K. Schönhammer

Electronic structure of cerium and light rare-earth intermetallics

Abstract We give an overview of the use of the impurity Anderson Hamiltonian to describe the spectroscopic and low-energy thermodynamic properties of cerium intermetallics, with emphasis on interpreting 4f photoemission spectra. We show Ce valence-band resonant photoemission, Bremsstrahlung isochromat and 3d X-ray photoemission spectra for CeRu2, CeNi2, CeIr2 and CeAl, and give a complete theoretical analysis of the spectra. We summarize the relation between the large and small energy scale properties. For each system, all the spectra, as well as the static magnetic susceptibility and Kondo temperature, can be described by the model using essentially the same parameters. We also present details of resonant photoemission spectra for five other cerium compounds, CeSi2, CeOs2, CePd3, CeCo2 and CeNi5, and discuss generally the problem of obtaining the experimental 4f spectrum. Alternative theories of 4f photoemission are examined critically and we give applications of the Anderson Hamiltonian theory to CeAl2,...

On the theory of chemisorption

A solution of the Anderson model for chemisorption including correlation effects is presented. The solution becomes exact in three limiting cases (largeU, smallU i.e. Hartree-Fock and the so-called symmetric strong coupling limit) of the intra atomic Coulomb repulsionU and the hopping termV. Excellent agreement of our approximation with Schrieffers exact numerical results for the binding energy of a small model system is found. Our results for the binding energy and the charge of the adsorbate show a significant improvement over the Hartree-Fock approximation. The single particle spectral density of the H-F approximation turns out to be even qualitatively wrong in the largeU strong coupling limit.

Correlation effects on core level spectra of adsorbates

Abstract The spectrum of core levels in adsorbates is calculated for a model which allows the transfer of screening charge to the adsorbate. Spin degeneracy and the Coulomb interaction of the valence electrons is included. The core level spectrum is calculated within a time dependent Hartree-Fock scheme for broad resonances and from a perturbation expansion for the core hole self-energy in the case of a very narrow valence resonance.

THEORY OF EXCITON-EXCITON CORRELATION IN NONLINEAR OPTICAL RESPONSE

We present a systematic theory of Coulomb interaction effects in the nonlinear optical processes in semiconductors using a perturbation series in the exciting laser field. The third-order dynamical response consists of a phase-space filling correction, a mean-field exciton-exciton interaction, and two-exciton correlation effects expressed as a force-force correlation function. The theory provides a unified description of effects of bound and unbound biexcitons, including memory effects beyond the Markovian approximation. In the degenerate four-wave-mixing experiments, correlation effects are shown leading to polarization mixing, ringing, etc. The strong interaction, a nonperturbative theory of the correlation function, is numerically evaluated for a one-dimensional model. Approximations for the correlation function are presented.

Sticking and inelastic scattering at metal surfaces: The electron-hole pair mechanism

Abstract We present calculations of the contribution of the electron-hole pair creation to the inelastic scattering probability and the sticking coefficient of atoms or molecules on metal surfaces. For light chemically reactive adsorbates this mechanism can be very important. The total inelastic scattering probability for thermal He atoms on a copper surface due to e-h pairs on the other hand is only 10 −5 , i.e. for rare gas atoms the phonon mechanism is the dominating one.

Fermion–boson transmutation and comparison of statistical ensembles in one dimension

The theoretical description of interacting fermions in one spatial dimension is simplified by the fact that the low‐energy excitations can be described in terms of bosonic degrees of freedom. This fermion–boson transmutation (FBT) which lies at the heart of the Luttinger liquid concept is presented in a way which does not require a knowledge of quantum field theoretical methods. As the basic facts can already be introduced for noninteracting fermions they are mainly discussed. As an application we use the FBT to present exact results for the low‐temperature thermodynamics and the occupation numbers in the microcanonical and the canonical ensemble. They are compared with the standard grand canonical results.

Photoemission spectrum for hydrogen chemisorbed on Ni

Abstract Recent Xα-SW cluster calculations, which indicate the relative importance of the sp-bands compared to the d-bands for hydrogen chemisorption on Ni are used to parameterize a model Hamiltonian. The adsorbate Greens function is calculated and yields the experimental width of the extra peak seen in photoemission. It is shown that in the equilibrium geometry the qualitative changes in the spectrum due to many body effects are too small to be seen experimentally.

Exactly soluble limits of a model for core level spectra of adsorbates

Photoelectron spectra of core levels for adsorbed atoms or molecules are calculated for a model which allows the screening of the core hole both by coupling to the surface plasmons and by charge transfer to the adsorbate. Exact analytical expressions for the core spectra are given for the case where the charge transfer occurs from a filled substrate band.

D.C. conductivity and Anderson transition in disordered systems

Abstract The positions of the Anderson transition and the mobility edges for the Anderson model of cellular disorder are studied numerically working directly with the Kuboformula for the conductivity. Numerical results are given for 2- and 3-dimensional systems.

Improved Transport Equations Including Correlations for Electron–Phonon Systems: Comparison with Exact Solutions in One Dimension

Abstract We study transport equations for quantum many-particle systems in terms of correlations by applying the general formalism developed in an earlier paper to exactly soluble electron-phonon models. The one-dimensional models considered are the polaron model with a linear energy dispersion for the electrons and a finite number of electrons and the same model including a Fermi sea (Tomonaga-Luttinger model). The inclusion of two-particle correlations shows a significant and systematic improvement in comparison with the usual non-Markovian equations in Born approximation. For example, the improved equations take into account the renormalization of the propagation by the self-energies to second order in the coupling.