the Al $L_{2,3}$ and the Rh and Pd $M_{4,5}$ levels and compared with recent experiments. In the case of Rh $M_{4,5}$, the level-dependent effects are strong leading to an increased asymmetry, a much increased lifetime width of the $M_4$ level, and deviation from Lorentzian lifetime broadening. In Pd, the effects are much weaker owing to a much smaller $d$ density of states at the core-hole atom, and it is almost negligible in Al, the most noticeable effect being an increased width of the lower $L_2$ component. The effects of crystal fields at low symmetry core-hole sites are also considered. We show that it is essential to treat the exchange part using the full core-valence exchange operator in order to describe the crystal fields in a satisfactory manner.

Temperature effects on surface core level spectra

Stefano de Gironcoli

INFM-DEMOCRITOS and Scuola Internazionale Superiore di Studi Avanzati, Trieste, Italy

I report on some recent ab-initio studies on the effects of temperature on the peak position and width of core level spectra for metallic surfaces. Calculations for the vibrational broadening of core level peaks in Be (0001) and Rh (001) will be presented. Calculation reproduces very well the multi-phonon replicas recently observed in low-temperature high-resolution photoemission spectra emerging from bulk and inner surface layers in Be (0001). The absence of marked multi-phonon replicas in the photoemission spectrum from the topmost surface layer is traced back to a stronger lattice relaxation around surface core-hole defect and to its coupling to surface phonons.
Moreover by comparing recently measured temperature-dependent surface core level shifts and surface state positions to values calculated with Density Functional Theory for different surface geometries it has been possible to determine the multilayer thermal expansion of the first three interlayer spacings of the (0001) surface of beryllium. These results reveal that, in the temperature range from 300 to 700 K the first-to-second, second-to-third and third-to-fourth interlayer distances expand by 88$\pm$15, -10$\pm$15 and -6$\pm$20 $\times 10^{-6}$ K$^{-1}$, respectively. This work confirms a previous Low Energy Electron Diffraction study which reported a strong thermal surface expansion in Be(0001) but it is in disagreement with the most advanced theoretical calculation available at present.
In Rh (001) the comparison of calculated position and vibrational broadening of layer-specific core level peaks with experimental temperature-dependent photoemission spectra allows to show the composite nature of the supposedly bulk peak. These findings have been confirmed by recent experimental work.

Quasiparticle band structure of the As vacancy on GaAs(110)

Magnus Hedström, Arno Schindlmayr, and Matthias Scheffler

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

We report ab initio calculations of the structural and electronic properties of the As vacancy on GaAs(110). Emphasis is put on accurate convergence testing of atomic relaxations and the energy of the defect levels and their dispersion with respect to surface cell size and $k$-point sampling. Under p-type conditions the stable charge state of the vacancy is predicted to be +1 in agreement with other calculations and experiment.
The vacancy introduces two defect levels in the band gap. Since an LDA description for states in the band gap is insufficient we address this problem by performing $GW$ calculations. We focus on the lower of these states, which may be unoccupied, singly or doubly occupied depending on the position of the Fermi level. This defect state consists of Ga-Ga bonds which are not present in neither the bulk nor the clean surface. As a consequence we found the $GW$ corrections to be non-trivial and different from the corrections to the bulk band gap.
The energy of the defect level is extracted at the special $k$-point and the accuracy of this procedure is evaluated by a tight-binding fit to the ab initio calculated dispersion relations.

Ab initio calculation of the anisotropic dielectric tensor of GaAs/AlAs superlattices

Silvana Botti$^1$, Nathalie Vast$^1$,Lucia Reining$^1$, Valerio Olevano$^1$, and Lucio Claudio Andreani$^{2}$

$^1$Laboratoire des Solides Irradiés, CNRS-CEA, Ecole Polytechnique, Palaiseau, France
$^2$Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica "A. Volta", Università di Pavia, Italy

The size reduction achieved in one, two or three dimensions in heterostructures at the nanoscale level leads to electronic ground and excited states widely different from those of the bulk crystals, and has opened the way to a new generation of optoelectronic and photonic devices. GaAs/AlAs superlattices (SL's) are extensively studied examples. While the GaAs and AlAs bulk semiconductors present an isotropic optical response, in the GaAs/AlAs SL the lowering in the crystal symmetry gives rise to an optical anisotropy.
Here, we will present a study of the dielectric tensor of (001) (GaAs)$_p$/(AlAs)$_p$ SL's as a function of their period $p$ for $1 \le p \le 12$, starting from Density Functional Theory. We have performed both ab initio [1] and semi-empirical calculations [2], and we will compare their results. The simple picture of the macroscopic dielectric function being a sum of independent transitions between one-electron states ignores contributions which may be especially important when the scale of the system is reduced and the inhomogeneity of the medium more pronounced. In particular, the crystal local field effects are expected to be relevant, since they reflect the charge inhomogeneity of the responding material. This point turns in fact to be crucial for the explanation of the SL birefringence, as we will illustrate by discussing the following results: - Calculations neglecting local field effects account neither for the experimentally observed value of the static birefringence [4], nor for its decrease with decreasing SL period $p$, even qualitatively. - The behavior of the dielectric tensor is determined by the interplay between quantum confinement and the local fields effects. The use of the effective medium approach [3] is shown to be justified in the growth direction even for small periods, whereas the direct effect of quantum confinement is found to be larger in the in-plane direction. - The static birefringence is drastically enhanced by the anisotropy of the local fields. Including the latter, the qualitative behavior of the experiment [4] can be reproduced. The quantitative agreement is improved by the inclusion of further many-body effects, using the Slater-Koster contact exciton model [5].

[1] S. Botti, N. Vast, L. Reining, V. Olevano, L.C. Andreani, submitted.
[2] S. Botti and L.C. Andreani, Phys. Rev. B 63, 235313 (2001).
[3] D. Bergman, Physics Report 43, p. 291 (1978); M.G. Cottam and D.R. Tilley, in Introduction to surface and superlattice excitations, (Cambridge University Press, Cambridge, 1989), p. 267; E. Jahne, Phys. Stat. Sol. (b) 194, 279 (1996).
[4] A.A. Sirenko, P. Etchegoin, A. Fainstein, K. Eberl and M Cardona, Phys. Rev. B 60, 8253 (1999).
[5] J.E. Rowe and D.E. Aspnes, Phys. Rev. Lett. 25, 162 (1970).

Ab initio calculations of the dielectric response of graphite and small-diameter carbon nanotubes

A.G. Marinopoulos$^1$, Lucia Reining$^1$, Valerio Olevano$^1$, and Angel Rubio$^2$

$^1$Laboratoire des Solides Irradiés, CNRS-CEA, Ecole Polytechnique, Palaiseau, France
$^2$Departamento de Física de Materiales, Facultad de Ciencias Químicas and DIPC, Universidad del Pais Vasco, San Sebastián, Spain

The dielectric response functions and optical absorption spectra of graphite and of small-diameter carbon nanotubes (4 Å) were determined within the framework of time-dependent density-functional theory in both the RPA and adiabatic LDA approximations.
For graphite, the dispersion of the two valence plasmons and, in general, the lineshape of the electron-hole excitation continuum in the loss spectrum was determined for a wide range of momentum-transfer orientations with respect to the basal planes. Our findings show important anisotropic behavior of the dielectric response and strong effects of the interlayer Coulomb interaction on the loss spectra, notably on the frequency of the higher-frequency $\pi + \sigma$ plasmon. Incorporation of local fields in the response has an important influence on the spectra for momentum orientations approaching the crystallographic c-axis direction [1]. Similarly, the calculated spectra of the small tubes display strong anisotropy and dependence on the inter-tube interaction.

[1] A.G. Marinopoulos, L. Reining, V. Olevano, A. Rubio, T. Pichler, X. Liu, M. Knupfer and J. Fink, Phys. Rev. Lett., to appear (2002).

Optical properties of disordered alloys

A. Mookerjee, K.K. Saha, and T. Saha-Dasgupta

S.N. Bose National Centre for Basic Sciences, Kolkata, India

We shall present here a formulation for the study of optical conductivity in disordered alloys. The efect of disorder will be introduced via a multiple scattering formulation within the Augmented Space formalism introduced by us earlier. We shall propose a calculation based on the TB-LMTO as a starting point of a more accurate GW formulation for the alloy system.

POSTERS

A first-principles study of electronic properties of ZrO$_2$: preliminary results.

Philippe Baranek, Nathalie Vast and Lucia Reining,

Laboratoire des Solides Irradiés, CNRS-CEA, Ecole Polytechnique, Palaiseau, France

Zirconia (ZrO$_2$) has a wide range of materials applications because of its high strength and stability at high temperature, and its high resistance to the irradiations. A prospective application of particular current interest is its possible use as host matrix of targets in the transmutation process. Its electronic structure is therefore the subject of intensive experimental and theoretical studies.
We present preliminary results on an ab initio study of the electronic properties and of the electron energy loss spectrum (EELS) of ZrO$_2$. The calculations have been carried out within the density-functional theory framework in the local density approximation using plane-wave pseudopotentials techniques. The fully relaxed structural parameters of the three low-pressure (cubic, tetragonal and monoclinic) phases of zirconia and there equation of states are in good agreement with experiments and with previous theoretical works. The relative stability of the different phases is found to be correctly reproduced. The EELS spectrum of the cubic phase, determined within the random phase approximation (RPA) including the local-fields, is discussed and compared to the EELS spectrum of TiO$_2$.

Static Dielectric Tensor of a Periodic Arrays of GaAs Quantum Wires Embedded in an AlAs Matrix

F. Bruneval, S. Botti, N. Vast, L. Reining

Laboratoire des Solides Irradiés, CNRS, CEA, Ecole Polytechnique, Palaiseau, France

Former calculations on (001)(GaAs)$_p$/(AlAs)$_p$ superlattices performed in our laboratory [1] stated that the classical effective medium limit for the static dielectric constant is surprisingly close to the quantum, ab initio value for light polarized in the growth direction, even for the thinest superlattices. We wonder whether the same property holds for a different geometry, namely periodic arrays of quantum wires. The static dielectric constant has therefore been calculated both from a semi-empirical band structure, and within an ab initio scheme, the Density Functional Perturbation Theory. Our work deals with (101) oriented quantum wires with a filling ratio $f=\frac{1}{4}$ and a range of diameters up to 9 Å. We conclude that the behavior of the dielectric constant as a function of the system size is very similar for quantum wires and superlattices.

[1] S. Botti, N. Vast, L. Reining, V. Olevano et L. C. Andreani, soon published in Phys. Rev. Lett. (2002).

Total Energies of Jellium Spheres Using Many-Body Perturbation Theory

K. T. Delaney$^{1}$, P. Rinke$^{1}$, Tim Gould$^{1}$, P. García-González$^{2}$, and R. W. Godby$^{1}$

$^1$Department of Physics, University of York, United Kingdom
$^2$Departamento de Física de la Materia Condensada Universidad Autónoma de Madrid, Spain

Many-Body Perturbation Theory (MBPT) is a method which allows the properties of a many electron system to be studied explicitly. MBPT calculations within Hedin's GW approximation have been performed for many years, permitting excited state properties of materials to be studied very accurately. Using the recently developed Space-Time method [1], such calculations have become computationally feasible for a wide range of systems including extended, surfaces and clusters.
Recently, interest has developed in the field of ab-initio total energy calculations from MBPT. In the GW scheme, it is possible to extract the total energy directly from the Green's function of the electron system. Tests on the 3D homogeneous electron gas and jellium slabs have proven to be very successful [2]. We present total energies obtained for jellium spheres, another class of model inhomogeneous electron systems with a wide range of properties, and discuss the feasibility of the GW scheme for developing accurate total energy methods.

[1] H.N. Rojas, R.W. Godby and R.J. Needs, Phys. Rev. Lett 74 1827 (1995) [2] P. García-González and R. W. Godby, Phys. Rev. B 63 075112 (2001)

The GW space-time method in adaptive curvilinear coordinates

P. Eggert, A. Schindlmayr, and M. Scheffler

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

The calculation of quasiparticle energies within the $GW$ approximation is a challenging task for complex materials. By choosing the representation of the equations most suitable for each computational step, the $GW$ space-time method shows an efficiency gain in comparison with traditional implementations in reciprocal space. At present the principle obstacle that prevents applications to bigger and more complex systems is the rapid increase of the plane-wave basis set with the system size. Furthermore, in inhomogenous systems the atom that needs the highest cutoff energy determines the cutoff energy of the hole supercell. To overcome these problems we use adaptive curvilinear coordinates, which allow a local variation of the effective cutoff energy. In this approach the drawbacks of conventinoal plane waves can be overcome wihout giving up the advantages (orthogonality, systematic convergence and the possibility to use FFT algorithms). Results for quasiparticle band structures of H$_2$ on Si(001)(2x1) will be presented together with a discussion of the efficiency of the new basis set

Many-body properties of semi-infinite jellium surfaces

G. Fratesi $^{1}$, G.P. Brivio $^{1}$, G. Onida $^{2}$, and P. Rinke$^{3}$

$^{1}$Istituto Nazionale di Fisica della Materia and Dipartimento di Scienza dei Materiali, Università degli Studi di Milano, Italy
$^{2}$Dipartimento di Fisica, Università degli Studi di Milano, Italy
$^{3}$Physics Department, University of York, United Kingdom

Investigations of general and relatively simple systems may help one to propose efficient self-energy models, which nevertheless retain the main features of the exact operator. We have investigated the jellium surface, a highly inhomogeneous system for which Density Functional Theory (DFT) in the usual Local Density Approximation (LDA) fails. For this geometry the LDA effective potential and the LDA interaction energy with a second jellium surface show a long-distance behaviour, which is qualitatively incorrect. The real behaviour, as determined by the many-body nature of the electron system, is well described by Hedin's $GW$ approximation, as shown in [1] and [2]. In those as well as in many other $GW$ calculations a finite slab geometry is used for computational convenience, which results in a discrete electronic spectrum. In contrast, we describe a truly semi-infinite jellium substrate within a Green's-function formalism, thus keeping the spectrum continuous. We employ the embedding method and analytically continue from imaginary to real frequencies.

[1] A. G. Eguiluz, W. Hanke, Phys. Rev. B 39, 1043 (1989).

[2] P. García-Gonzalez, R. W. Godby, Phys. Rev. Lett. 88, 056406 (2002).

Ab initio calculation of optical spectra of condensed argon

S. Galamic-Mulaomerovic and C.H. Patterson

Department of Physics, University of Dublin, Ireland

The dielectric function and loss function of condensed argon are calculated by solving the Bethe-Salpeter equation using a method similar to that reported by Rohlfing and Louis$^1$ and by conventional RPA techniques. Calculations are done in a Gaussian orbital basis. Tests of Gaussian orbital bases using silicon show excellent agreement between dielectric function matrix elements calculated using a large plane wave bases and smaller Gaussian bases.

[1] M. Rohlfing and S.G. Louie, Phys. Rev. B 62, 4927 (2000).

Total energies from ab initio GW methods for inhomogeneous, periodic systems

Tim Gould $^{1}$ P. García-González $^{2}$, K. T. Delaney $^{1}$, and R. W. Godby $^{1}$

$^{1}$Department of Physics, University of York, United Kingdom
$^{2}$Departamento de Física de la Materia Condensada, Universidad Autnoma de Madrid, Spain

We present work covering total energies of inhomogenous, periodic systems. These ab initio calculations have been performed in the $GW$ approximation using the Galitzki-Migdal formula for total energy. Using this method it is possible to obtain both the non self-consistent $G_0 W_0$ energies as well as enabling us to generate self-consistent $GW$ total energies. Calculations have been performed using the Space-Time method of [1-2] to generate the self-energy $\Sigma$ for crystalline Silicon. We then use a method in the LDA space which allows calculation of the total energy without the usual problems associated with the large $\omega$ behaviour of the Greens functions $G$ and $G_0$. Results are compared to those of the LDA and we discuss the associated convergence problems.

[1] "Space-time method for ab initio calculations of self-energies and dielectric response functions of solids", H.N. Rojas, R.W. Godby and R.J. Needs, Phys. Rev. Lett. 74 1827 (1995)
[2] "The GW space-time method for the self-energy of large systems", Martin M. Rieger, L. Steinbeck, I.D. White, H.N. Rojas and R.W. Godby, Computer Physics Communications 117 211-228 (1999)

Orientation dependent EELS spectra of carbon based materials

Kevin Jorissen$^{1}$, John Titantah$^{2}$, and Dirk Lamoen$^{2}$

$^{1}$EMAT, University of Antwerpen (RUCA), Belgium
$^{2}$Department of Physics, University of Antwerp (RUCA), Belgium

We will present ab initio calculations on the fine structure of ionization edges of carbon based materials (e.g. graphite and nanotubes). The ab initio calculations are performed with the FP-LAPW method as implemented in the WIEN2k package [1], which is based on density functional theory and uses periodic boundary conditions. We look at the orientation dependence of the energy-loss near edge structure (ELNES)[2] of these materials and compare the results with experimental angular-resolved EELS [3]. We consider the separate contributions coming from s to $\pi^\star$ and s to $\sigma^\star$ excitations to the ELNES. We also study the effect of the core hole excitation on the spectrum of graphite by using a supercell model. The introduction of the core hole effect appears to be necessary to recover the experimental spectrum.

[1] P.Blaha et al., WIEN2k, (Kh Schwarz, Techn. Universitaet Wien, Austria), 2001.ISBN 3-9501031-1-2
[2] M.Nelhiebel et al., Phys.Rev.B 59, 12807 (1999)
[3] A.J. Papworth et al., Phys. Rev. B 62, 12628 (2000), K. Suenaga et al., Phys. Rev. B 63, 165408-1 (2001)

Ab-initio Gutzwiller method for correlated electrons

J.-P. Julien$^{1}$ and J. Bouchet$^{2}$

$^{1}$CNRS-LEPES, Grenoble, France
$^{2}$Los Alamos National Laboratory, Los Alamos, New Mexico, USA

Except for small molecules, it is impossible to solve many electrons problems without imposing severe approximations. However, for materials with the kinetic energy greater than Coulomb interaction, computations based on density functional theory (DFT) associated with the local density approximation (LDA) give satisfying qualitative and quantitative results to describe ground state properties. These solids have weakly correlated electrons presenting extended states, like sp materials or covalent solids. The application of this approximation to systems where the wave functions are more localized (d or f-states) as, i.e., transition metals and their oxides or rare earths, is more questionable and can even lead to unphysical results. Other theoretical approaches (i.e. slave bosons, Green functions decoupling, and more recently dynamical mean field theory in infinite dimension) treat in a better way correlation effects than the DFT-LDA, but the price to be paid is an oversimplification of the system, generally reducing the number of involved orbitals and using parametrized Hamiltonians (like Hubbard model) where the ab-initio aspect of the DFT-LDA is lost.
Among numerous theoretical approaches, the Gutzwiller method provides a transparent physical interpretation: applied to the one-band Hubbard model, the trial Gutzwiller wave function is built starting from the Slater (Hartree) determinant by reducing the weight of configurations ith double occupied lattice sites. The method corrects the Hartree approach for which up and down spin electrons are independent.
Using a density matrix approach to Gutzwiller method, first proposed by Nozières to one-band Hubbard model, we generalize it to (multiband) degenerate Hubbard Hamiltonian, within a Tight-Binding basis. In model academic cases (for e.g. double degenerate band, like Eg. symmetry half filled bands) we retrieve quite directly results (quasi-particles effective mass, metal-insulator transition, respect of Hund rules) that have been obtained by other groups using slave bosons or traditional way of handling Gutzwiller wave functions. Moreover, we have established a method that allows to obtain an effective Hamiltonian (renormalization of hoppings as well as on-site levels). We have applied this approach to d -Plutonium, where the f electrons present a correlated character. The TB-LMTO (Tight-Binding Linear Muffin-Tin Orbitals) provided an ab-initio Tight-Binding Hamiltonian. As this method uses the LDA, we have subtracted the mean interaction between f electrons, already taken into account within the LDA, and we have then re-added an Hubbard-like interaction term and treated the full obtained Hamiltonian within the above-described multiband Gutzwiller approach. That way, one can conciliate on the same footing the first principle ab-initio aspects with a realistic description of the whole set of bands, without adjustable parameters, and however describe in a better way the strong correlation aspect (or, at least, the coherent part of the spectrum).
For d-Plutonium, this approach gave an electronic specific heat contributiong very close to the experimental value, whereas it is usually underestimated by a factor of 10 by standard LDA calculations. Other aspects, like the underestimation of 35% of the equilibrium volume by LDA calculations (one can expect that the reduction of the kinetic energy, due to correlations, gives the correct trend to increase the computed volume), or the more probable atomic configuration, are still under investigation.

First principles calculations of H sites in titania

Marina V. Koudriachova$^{1}$, Nicholas M. Harrison$^{2}$, and Simon W. de Leeuw$^{1}$

$^{1}$Computational Physics, TU Delft, The Netherlands
$^{2}$CLRC, Daresbury Laboratory, United Kingdom

H impurities are found in most as-grown oxidic crystals. Their presence can be revealed by the IR absorption band of the OH stretching mode. At elevated temperature the protons become mobile and contribute to the ionic diffusion and electrical conductivity and also can affect the refractive index. Because of the influence of hydrogen on device applications, it has been a subject of many recent investigations in oxidic materials. Apart from being a nature defect, H can also be introduced artificailly beings a useful "probe" for the study of a variety of optical, electronic and thermodynamic properties of the host system. Numerous investigation of H-sites in TiO$_2$ rutile has been carried out with different experimental and theoretical techniques. They are summorised in two different structural models, which contradicts each other and are supported by different observations. Both models are not entiely satisfactory, since the suggested geometry does not correspond to the observed frequency downshift of the OH stretching from its nominal value. Here we present results of first principle calculations of H-sites in pure and doped rutile. Despite the small size H-ions strongly modify the local structure due to the charge transfer to the neighbouring oxygen ions. The predicted geometry and calculated frequency of the OH strenching vibration are in good agreement with the observed value. The influence of dopants and their concentrations on the IR absorption is disscussed. Calculated results allows to resolve the contradiction in the reported experimental data.

First-Principles Calculations of the Optoelectronic Properties of Silicon Nanodots

Marcello Luppi$^{1}$, Elena Degoli$^{2}$, Rita Magri$^{1}$, and Stefano Ossicini$^{2}$

$^{1}$INFM-S3-Dipartimento di Fisica, Università di Modena e Reggio Emilia, Italy
$^{2}$INFM-S3-DISMI, Università di Modena e Reggio Emilia, Italy

The optoelectronic properties of Si nanodots have been investigated using ab initio total energy calculations, within the Density Functional Theory. Structural relaxations have been considered. We have studied two types of nanodots: isolated clusters covered by H, studying the substitution of Si-H bonds with different Si-O bonds; and nanocrystals embedded in SiO2 matrix. In the first case we find that the optoelectronic properties strongly depend on the type and the number of Si-O bonds, especially for the gap value and the arrangement of the energy levels. In the second the close interplay between chemical and structural effects is pointed out. Comparison with experiments related to porous silicon and Si nanocrystals in SiO2 has been performed.

Tunnelling through a Metal-Vacuum-Metal Junction: Self-Consistent Maximum Entropy Approach

Hector Mera$^{1}$, Peter Bokes$^{1,2}$ and Rex Godby$^{1}$

$^{1}$Department of Physics. University of York, United Kingdom
$^{2}$Department of Physics, Slovak Technical University (FEI STU), Bratislava, Slovakia

We present the first application of the maximum entropy approach to non-equilibrium current-carrying steady state [1], for a jellium model of a Metal-Vacuum-Metal (MVM) junction. Self-consistent densities and potentials are discussed in comparison with the benchmark calculation of McCann and Brown [2] which is the reference point for most day ab-initio quantum transport calculations. We show that both methods give similar result for small applied potential while subtantial differences are seen at larger bias, and in the resonant tunnelling regime.

[1] Peter Bokes and Rex Godby. To be presented at this conference.
[2] McCann and Brown, Surf.Sci. 194, 44 (1988)

Theoretical phonon studies of semiconductor structures

P. Palacios, C. Tablero, and P. Wahnón

ETSI Telecomunicacion, UPM, Madrid, Spain

The main goal of the work that we present here is to study the phonon dispersion diagram of several semiconductors, specially of the type III-V (GaAs,GaP,...) and its alloys specially with Titanium atom, TiGaX where X=As or P.
In a previous work [1], our group have demostrated that this kind of alloys presents an isolated half filled intermediate band. Our final purpose of this work is the calculation of absorptions coefficients for this kind of systems.
In this work we perform accurate calculations of phonon dispersion using the first principles computed codes SIESTA [2] and ABINIT [3], within DFT [4,5] approximation and with LCAO and PW basis set respectively. We use the local density (LDA) and generalized gradient (GGA) approximation for the exchange-correlation potencial and norm conserving pseudopotencials for core electrons of the all atoms. We use different basis set to study its effects and comparing them with the experimental results.

[1] P. Wahnón and C. Tablero, Phys. Rev. B, 65, 165115.
[2] P. Ordejón, E. Artacho and J. M. Soler, Phys. Rev. B 53 (1996) 10441; D. Sanchez Portal, P. Ordejón, E. Artacho and J. M. Soler, Int. J. of Quant. Chem. 65 (1997) 453.
[3] The ABINIT code is a common project of the Université Catholique de Louvain, Corning Incorporated, the Université de Liège and other contributors (URL http://www.abinit.org).
[4] R. G. Parr and W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, Oxford, 1993
[5] Electronic Density Functional Theory. Recent Progress and New Directions, Plenum Press, 1998

Strongly bound exciton at the C(100) surface

M. Palummo$^1$, O. Pulci$^1$ , A. Marini$^1$, R. Del Sole$^1$, V. Olevano$^{2}$, and L. Reining$^{2}$

$^1$Laboratoire des Solides Irradiés, CNRS-CEA, Ecole Polytechnique, Palaiseau, France
$^2$Istituto Nazionale per la Fisica della Materia - Dipartimento di Fisica dell'Università di Roma Tor Vergata, Italy

We investigated in an {\it ab-initio} framework the optical and the electron energy loss spectrum of the ideal C(100) 2x1 surface. The inclusion of many-body effects, like self-energy local-fields and electron-hole interaction, deeply influence the dielectric response. With the complete description of all these effects a bound exciton of about 1 eV, has been found. Our theoretical results are well confirmed by a comparison with an experimental electron energy loss spectrum available for this surface. A comparison with the Si(100) surface optical properties, is also given.

Optical absorption spectra of a substituted porphyrin and its zinc complex

Oana Cramariuc, Terttu I. Hukka and Tapio T. Rantala

Tampere University of Technology, Finland

Molecular optoelectronic devices in mind we are interested in photon absorption induced electron transfer processes in large organic molecular complexes. As a first step towards understanding electron donating-accepting complexes and using more sophisticated methods we have carried out straightforward DFT-GGA calculations for an asymmetric meso-substituted porphyrin and its zinc complex to approximate their optical absorption spectra.
We present detailed molecular structures and their relation to the electronic structures. The ground state electronic stuctures are used to approximate the excited states in the one-electron picture and related absorption spectra have been evaluated. Results are compared with those from other first principles calculations like TDDFT and experiments, where available. The simple method is shown to work surprisingly well for transition energies but to fail for intensities.

Optical properties of perovskite alkaline earth titanates: a formulation

Kamal Krishna Saha, Tanusri Saha-Dasgupta, and Abhijit Mookerjee

S. N. Bose National Centre for Basic Sciences, Kolkata, India

We suggest a formulation of the optical conductivity as a convolution of the energy-resolved joint density of states and an energy-frequency dependent transition rate. The need is to go beyond the usual reciprocal space based formulations and obtain an expression which we can immediately generalize for disordered systems. This would require labelling states by energy and the angular momentum labels ($l,m$) alone. Once we derive this expression we shall find a representation for the optical conductivity in the minimal basis set of the tight-binding linearized muffin-tin orbitals (TB-LMTO). The generalization to disordered systems will be carried out through the augmented space recursion (ASR) introduced by us earlier for the study of electronic properties of disordered systems [1-3]. The ASR carries out the configuration averaging essential to the description of properties of disordered systems, going beyond the usual mean-field approaches and taking into account configuration fluctuations. The input into the ASR method includes the Hamiltonian parameters of the pure constituents, as the starting point of the local-spin density approximation (LSDA) iterations for the alloy. It also includes the information about the transition rates of the pure constituents, expressed as functions of the initial- and final-state energies. Our aim is to reformulate the reciprocal-space representation of the transition rate and re-express it in the energy-frequency label representation for the pure constituents. Only when we are confident that this works, can we proceed with the full calculations for the disordered alloy. In order to gain confidence in our formulation, we apply the formulation to three alkaline earth titanates $CaTiO_3$, $SrTiO_3$ and $BaTiO_3$ and compare our results with available experimental data on optical properties of these systems.

[1] Mookerjee A., 1973 JPC 6 1340
[2] Saha T., Dasgupta I. and Mookerjee A., 1996 JPCM 8 1979
[3] Dasgupta I., Saha T. and Mookerjee A., 1997 JPCM 9 3529

Dynamical properties of the hcp metals Sc, Y, and Ru

Wolf-Dieter Schöne$^{1,2}$ and Walter Ekardt$^{1}$

$^{1}$Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
$^{2}$Freie Universität Berlin, Fakultät für Physik, Berlin

In this paper we perform a comparative study of the dynamical response of the three hcp metals Sc, Y, and Ru for small- and medium-sized wave vectors up to 50 eV. The calculations are based on ground states which are determined using ab initio pseudopotentials together with a plane-wave expansion of the wave functions. We consider many-body as well as crystal local-field effects which both turn out to be small. In all of the three elements we find a strong anisotropy of the response with respect to the direction of the wave-vector transfer. Furthermore, we obtain almost undamped plasmon excitations for small wave vectors in the direction normal to the hexagonal planes for the response of Sc and Y.

Metastable photo-induced structural changes in chalcogenide glasses

Sergei I. Simdyankin and Stephen R. Elliott

Department of Chemistry, University of Cambridge, United Kingdom

Chalcogenide glasses are semiconducting materials with bandgaps typically in the range 1-3 eV, depending on the constituents and composition. Perhaps the most interesting opto-electronic behaviour exibited by chalcogenide glasses is the metastability resulting from the absorbtion of (near bandgap) light. Although the experimental picture of photo-induced metastability in glassy chalcogenides is becoming clearer, theoretical understanding of the microscopic mechanisms involved in these phenomena has been very limited. The first tentative steps towards a quantitative theoretical understanding have involved ab initio calculations in which electron occupancy is increased or decreased by one electron or electron-hole excitations are introduced. In both cases, configuration changes were observed. We are currently in active search for efficient ab initio computational methods the use of which would allow us to further understand the effects of photo-induced excitation on the atomic and electronic structure.

Long-range Hartree contribution to the absorption and
electron-energy-loss spectra: finite versus infinite systems

F. Sottile$^{1}$, V. Olevano$^{1}$,  L. Reining $^{1}$, and A. Rubio$^{2}$

$^{1}$Laboratoire des Solides Irradiés, CNRS, CEA, Ecole Polytechnique, Palaiseau, France
$^2$Departamento de Física de Materiales, Facultad de Ciencias Químicas and DIPC, Universidad del Pais Vasco, San Sebastián, Spain

We analyse in detail the role of the long-range part of the Hartree potential in the dielectric response of finite and infinite systems in order to assest the differences between optical and electron energy loss spectra EELS (in the long wavelength limit q->0). We analyse all the ingredients contributing to the calculations of the miscroscopic dielectric function that controls both absorption and EELS spectra. In particular for finite systems we show numerically and analytically that the two spectra coincide. In the case of extended systems the long-range part of the electron-electron interaction is responsable for the differences between absorption and EELS. This subtlety puts in evidence the physical difference betweeen the two spectra through an effective screening in EELS formaly absent in absorption. We compare the most widely used computational approaches (real and reciprocal space) to calculate response functions. We illustrate the discussions for the specific case of a set of interacting and non-interacting Beryllium atoms.

Exchange-correlation kernels for inhomogeneous systems

K.Tatarczyk, A.Schindlmayr, and M. Scheffler

Fritz-Haber-Institut, Berlin-Dahlem, Germany

Although the time-dependent density-functional theory makes it possible to study excited states, the task remains challenging in practice. Usually the density-density response function of the Kohn-Sham system is calculated first and then renormalised through Dyson's equation. This involves the dynamic response of the exchange-correlation potential, the exchange-correlation kernel. Like the potential, the kernel is not known exactly and usually parametrised for the homogeneous electron gas. However, as the kernel is a nonlocal quantity, this leads to conceptual problems when inhomogeneous systems are studied, because the definition of the local density is then ambiguous. To address this issue we have studied the plasmon dispersion of bulk alkali metals. We have tested not only the performance of various parametrisations for the exchange-correlation kernel but also several proposals circulated in the literature for applying these parametrisations to inhomogeneous systems.

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