IFF
Scientific Report 1998/1999

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 keeps the macroscopic consequences in mind. The analytical and numerical investigations are in many ways closely connected with experimental studies performed in other groups of IFF, but also with activities in other institutes of the Research Center Jülich.

Central points of interest for the research in Theory III are in the field of electronic structure of solids (F&E-Nr. 23.20.0). Materials classes under considerations are metals and semiconductors specifically with respect to their importance for information technology (F&E-Nr. 23.42.0). A second mainstream is formed by cooperative phenomena in condensed matter (F&E-Nr. 23.15.0). Questions here aim at dynamics of structure and pattern formation and the statistical mechanics of order and disorder processes. Specific activities in the field of complex fluids (F&E-Nr. 23.30.0) 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 code 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 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 a central interest of our research a main concern now is directed towards the understanding of surface and interface properties.

Atoms of 3d-transition metals adsorbed on the 001 surface of iron have been investigated with detailed ab-initio studies by KKR-Greenfunction techniques. Different configurations of impurities adsorbed at the surfaces have been calculated with respect to direct exchange and interaction processes. These first principles results are in good agreement with experimental studies by scanning tunneling microscopy for chromium adsorbed on iron surfaces, performed by the NIST-group, which gives confidence in the predictive power and numerical accuracy of these calculations.

Macroscopic structures are formed by growth processes. The surface of a growing crystal plays a critical role in a solidification process. The study of kinetic properties of surfaces starting from the electronic structure of the material requires an enormous amount of computing time. For this reason massively parallel computers are appropriate tools for the calculation of these phenomena. A parallelized ab-initio computer code combining norm-conserving pseudo-potential calculations with molecular dynamics simulations for the molecular motion has been developed. The materials under consideration are important for information technology: semiconductors like silicon, germanium, galliumarsenide, etc.. Detailed studies of the speed-up of this computation on vector and parallel computers have been performed. The developed computer code will be a sound basis for future investigations of adsorption and surface transport of the semiconductor materials.

The properties of glasses represent a point of interest since several years. In our previous studies the effective interaction between the molecules have been assumed to be of relatively short range. In addition to these direct interatomic interactions indirect long range forces originating for example from electric or elastic dipols may significantly change the systems collective behavior. We therefore have studied a model system with frozen-in disorder where the elements of the medium interact with dipolar forces. As a basic result it was found that the eigenstates of the corresponding Anderson model develop a multifractal structure. The results also give interesting contributions to the theory of large random matrices, where drastic differences are found compared to the Wigner-Dyson random matrix theory.

Ion tracks in glassy materials have been studied also since a few years with remarkable success. In the past year a viscoelastic theory has been formulated to capture the various time scales in this problem from a unified point of view. The solutions of this viscoelastic field equations reproduce all characteristic features of the observed deformation phenomena when energetic heavy ions are penetrating into glassy materials.

Particular long range interactions between atoms or molecules are introduced by the motion of the molecules in a flow-field. The investigation of tethered polymers in flow has started about two years ago. It has now become possible to include into the calculation the feedback effect of all segments of the molecule onto the other segments via the flow. It turned out, that these feedback effects are crucial to understand the behavior of large molecules in a hydrodynamic environment. This extends the previous linear viscoelastic theory for Non-Newtonian fluids towards the incorporation of nonlinear effects. (This interesting field of research will not be continued in Theory III since the responsible scientist Prof. W. Zimmermann has accepted a professor position at university Saarbrücken.)

An old problem in the field of order-disorder transitions is the question of pinning and depinning of an advancing front over a background of randomly frozen impurities. A particularly characteristic example of such a process is the high speed phase transition (solidification) of an alloy after laser-induced surface melting. In a combination of analytical methods and numerical large scale Monte-Carlo simulations new insights into this problem have been gained. A central result is the quantitative confirmation of a scaling hypothesis for the dependence of the growth rate upon interaction energies, concentration and strength of impurities, temperature and driving force.

The increasing possibility of realistic calculations of materials properties from first principles has again increased the interest in collective properties of solid surfaces. A technologically very important and theoretically highly challenging problem is the formation and propagation of cracks in solids. A new theory has combined previous concepts on crack formation and surface deformations. It was found that cracks will be susceptible to surface instabilities which may eventually be responsible for directional instabilities during crack propagation.

H. Müller-Krumbhaar