My research work has focussed on the calculation of electronic and structural properties of solids and atomic clusters, using numerical approaches at different level of sophistication. The systems studied include metallic glasses, surfaces of metals and semiconductors, layered systems, as well as small metal and semiconductor clusters.
The structure and dynamics of amorphous systems has been investigated by a calculation of the dynamical structure factor for the metallic glass Mg_70Zn_30 from its frequency moments[1].
Reconstruction and multilayer relaxation of clean and adsorbate-covered metal surfaces has been investigated using a parametrized tight-binding Hamiltonian[14,20,21]. Ab initio techniques have been used to relate surface modifications due to adsorbates to hydrogen-induced embrittlement in bulk metals[51,62] and to predict the low probability of "cold fusion"[37].
In an attempt to understand catalytic processes, potential energy surfaces have been calculated for the the adsorption of atoms and dissociation of molecules on transition metal surfaces, using parametrized and ab initio methods[12,16,19,27,44,46]. Similar methods were applied to understand the direct and indirect interactions between adsorbates[25,26] on transition metal surfaces and the field desorption process[18,22].
The driving forces of surface segregation in alloys have been determined in terms of differences in surface tension and atomic size of the alloy components, which can be modified by adsorbates or absorbed hydrogen[4,17]. Using a similar formalism for total energies, core-level binding-energy shifts, which had been observed at surfaces and in adsorbates, have been interpreted within the equivalent cores approximation[2,3,5-9,11,13].
The shortcoming of the Local Density Approximation to reproduce quasiparticle energies has been corrected in first-principles "GW" calculations of band offsets in semiconductor heterojunctions[30,35] and superlattices[36,39].
A first principles calculation of Scanning Tunneling Microscopy images of graphite surfaces has been performed in order to explain anomalous features in experimental data[28,29]. Similar calculations have been used to predict Atomic Force Microscopy images of graphite and their relation to local rigidity[38,41-43] and atomic-scale friction[40,48,60].
Structural and electronic properties of novel materials such as BC_3 [31,32] or C_60-"buckyball" derived systems [49,50,52-59,61] have been predicted using parametrized and first-principles techniques.
The equilibrium properties of semiconductor and metal clusters have been calculated in search for new phenomena (stable structures, structural transitions at large cluster sizes, segregation, electronic and thermal excitations) in these systems, which show a combination of atomic, surface and bulk properties[10,15,23,24,33,34,45,49,50,52-59].
I intend to continue my present research in the field of Computational Condensed Matter Physics also in the foreseeable future. As in the past, my research interests will be motivated by a desire to promote the fundamental understanding of electronic and structural properties of new materials, specifically low-dimensional systems (surfaces, interfaces, layered structures, clusters). In the following, I will discuss three research areas in which I am most actively involved.
1. Hydrogen Interaction with Metals (supported by the U.S. Office of Naval Research)
The objective of this project is to obtain microscopic understanding of the (presumably common) origin of hydrogen induced embrittlement in bulk metals and abrupt structural changes at surfaces (surface reconstruction). The static part of this calculation is based on ab initio Local Density Functional (LDA) calculations of the total energy and electronic structure of the system as a function of geometry. Most of our calculations for the H- loaded bulk Pd metal and the H-covered Pd(110) surface have been completed and published. The theory is presently being extended to technologically more relevant systems such as Fe with an open bcc structure. The dynamical aspect of these systems (H-induced phonon softening, melting transition, crack formation under tensile stress) will be calculated using our extension of Nose molecular dynamics (MD). Our recently published many-body alloy potentials, based on nearest-neighbor isotropic hopping integrals, are being extended to describe anisotropic hopping (e.g. in bcc metals) and used for the forces in the MD calculations. These potentials will be used in real-time MD calculations for realistically large systems, using the coming generation of massively parallel computers.
2. Atomic Clusters (supported by the U.S. National Science Foundation)
The objective of this research is to understand on the microscopic level the equilibrium structure and excitation spectra of small atomic clusters, especially Na_n and C_n. Using both ab initio LDA and parametrized tight-binding formalism for the electronic states and the Random Phase Approximation for the excitations, we are investigating the cluster size dependence of the Mie plasmon frequency and the damping mechanism of this excitation in Na_n clusters. My research on C_n clusters focuses on the formation path and stability of very large "fullerene" type structures (isolated clusters, cluster aggregates, negatively curved graphite "foam"), on the uncommon electronic excitation spectra of these structures, on the stability and on mechanisms leading to superconductivity of the corresponding solids. As mentioned above, I am actively involved in a project which will use massively parallel computing to address the dynamics of cluster condensation from a system of ca.10^6 isolated carbon atoms (per unit cell, in a system with periodic boundary conditions) as a function of temperature and cooling conditions.
3. Properties of Surfaces at the Atomic Level (partly supported by the Swiss National Science Foundation)
I am involved in exploring new uses of Scanning Probe Microscopy techniques to study surfaces, in collaboration with the experimental group of Professor H.-J. Guentherodt in Basel. My current interests include studies of the local surface rigidity and of atomic-scale friction without wear on graphite.
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