IFF

 ZCH

 ISG

 ZAT

In charge: Prof. H. Müller-Krumbhaar, IFF, h.mueller-krumbhaar@fz-juelich.de

Aims and Objectives

The research aims at detecting and explaining the phenomena and mechanisms originating from the interplay of the electronic, atomic and molecular constituents of condensed matter. Multiscale processes form the core of the investigations. The results can be expected to influence wide areas of science and technology. Focus-points of the program are "Electronic and Magnetic Phenomena", "From Matter to Materials", and "Soft Matter and Biophysics". The investigations concern electronic and magnetic quantum-states and properties, super-conductivity, phase-transitions and transport-processes, glassy states, and finally structure-formation and self-organisation. The spectrum of the materials under consideration encompasses the whole range of metals, semiconductors and ceramics, extending over macromolecular systems to biological membranes and cells. A large number of experimental and theoretical methods of investigation are employed and advanced: Central experimental techniques are neutron-scattering and synchrotron-radiation, in the theoretical studies large-scale numerics are used and advanced in addition to analytical methods.

Significant Results in 2003

Electronic and Magnetic Phenomena

• Research on spin dynamics in ultra-thin epitactical films is of eminent importance for the understanding of magnetic properties in nanostructured matter. Such understanding in turn is a prerequisite for the development of nanoscale devices which make use of the electron spin as the carrier of information. It has been hitherto impossible to investigate the coupling dispersion of spin waves at surfaces and in ultra-thin films since the sensitivity of the classical method, neutron scattering, is insufficient for studying surfaces and thin films. Based on the long tradition in high resolution inelastic electron scattering in the ISG3 ( formerly IGV) a novel electron spectrometer has been developed and applied to the investigation of spin waves in ultra thin cobalt filmes on copper surfaces, in collaboration with the Max Planck Institute of Microstructure Physics in Halle. For the first time, the dispersion of spin waves has been measured in thin film systems. The construction of an advanced spin spectrometer for the use in Juelich within the program Condensed Matter is in preparation.
• Understanding the microscopic processes which govern the dynamical behavior of the magnetization is of great importance for the development of fast magnetic switching strategies in magnetic data storage. We have advanced the technique of photoemission microscopy to investigate the dynamics of magnetization reversal processes with both high lateral and time resolution (100 ps). The results obtained on Permalloy microstructures revealed the onset of incoherent magnetization rotation processes which are excited by fast magnetic field pulses.
• Electronic devices of the future may employ organic molecules as switches between metallic leads, at least for some special tasks, for example as sensors of biological activities. This requires a basic understanding of the adsorption and the tunability of such molecules. Small organic molecules adsorbed on substrate surfaces of metals like copper or semiconductors therefore have been studied theoretically, using ab initio calculations based on our projector-augmented wave (PAW) computer code. Modifications of the electronic states of the molecules by the substrate can lead to a variety of phases of the organic adsorbate. Glycinate molecules for example are lying flat on the surface, both functional groups binding to Cu atoms. For the densest packing the calculations show that the heterochiral arrangement with both left and right oriented molecules are more stable than the homochiral arrangement.

From Matter to Materials

• It has been known since long that processes of diffusion, ripening and self assembly at the solid-electrolyte interface depend on the electrode potential. Nevertheless the physical basis for this "electrochemical annealing" was not understood. The first quantitative experiments on these phenomena were conducted in the ISG in the years 2000-2003. Now a major break through has been achieved on the theoretical side. We were able to develop a generic theory of the potential dependence of surface defect energies as well as of diffusion processes, of ripening and self assembly. This theory permits even quantitative predictions for particular experimental conditions (absence of specific adsorption processes).
• Lanthanum aluminate LaAlO₃ has received much attention as a ceramic material due to its promising use as filter material in microwave applications and as a substrate for high temperature super conductor thin films. Different preparation methods have been developed here which are producing ceramics of high quality. In particular the chemical preparation methods, nitrate decomposition and sol-gel processing, were found to yield ceramics of sufficient density and specifically desired high purity. Results of morphological and electrical investigations show significant correlations between the specific methods of preparation and the morphological properties. The purity of the probes is identified as the quality controlling factor even before the morphological details.
• Atoms adsorbed at the surface of a substrate interact by the elastic forces mediated through the substrate. A crystal surface, faceted at low temperatures, may undergo a roughening transition at elevated temperature but below the melting point. Above that roughening temperature the facet has disappeared and the surface develops long-wavelength fluctuations under equilibrium conditions. We have studied the effect of elastic interactions among adatoms and islands forming on the surface of an otherwise homogeneous crystal, using a multigrid Monte Carlo method and scaling arguments. Depending on the relative strength of the elastic repulsive interaction, correlation effects among the surface fluctuations may increase the roughening temperature drastically as compared to a system with only short-range forces.

Soft Matter and Biophysics

• Fluorescence correlation spectroscopy (FCS) has become an important tool for investigating the dynamical properties of single molecules in solution. To extract parameters characterizing the intramolecular dynamics of liner macromolecules like DNA or actin filaments, a theoretical expression for the FCS correlation function is required. On the basis of a previously developed Gaussian semiflexible chain model, the FCS correlation function has been calculated. The FCS signal contains contributions from the diffusive dynamics of the whole molecule as well as of the intramolecular dynamics. The intramolecular dynamics is observed at small time scales and the overall diffusion at very large times. The experimental time window is, however, typically in the transition range where both contributions are important. Comparison of the theoretical model with experimental data on DNA molecules yields good agreement.
• Clathrates are used broadly because of their catalytic activity, their selectivity with respect to isomers, their storage properties (energy, radioactive waste), etc. Using rotational tunnelling of small molecules the potential surface of the structural cages may be characterized in great detail. Such highly resolved neutron spectra of methyliodid in cubic II water clathrate displays three different adsorption positions. They relate with three different H-bridges within the cages. The transition of the metastable tunnelling states into thermal equilibrium relates to the dynamics of the host H-bridges.
• The amount of oil and water that can be mixed by a given limited portion of surfactant is a measure for the efficiency of that surfactant. In the considered model system the resulting mixture is a bicontinuous microemulsion. A small amount of a surfactant-analogous block-copolymer is able to boost the efficiency. However, addition of the separated blocks (homopolymers) reduces the emulsification power. According to theory a polymer induced modification of the interface bending modulus determines the efficiency changes. By using SANS and NSE the polymer influence on the bending moduli has been directly measured. Comparing SANS and NSE results also sheds light on the bending modulus renormalisation due to fluctuations.
• The shape and the motility of an adherent biolgical cell has been studied using Monte Carlo simulations. Using the simulation data we established scaling laws between the size of the cell and the enclosed size of the polymerizing actin cytoskeleton. With increasing concentration of actin molecules the cell exhibits a discontinuous transition between a motile and a static phase. It is shown that the velocity of the cell is maximal for an optimal choice of the depolymerization rate.
• Adhesion between living cells is mediated by molecular recognition between biomolecules located at the surfaces of the adherent cells. Our research aims at a basic understanding of the mechanic strength of adhesive contacts between cells, a question of utmost importance for many physiological processes. Previous work attempted to correlate mechanic strength with the physicochemical properties of bonds between cell adhesion molecules which are dissolved in aqueous buffer. Here we performed a systematic experimental study of the additional influence of surface anchorage of these molecules on the dynamics and strength of adhesive contacts. We found a dramatic influence of this up to now neglected factor: Bonds between surface attached biomolecules exhibit a dissociation rate which is approximately 100 times higher as compared to bonds between identical but not surface connected molecules. This surprising effect was explained by basic principles of statistical mechanics.