IFF
Scientific Report 1999/2000

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 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 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.

In recent years, metals and semiconductors have been treated with different computational methods. In order to crosscheck our methods for investigation of semiconductor properties, a method usually employed for metals, the KKR-Green's function technique, and a typical procedure used for semiconductor properties, the pseudopotential method, have been compared. The electronic and geometrical structures of complexes of Cd or In acceptors with P, As, or Sb donors in Si and Ge have been analyzed. Behind this problem there is some technological interest, since the Coulomb attraction between oppositely charged defect atoms in Silicon or Germanium leads to the formation of stable acceptor-donor complexes, which can cause serious technological problems like uncontrolled annihilation or creation of charge carriers. Observables which are accessible to both methods agree well, for example the lattice relaxations around defect pairs in Silicon. In addition to the physical and technological interest in this problem, the results increase confidence in our numerical methods.

The properties of semiconductor crystals depend sensitively on their growth mechanism. One tries to avoid island-growth of the crystals because of the increased probability of impurity absorption at the step-edges around islands. Instead one would like to have layer-growth as far as possible. It is known since some time that the use of group-V elements like As or Sb on Silicon or Germanium surfaces act as surfactants favoring by layer growth of the underlying crystal. A first principles pseudopotential study of surface steps of As terminated Silicon or Germanium crystals has been performed. Many details of structure and energetics of the step-edge and the activation barriers for the growth processes have been obtained.

The dynamics of glasses and other amorphous materials still pose a significant number of open questions. During the last year the isotope effect upon diffusion in a monatomic Leonard Jones liquid has been calculated by molecular dynamics simulations. The isotope effect is the most direct measure of collectivity of motion. It was found for example, that the isotope effect depends rather sensitively upon the density and temperature of the material. These results are in good agreement with experiment.

Another class of related problems of disordered materials concerns the dynamics of polymers. A specifically difficult problem is the influence of long range forces between the individual parts of a long polymer chain. Such long range forces occur for example through hydrodynamic effects in polymer solutions. Viscoelasticity and turbulent drag reduction are examples for macroscopic consequences. Detailed numerical and analytical model studies of a strongly deformed polymer in solutions have led to a deeper understanding of correlation effects in the system.

A completely different source of a long range effective interaction in condensed matter physics is due to elastic deformations of solids. A particularly difficult problem hereby is the transition from coherent elastic deformation to incoherent deformations through production of dislocations. A puzzling experiment on SiC under helium gas implantation showed the formation of cracks and the sudden expulsion of dislocation loops near the crack tips. This complex phenomenon was analyzed theoretically in some detail and gave new insights in collective effects in crack propagation and loop formation.

A fundamental assumption in the theory of elasticity is that sufficiently far away from a point or line dislocation the elastic strain field can be described by linear theory of elasticity. In contrast it was now discovered that there are systems, where the strain field of an edge dislocation must be described in a nonlinear theory even far away from the core of the dislocation. In addition an exact analytical solution to this specific problem was found. This is one more example for the observation that elasticity problems even today are still full of surprises.

H. Müller-Krumbhaar