Metamagnets have been playing an important role as model systems in the physics of phase transitions for 30 years. Especially the isolating substances FeCl2 and FeBr2 have attracted much interest both in experimental and theoretical physics. In this thesis we focus on experimental investigations of Fe1-xMgxBr2 with x = 0 and 0.05. By SQUID-magnetometry, SQUID-susceptometry, elastic neutron scattering and calorimetry we studied the magnetic phase boundaries and especially the recently found new phase transition at H=H1(T) (Aruga Katori et al. 1996). Also we arrived to a physical understanding of the non-critical fluctuations at H=H-(T) in terms of a smeared 2-dimensional phase transition of the spin down sublattice.

In the present thesis the structural and magnetic properties of 3d transition metals and their alloys have been investigated within the framework of density functional methods. The calculations have been made mainly to understand magneto-volume instabilities and martensitic phase transformations in iron-nickel and iron-manganese alloys. Many aspects of the alloys can already be understood from the discussion of elemental iron, which forms the essential part of this work. A simple thermodynamical approach combined with the ab-initio results allows the discussion of the physical properties to be extended to finite temperatures. The thermal expansion coefficients calculated within this model coincide well with experiment. Besides the magnetic iron alloys the nonmagnetic Al-Cu-Zn system has been investigated. This ternary alloy undergoes several martensitic phase transformations and spinodal decomposition has also been observed on the Al-Zn-rich side of the phase diagram. The spinodal decomposition is relieved by incommensurable ordering if the aluminium atoms are increasingly replaced by copper. The mixing behavior and the structural phase stability at T = 0 K are obtained from ab initio electronic structure calculations for ordered compounds. The experimental trends are well reproduced. - In dieser Arbeit werden die strukturellen und magnetischen Eigenschaften von 3d-Übergangsmetallen und ihren Verbindungen mit Hilfe verschiedener Methoden der Dichtefunktionaltheorie behandelt. Dabei werden insbesondere Magnetovolumen-Instabilitäten und martensitische Phasenübergänge in Eisen-Nickel und Eisen-Mangan-Verbindungen diskutiert. Einen zentralen Punkt der Arbeit bildet die Untersuchung von elementarem Eisen, da viele Eigenschaften der Legierungen bereits dadurch verstanden werden können. Die Verknüpfung der ab-initio-Ergebnisse mit einem einfachen thermodynamischen Ansatz ermöglicht eine Diskussion bei endlichen Temperaturen. Die im Rahmen dieses Modells berechneten thermischen Ausdehnungskoeffizienten zeigen bereits eine gute Übereinstimmung mit den experimentellen Daten. Neben den Eisenlegierungen wird das nichtmagnetische System Al-Cu-Zn untersucht, welches in bestimmten Konzentrationsbereichen ebenfalls martensitische Phasenübergänge aufweist und auf der Al-Zn-Seite spinodale Entmischung zeigt, die mit zunehmender Substitution von Aluminium durch Kupfer in inkommensurable Ordnung übergeht. An Hand von ab-initio-Berechnungen der elektronischen Struktur geordneter Verbindungen wird das Mischungsverhalten sowie die strukturelle Phasenstabilität des Systems für T = 0 K untersucht. Die Ergebnisse spiegeln die experimentell gefundenen Tendenzen gut wieder.

Particles in granular systems collide inelasticly and kinetic energy is dissipated in the granular system. Granular temperature measures the unordered relative motion of the particles. As a result of the inelastic collisions granular temperature decreases, this process is called collisional cooling. In most cases granular particles are charged. This thesis studies the influence of electrical charges on the collisional cooling by using computer simulations and kinetic theory. It is shown, that electrical charge modifies the dissipation rate by a Boltzmann-factor. The energy barrier of the Boltzmann-factor is given by the electrostatic interaction of two colliding particles. In dense systems this energy barrier is reduced due to the interactions with the particles, that do not take part in the collision. A quantitative expression is given for the effective reduction of the energy barrier. The results found for homogeneous systems is expanded for the local description of inhomogeneous systems.

Past and recent investigations of traffic dynamics mostly rest on averaged data taken over a short period of time, e.g. one minute. They make a qualitative distinction between the generally accepted traffic states called "free flow", "synchronised flow" and "Stop-and-Go traffic" possible. However, the essential car-car-interactions on the microscopic level are concealed. With extensive statistical examinations of single-vehicle traffic data presented in this work one gains new insights especially from a microscopic point of view. By means of time series analyses, correlation functions and by use of histogram methods new evaluation methods for driving behaviour are introduced. These quantities show a strong dependency on the present traffic state, the observed interval of density and the environment (e.g., freeway or city traffic). Microscopic features like synchronisation of velocities ranging over a number of vehicles or decreasing time headways smaller than one second noticeably influence macroscopic proper ties of traffic as expressed in fundamental diagrams. Moreover, on the basis of cross correlation functions connecting flow and density some quantities can be defined to discriminate between the several traffic states quantitatively. These empirical results have impacts on modelling and simulation of traffic flow. A modified cellular automaton comprising aspects of anticipation is introduced and discussed in great detail. Metastable states can be formed and all other criteria are also met to indicate a well-operating traffic flow model. A lot of simulations were done in order to analyse single-lane and multi-lane behaviour and to find out the specific problems coming along with event-driven measuremens methods using "inductive loops". Another focus are density waves to estimate their speed and to elucidate the phase separation and the transition between different traffic states.

In this work structural properties of bulk, nano-particles, and thin films were investigated by using molecular-dynamics simulations. The focus was on the investigation of martensitic transformations in those systems, mainly consisted of Fe and Ni. For the describtion of the interatomic forces the Embedded-Atom Method was used. The calculation of the free energy as a function of temperature gave insight into the thermodynamics of the system, and led to a correct interpretation of the structural transformation from a closed packed structure (face-centered-cubic, hexagonal-closed-packed) to the body-centered-cubic structure and vice versa. Pre-existing lattice defects turned out to be the dominant factor for the martensitic transformation at low temperatures, whereby the austenitic transformation at high temperatures is less affected by defects. The explanation of the different behavior of the martensitc transformation process and the austenitic transformation process could be given by a detailed examination of the free energy along the Bain-path. The study of the crystallographic orientational relationships of the austenitic and martensitic phases gave insight into the transformation process on the atomic-scale. The transformation process observed in the molecular-dynamics simulations can be described in terms of the Wechsler-Lieberman-Read theory as a combination of Bain-transformation, rotation, and lattice invariant shear due to stacking faults and twinning. Simulations of very large supercells containing up to eight million atoms facilitated the study of the homogeneous and heterogeneus nucleation process of structural phase transitions within the solid state. Molecular-dynamics simulations of shock-induced austenitic transformations gave valueable insight into the grain-boundary dynamics of the developing austenitic grains. The heterogeneous nucleation process at different types of defects in nano-particles was studied. The burst-type growth of the martensitc phase starts at pre-existing defects with further growth of the twinned martensitic phase into the austenite-matrix. With decreasing size of the nano-particle, transition temperatures decreased as revealed in the few experiments that exist. In the framework of the used Embedded-Atom Method-potential, this effect is due to the different surface energies of the austenitic and the martensitic phases. The interplay between the structure of films and an underlying substrate was intensively studied for the well known Fe on Cu-system. Experimental observations, like the increasing tendency for a structural transformation from the face-centered-cubic structure to the body-centered-cubic structure with increasing film thickness and decreasing temperature, were confirmed. Simulations of the growth process gave insight into recently performed experiments of the dependence of the structural stability of face-centered-cubic Fe-films on Cu(111)-substrates as a function of the deposition process like thermal deposition and pulsed laser deposition.

Boron and boron rich solids are known to have a high concentration on intrinsic structural imperfections. From known structure data of real crystals and known band structure calculations of perfect ideal crystals a correlation between intrinsic structure defect concentration and electron deficit in the valence band is concluded. This correlation forms the basis for the following theses: 1. The electron deficit in the valence band of a perfect crystal is the driving force for the intrinsic structure defects in a real crystal. 2. The small electron deficit becomes compensated by the structure defects - this explains the semiconducting behavior. 3. The structure defects are the reason for the high density of localized electronic states in the band gap. The photoluminescence of beta-rhombohedral boron in the range 0.75eV to 1.4eV under interband excitation was investigated systematically and was interpreted using the one-dimensional Franck-Conden-Model. The only partially occupied B13-position in beta-rhombohedra l boron is suggested to be the reason for the localized electronic state, which is involved in the photoluminescence process. Together with an investigation of the time-depending photoconductivity under interband excitation the energy band schema of beta-rhombohedral boron is improved. The improved energy band schema is able to explain all known experimental data including the fatiguing of photoluminescence. An investigation of FIR-spectra of boron carbide and metal doped beta-rhombohedral boron by Kramers-Kronig-Analysis gives information on the main transport processes. Beside hopping conduction of localized electrons, band conduction of delocalized electrons were found. While holes in the valence band are the delocalized charge carriers in boron carbide, in vanadium doped beta-rhombohedral boron delocalized electrons in an extrinsic impurity band are suggested

Today, peer-to-peer services can comprise a large and growing number of services, e.g. search services or services dealing with heterogeneous schemas in the context of Digital Libraries. For a given task, the system has to determine suitable services and their processing order (execution plan). As peers can join or leave the network spontaneously, static execution plans are not sufficient. This paper proposes a logic-based approach for dynamically computing execution plans: Services are described in the DAML-S language. These descriptions are mapped onto Datalog. Finally, logical rules are applied on the service description facts for determining matching services and finding an optimum execution plan.

Proceedings of the Second INEX Workshop, Dagstuhl, Germany, December 15-17, 2003. International Conference and Research Center for Computer Science

<H4>In:</H4>Intelligent Search on XML Data : Applications, Languages, Models, Implementations, and Benchmarks / Henk Blanken ... [et al.] (eds.)- Berlin : Springer, 2003. - ISBN 3-540-40768-5, S. 59-75

<H4>In:</H4>SIGIR 2003 : proceedings of the 26th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, July 28 to August 1, 2003, Toronto, Canada / edited by Jamie Callan ... - New York, 2003