Institute Theory III
General Overview
The institute Theory III investigates the mechanisms of the formation of structures and their consequences in condensed matter. The research starts from electronic properties which define the shortest length and time scales, but it also encompasses the macroscopic consequences. The analytical and numerical investigations are in many ways closely connected with experimental studies performed in other groups of the IFF, but also with activities in other institutes of the Research Center Juelich.
Central points of interest for the research in Theory III are in the field of electronic structure of solids. Materials classes under consideration are metals and semiconductors, specifically with respect to their importance for information technology. A second mainstream is formed by cooperative phenomena in condensed matter. Questions here aim at the dynamics of structure and pattern formation and the statistical mechanics of order and disorder processes. Specific activities in the field of complex fluids are concerned with structure and dynamics of soft matter.
The research of Theory III employs all analytical and numerical techniques applicable to many-body problems in condensed matter. In addition the development of new methodological concepts and numerical procedures is part of our research interest. The development of parallel program codes adapted to massively parallel computers has received special attention in recent years.
The explanation of the microstructure and dynamics of real solids requires the understanding of the electronic properties. One of the most important methods for the calculation of the electronic structure of real solids is the density functional theory in connection with appropriate numerical procedures. While in recent years bulk properties of metals and semiconductors have been at the center of our research a main concern now is directed towards the understanding of surface and interface properties, with particular emphasis on magnetism.
Many properties of metals in practical use depend on the structure and properties of their surfaces. From firstprinciples calculations it was found that the rearrangement of the electronic charge for noble metals and FCCtransition-metals does not lead to a significant change of the remaining bonds, when a bond is broken. This novel finding can lead to the development of simple models to describe the energetics of a surface like step and kink formation, crystal growth, alloy formation, equilibrium shape of mesoscopic crystallites and surface facetting.
The simulation of scanning-tunneling-microscope(STM)-images by density-functional methods with pseudopotentials help to clarify the detailed structure of inhomogeneities on Si(111)-surfaces covered by As, which are relevant for epitaxial growth. Steps on Si(I I1):AS always have the height of a double layer. For both typical step orientations the exposed second layer Si-atoms at the step-edge are replaced by As-atoms, which means Aspassivated steps. The structures are much more prominently exhibited for negative bias-voltage of the STM as compared to positive bias.
For the formation of epitaxial layers on crystalline substrates a model which is capable to incorporate elastic strain was formulated and studied by Monte Carlo Methods. The. description based on rate-equations leads to scaling predictions for cluster-statistics and diffusion rates. A particular result is that elastic repulsion between the adsorbed particles shifts the formation of islands to higher values of the coverage.
The pressure-dependence of the chemical diffusion constant of a glass was calculated by molecular dynamics simulations of a binary Lennard-Jones model. Four temperature-regimes were observed. The apparent activation drops from high values in the hot liquid to a plateau-value. Near the critical temperature of the mode-couplingtheory it rises steeply, but in the glassy state values similar to the liquid state are observed.
A system of parallel cracks in a uniaxially strained solid allows for stress-relaxation under coarsening of the cracks. A conventional mean-field theory breaks down and several independent lengths have to be taken into account. Scaling laws for this coarsening behavior have been derived which differ markedly form the conventional coarsening theories.
The drag-coefficient for a polymer-chain in a flowing medium depends in a nonlinear way on the velocity of the flow, in contrast to the linear Stokes-law for simple bodies. This effect is due to both an intra-chain and an interchain interaction mediated by hydrodynamics. Computer-simulations supported by analytical calculations have given an explanation of this effect and of its dependence upon fluctuations.
A phase-field method for the treatment of hydrodynamic flow with free interfaces was developed and applied to the phase-transition between partial wetting and dewetting on a substrate, At low Reynolds numbers the calculation agrees with the creep-flow approximation, at higher Reynolds numbers hydrodynamic vorticity leads to significant deviations like surface ripples.
Finally we are pleased that Dr. Roland Rzehak received the Dr.-Eduard-Martin-award for his doctoral thesis at university Saarbrucken during this year.
H. Müller-Krumbhaar