## Physik

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- The production of _j63 [eta] and {_w63 [omega] mesons in 3.5 GeV p+p interaction in HADES (2011)
- The study of meson production in proton-proton collisions in the energy range up to one GeV above the production threshold provides valuable information about the nature of the nucleon-nucleon interaction. Theoretical models describe the interaction between nucleons via the exchange of mesons. In such models, different mechanisms contribute to the production of the mesons in nucleon-nucleon collisions. The measurement of total and differential production cross sections provide information which can help in determining the magnitude of the various mechanisms. Moreover, such cross section information serves as an input to the transport calculations which describe e.g. the production of e+e− pairs in proton- and pion-induced reactions as well as in heavy ion collisions. In this thesis, the production of ω and η mesons in proton-proton collisions at 3.5 GeV beam energy was studied using the High Acceptance DiElectron Spectrometer (HADES) installed at the Schwerionensynchrotron (SIS 18) at the Helmholtzzenturm f¨ur Schwerionenforschung in Darmstadt. About 80 000 ω mesons and 35 000 η mesons were reconstructed. Total production cross sections of both mesons were determined. Furthermore, the collected statistics allowed for extracting angular distributions of both mesons as well as performing Dalitz plot studies. The ω and η mesons were reconstructed via their decay into three pions (π+π−π0) in the exclusive reaction pp −→ ppπ+π−π0. The charged particles were identified via their characteristic energy loss, via the measurement of their time of flight and momentum, or using kinematics. The neutral pion was reconstructed using the missing mass method. A kinematic fit was applied to improve the resolution and to select events in which a π0 was produced. The correction of measured yields for the effects of spectrometer acceptance was done as a function of four variables (two invariant masses and two angles). Systematic studies of the acceptance for different input distributions were performed. The measured yields were normalized to the number of measured events of elastic scattering. Systematic errors due to the methods of the data analysis and the background subtraction were investigated. Production angular distributions of ω and η mesons were measured. Both mesons exhibit a slightly anisotropic angular distribution. The Dalitz plot of ω meson production shows indications of resonant production. However, the deviation of the distribution from the one expected by phase space simulations is not large. The Dalitz plot of η meson production shows a signal of the production via the N(1535) resonance, The contribution of N(1535) to the production was quantified to be about 47%. The angular distribution of η mesons does not show significant differences between resonant and non resonant production. The total production cross section of ω mesons in the reaction pp −→ ppω was determined to be 106.5 ± 0.9 (stat) ± 7.9 (sys) [μb] where stat indicates statistical error and sys indicates systematic error, while that of η mesons was determined to be 136.9 ± 0.9 (stat) ± 10.1 (sys) [μb] in the reaction pp −→ ppη

- A numerical renormalization group approach to dissipative quantum impurity systems (2011)
- The miniaturization of electronics is reaching its limits. Structures necessary to build integrated circuits from semiconductors are shrinking and could reach the size of only a few atoms within the next few years. It will be at the latest at this point in time that the physics of nanostructures gains importance in our every day life. This thesis deals with the physics of quantum impurity models. All models of this class exhibit an identical structure: the simple and small impurity only has few degrees of freedom. It can be built out of a small number of atoms or a single molecule, for example. In the simplest case it can be described by a single spin degree of freedom, in many quantum impurity models, it can be treated exactly. The complexity of the description arises from its coupling to a large number of fermionic or bosonic degrees of freedom (large meaning that we have to deal with particle numbers of the order of 10^{23}). An exact treatment thus remains impossible. At the same time, physical effects which arise in quantum impurity systems often cannot be described within a perturbative theory, since multiple energy scales may play an important role. One example for such an effect is the Kondo effect, where the free magnetic moment of the impurity is screened by a "cloud" of fermionic particles of the quantum bath. The Kondo effect is only one example for the rich physics stemming from correlation effects in many body systems. Quantum impurity models, and the oftentimes related Kondo effect, have regained the attention of experimental and theoretical physicists since the advent of quantum dots, which are sometimes also referred to as as artificial atoms. Quantum dots offer a unprecedented control and tunability of many system parameters. Hence, they constitute a nice "playground" for fundamental research, while being promising candidates for building blocks of future technological devices as well. Recently Loss' and DiVincenzo's p roposal of a quantum computing scheme based on spins in quantum dots, increased the efforts of experimentalists to coherently manipulate and read out the spins of quantum dots one by one. In this context two topics are of paramount importance for future quantum information processing: since decoherence times have to be large enough to allow for good error correction schemes, understanding the loss of phase coherence in quantum impurity systems is a prerequisite for quantum computation in these systems. Nonequilibrium phenomena in quantum impurity systems also have to be understood, before one may gain control of manipulating quantum bits. As a first step towards more complicated nonequilibrium situations, the reaction of a system to a quantum quench, i.e. a sudden change of external fields or other parameters of the system can be investigated. We give an introduction to a powerful numerical method used in this field of research, the numerical renormalization group method, and apply this method and its recent enhancements to various quantum impurity systems. The main part of this thesis may be structured in the following way: - Ferromagnetic Kondo Model, - Spin-Dynamics in the Anisotropic Kondo and the Spin-Boson Model, - Two Ising-coupled Spins in a Bosonic Bath, - Decoherence in an Aharanov-Bohm Interferometer.

- Mechanisms of nanofractal structure formation and post-growth evolution (2011)
- Nanotechnology is a rapidly developing branch of science, which is focused on the study of phenomena at the nanometer scale, in particular related to the possibilities of matter manipulation. One of the main goals of nanotechnology is the development of controlled, reproducible, and industrially transposable nanostructured materials. The conventional technique of thin-film growth by deposition of atoms, small atomic clusters and molecules on surfaces is the general method, which is often used in nanotechnology for production of new materials. Recent experiments show, that patterns with different morphology can be formed in the course of nanoparticles deposition process on a surface. In this context, predicting of the final architecture of the growing materials is a fundamental problem worth studying. Another factor, which plays an important role in industrial applications of new materials, is the question of post-growth stability of deposited structures. The understanding of the post-growth relaxation processes would give a possibility to estimate the lifetime of the deposited material depending on the conditions at which the material was fabricated. Controllable post-growth manipulations with the architecture of deposited structures opens new path for engineering of nanostructured materials. The task of this thesis is to advance understanding mechanisms of formation and post-growth evolution of nanostructured materials fabricated by atomic clusters deposition on a surface. In order to achieve this goal the following main problems were addressed: 1. The properties of isolated clusters can significantly differ from those of analogous clusters occurring on a solid surface. The difference is caused by the interaction between the cluster and the solid. Therefore, the understanding of structural and dynamical properties of an atomic cluster on a surface is a topic of intense interest from the scientific and technological point of view. In the thesis, stability, energy, and geometry of an atomic cluster on a solid surface were studied using a liquid drop approach which takes into account the cluster-solid interaction. Geometries of the deposited clusters are compared with those of isolated clusters and the differences are discussed. 2. The formation scenarios of patterns on a surface in the course of the process of cluster deposition depend strongly on the dynamics of deposited clusters. Therefore, an important step towards predicting pattern morphology is to study dynamics of a single cluster on a surface. The process of cluster diffusion on a surface was modeled with the use of classical molecular dynamics technique, and the diffusion coefficients for the silver nanoclusters were obtained from the analysis of trajectories of the clusters. The dependence of the diffusion coefficient on the system’s temperature and cluster-surface interaction was established. The results of the calculations are compared with the available experimental results for the diffusion coefficient of silver clusters on graphite surface. 3. The methods of classical molecular dynamics cannot be used for modeling the self-assembly processes of atomic clusters on a surface, because these processes occur on the minutes timescale, what would require an unachievable computer resource for the simulation. Based on the results of molecular dynamics simulations for a single cluster on a surface a Monte-Carlo based approach has been developed to describe the dynamics of the self-assembly of nanoparticles on a surface. This method accounts for the free particle diffusion on a surface, aggregation into islands and detachment from these islands. The developed method is allowed to study pattern formation of structures up to thousands nm, as well as the stability of these structures. Developed method was implemented in MBN Explorer computer package. 4. The process of the pattern formation on a surface was modeled for several different scenarios. Based on the analysis of results of simulations was suggested a criterion, which can be used to distinguish between different patterns formed on a surface, for example: between fractals or compact islands.This criteria can be used to predict the final morphology of a growing structure. 5. The post-growth evolution of patterns on a surface was also analyzed. In particular, attention in the thesis is payed to a systematical theoretical analysis of the post-growth processes occurring in nanofractals on a surface. The time evolution of fractal morphology in the course of the post-growth relaxation was analyzed, the results of these calculations were compared with experimental data available for the post-growth relaxation of silver cluster fractals on graphite substrate. All the aforementioned problems are discussed in details in the thesis.

- Emergent inert adjoint scalar field in SU(2) Yang-Mills thermodynamics due to coarse-grained topological fluctuation (2011)
- We compute the phase and the modulus of an energy- and pressure-free, composite, adjoint, and inert field φ in an SU(2) Yang-Mills theory at large temperatures. This field is physically relevant in describing part of the ground-state structure and the quasiparticle masses of excitations. The field φ possesses nontrivial S1-winding on the group manifold S3. Even at asymptotically high temperatures, where the theory reaches its Stefan-Boltzmann limit, the field φ, though strongly power suppressed, is conceptually relevant: its presence resolves the infrared problem of thermal perturbation theory.