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- Doctoral Thesis (45) (remove)

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- Measurement and interpretation of laser accelerated protons at GSI (2014)
- This thesis is structured into 7 chapters: • Chapter 2 gives an overview of the ultrashort high intensity laser interaction with matter. The laser interaction with an induced plasma is described, starting from the kinematics of single electron motion, followed by collective electron effects and the ponderamotive motion in the laser focus and the plasma transparency for the laser beam. The three different mechanisms prepared to accelerate and propagate electrons through matter are discussed. The following indirect acceleration of protons is explained by the Target Normal Sheath Acceleration (TNSA) mechanism. Finally some possible applications of laser accelerated protons are explained briefly. • Chapter 3 deals with the modeling of geometry and field mapping of magnetic lens. Initial proton and electron distributions, fitted to PHELIX measured data are generated, a brief description of employed codes and used techniques in simulation is given, and the aberrations at the solenoid focal spot is studied. • Chapter 4 presents a simulation study for suggested corrections to optimize the proton beam as a later beam source. Two tools have been employed in these suggested corrections, an aperture placed at the solenoid focal spot as energy selection tool, and a scattering foil placed in the proton beam to smooth the radial energy beam profile correlation at the focal spot due to chromatic aberrations. Another suggested correction has been investigated, to optimize the beam radius at the focal spot by lens geometry controlling. • Chapter 5 presents a simulation study for the de-neutralization problem in TNSA caused by the fringing fields of pulsed magnetic solenoid and quadrupole. In this simulation, we followed an electrostatic model, wherethe evolution of both, self and mutual fields through the pulsed magnetic solenoid could be found, which is not the case in the quadrupole and only the growth of self fields could be found. The field mapping of magnetic elements is generated by the Matlab program, while the TraceWin code is employed to study the tracking through magnetic elements. • Chapter 6 describes the PHELIX laser parameters at GSI with chirp pulse amplification technique (CPA), and Gafchromic Radiochromic film RCF) as a spatial energy resolver film detector. The results of experiments with laser proton acceleration, which were performed in two experimental areas at GSI (Z6 area and PHELIX Laser Hall (PLH)), are presented in section 6.3. • Chapter 7 includes the main results of this work, conclusions and gives a perspective for future experimental activities.

- Elektronische und magnetische Phasenseparation in EuB6 : Fluktuationsspektroskopie und nichtlinearer Transport (2013)
- Die Schwerpunkte dieser Arbeit sind elektrische, stationäre und zeitaufgelöste Transportmessungen an EuB6 sowie die Weiterentwicklung von Messmethoden und Analyseverfahren der Fluktuationsspektroskopie. Durch die Verwendung von modernen Computern und Datenerfassungskarten konnten die Messmethoden effektiver eingesetzt werden. Die ersten beiden Kapitel stellen die Grundlagen dar, die für diese Arbeit von Bedeutung sind. Der erste Teil dieser Arbeit wurde der Weiterentwicklung der bereits bekannten Messmethoden unter Verwendung einer schnellen Datenerfassungskarte gewidmet. Im Gegensatz zur Verwendung eines Signalanalysators bietet die Karte die Möglichkeit, auf die Rohdaten im Zeitraum zuzugreifen und sie anschließend mit einer selbst programmierten Software auszuwerten. Die technischen Methoden und der Aufbau der Software wurden in den Kapiteln 3 und 4 vorgestellt. Durch das Ersetzen des Signalanalysators kann bis zu 50% der Messzeit eingespart werden. Durch die Code-Erweiterung kann bereits nach zwei hintereinander gemessenen Spektren vorläufig bei tiefen Frequenzen ausgewertet und somit frühzeitig entschieden werden, ob eine längere Messzeit aussichtsreich ist. Außerdem wird durch Verwendung der Code-Erweiterung eine sehr viel höhere Spektrendichte (Anzahl von Messpunkten) erreicht. Da im Gegensatz zum Signalanalysator alle gemessenen Spektren gespeichert werden, können in jeder Messung die Spektren auf ihre Korrelation (Korrelationskoeffizient und Zweites Spektrum) hin untersucht werden, ohne zusätzliche Messzeit zu benötigen. Der zweite Teil dieser Arbeit befasst sich mit den elektrischen Transporteigenschaften von EuB6 und dem Verständnis der Kopplung zwischen Ladungs- und magnetischen Freiheitsgraden. Mittels Widerstands- und nichtlinearer Transportmessungen sowie Fluktuationsspektroskopie wurden Hypothesen von anderen Wissenschaftlern systematisch verifiziert, sowie neue, weiterführende Erkenntnisse gewonnen. Direkte experimentelle Hinweise für die Phasenseparation sowie das Auftreten von Perkolation aus Transportmessungen fehlten bisher. In dieser Arbeit wurden daher systematisch die elektrischen Transportseigenschaften des Systems in Abhängigkeit von der Temperatur und vom Magnetfeld untersucht. Mittels Fluktuationsspektroskopie konnte erstmals ein direkter Hinweis auf perkolatives Verhalten in den Transporteigenschaften beobachtet werden. Ein starkes nichtlineares Transportsignal (dritter harmonischer Widerstand, DHW) im Bereich von TMI und TC ist eine Signatur einer räumlich inhomogenen Stromverteilung auf der Mikroskala und ein weiterer deutlicher Hinweis auf magnetisch induzierte elektronische Phasenseparation. Insbesondere tritt nichtlinearer Transport bei H = 0 im FM Bereich auf und kann im PM-Bereich bei T > TMI durch externe Magnetfelder induziert werden.

- Dynamical effects and disorder in ultracold bosonic matter (2012)
- In this thesis, various aspects on the theoretical description of ultracold bosonic atoms in optical lattices are investigated. After giving a brief introduction to the fundamental concepts of BECs, atomic physics, interatomic interactions and experimental procedures in chapter (1), we derive the Bose-Hubbard model from first principles in chapter (2). In this chapter, we also introduce and discuss a technique to efficiently determine Wannier states, which, in contrast to current techniques, can also be extended to inhomogeneous systems. This technique is later extended to higher dimensional, non-separable lattices in chapter (5). The many-body physics and phases of the Bose-Hubbard is shortly presented in chapter (3) in conjunction with Gutzwiller mean-field theory, and the recently devised projection operator approach. We then return to the derivation of an improved microscopic many-body Hamiltonian, which contains higher band contributions in the presence of interactions in chapter (4). We then move on to many-particle theory. To demonstrate the conceptual relations required in the following chapter, we derive Bogoliubov theory in chapter (5.3.4) in three different ways and discuss the connections. Furthermore, this derivation goes beyond the usual version discussed in most textbooks and papers, as it accounts for the fact, that the quasi-particle Hamiltonian is not diagonalizable in the condensate and the eigenvectors have to be completed by additional vectors to form a basis. This leads to a qualitatively different quasi-particle Hamiltonian and more intricate transformation relations as a result. In the following two chapters (7, 8), we derive an extended quasi-particle theory, which goes beyond Bogoliubov theory and is not restricted to weak interactions or a large condensate fraction. This quasi-particle theory naturally contains additional modes, such as the amplitude mode in the strongly interacting condensate. Bragg spectroscopy, a momentum-resolved spectroscopic technique, is introduced and used for the first experimental detection of the amplitude mode at finite quasi-momentum in chapter (9). The closely related lattice modulation spectroscopy is discussed in chapter (10). The results of a time-dependent simulation agree with experimental data, suggesting that also the amplitude mode, and not the sound mode, was probed in these experiments. In chapter (11) the dynamics of strongly interacting bosons far from equilibrium in inhomogeneous potentials is explored. We introduce a procedure that, in conjunction with the collapse and revival of the condensate, can be used to create exotic condensates, while particularly focusing on the case of a quadratic trapping potential. Finally, in chapter (12), we turn towards the physics of disordered systems derive and discuss in detail the stochastic mean-field theory for the disordered Bose-Hubbard model.

- Modelling radiation fields of ion beams in tissue-like materials (2014)
- Fast nuclei are ionizing radiation which can cause deleterious effects to irradiated cells. The modelling of the interactions of such ions with matter and the related effects are very important to physics, radiobiology, medicine and space science and technology. A powerful method to study the interactions of ionizing radiation with biological systems was developed in the field of microdosimetry. Microdosimetry spectra characterize the energy deposition to objects of cellular size, i.e., a few micrometers. In the present thesis the interaction of ions with tissue-like media was investigated using the Monte Carlo model for Heavy-Ion Therapy (MCHIT) developed at the Frankfurt Institute for Advanced Studies. MCHIT is a Geant4-based application intended to benchmark the physical models of Geant4 and investigate the physical properties of therapeutic ion beams. We have implemented new features in MCHIT in order to calculate microdosimetric quantities characterizing the radiation fields of accelerated nucleons and nuclei. The results of our Monte Carlo simulations were compared with recent experimental microdosimetry data. In addition to microdosimetry calculations with MCHIT, we also investigated the biological properties of ion beams, e.g. their relative biological effectiveness (RBE), by means of the modified Microdosimetric-Kinetic model (MKM). The MKM uses microdosimetry spectra in describing cell response to radiation. MCHIT+MKM allowed us to study the physical and biological properties of ion beams. The main results of the thesis are as follows: MCHIT is able to describe the spatial distribution of the physical dose in tissue-like media and microdosimetry spectra for ions with energies relevant to space research and ion-beam cancer therapy; MCHIT+MKM predicts a reduction of the biological effectiveness of ions propagating in extended medium due to nuclear fragmentation reactions; We predicted favourable biological dose-depth profiles for monoenergetic helium and lithium beams similar to the one for carbon beam. Well-adjusted biological dose distributions for H-1, He-4, C-12 and O-16 with a very flat spread-out Bragg peak (SOBP) plateau were calculated with MCHIT+MKM; MCHIT+MKM predicts less damage to healthy tissues in the entrance channel for SOBP He-4 and C-12 beams compared to H-1 and O-16 ones. No definitive advantages for oxygen ions with respect to carbon were found.

- Verification of Monte Carlo transport codes by activation experiments (2012)
- With the increasing energies and intensities of heavy-ion accelerator facilities, the problem of an excessive activation of the accelerator components caused by beam losses becomes more and more important. Numerical experiments using Monte Carlo transport codes are performed in order to assess the levels of activation. The heavy-ion versions of the codes were released approximately a decade ago, therefore the verification is needed to be sure that they give reasonable results. Present work is focused on obtaining the experimental data on activation of the targets by heavy-ion beams. Several experiments were performed at GSI Helmholtzzentrum für Schwerionenforschung. The interaction of nitrogen, argon and uranium beams with aluminum targets, as well as interaction of nitrogen and argon beams with copper targets was studied. After the irradiation of the targets by different ion beams from the SIS18 synchrotron at GSI, the γ-spectroscopy analysis was done: the γ-spectra of the residual activity were measured, the radioactive nuclides were identified, their amount and depth distribution were detected. The obtained experimental results were compared with the results of the Monte Carlo simulations using FLUKA, MARS and SHIELD. The discrepancies and agreements between experiment and simulations are pointed out. The origin of discrepancies is discussed. Obtained results allow for a better verification of the Monte Carlo transport codes, and also provide information for their further development. The necessity of the activation studies for accelerator applications is discussed. The limits of applicability of the heavy-ion beam-loss criteria were studied using the FLUKA code. FLUKA-simulations were done to determine the most preferable from the radiation protection point of view materials for use in accelerator components.

- Design of the micro vertex detector of the CBM experiment : development of a detector response model and feasibility studies of open charm measurement
- The PhD addresses the feasibility of reconstructing open charm mesons with the Compressed Baryonic Matter experiment, which will be installed at the FAIR accelerator complex at Darmstadt/Germany. The measurements will be carried out by means of a dedicated Micro Vertex Detector (MVD), which will be equipped with CMOS Monolithic Active Pixel Sensors (MAPS). The feasibility of reconstructing the particles with a proposed detector setup was studied. To obtain conclusive results, the properties of a MAPS prototype were measured in a beam test at the CERN-SPS accelerator. Based on the results achieved, a dedicated simulation software for the sensors was developed and implemented into the software framework of CBM (CBMRoot). Simulations on the reconstruction of D0-mesons were carried out. It is concluded that the reconstruction of those particles is possible. The PhD introduces the physics motivation of doing open charm measurements, represents the results of the measurements of MAPS and introduces the innovative simulation model for those sensors as much as the concept and results of simulations of the D0 reconstruction.

- 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.

- Density functional theory and dynamical mean field theory: applications to correlated electron materials (2012)
- The study of systems whose properties are governed by electronic correlations is a corner stone of modern solid-state physics. Often, such systems feature unique and distinct properties like Mott metal-insulator transitions, rich phase diagrams, and high sensitivity to subtle changes in the applied conditions. Whereas the standard approach to electronic structure calculations, density functional theory (DFT), is able to address the complexity of real-world materials but is known to have serious limitations in the description of correlations, the dynamical mean-field theory (DMFT) has become an established method for the treatment of correlated fermions, first on the level of minimal models and later in combination with DFT, termed LDA+DMFT. This thesis presents theoretical calculations on different materials exhibiting correlated physics, where we aim at covering a range in terms of systems --from rather weakly correlated to strongy correlated-- as well as in terms of methods, from DFT calculations to combined LDA+DMFT calculations. We begin with a study on a selection of iron pnictides, a recently discovered family of high-temperature superconductors with varying degree of correlation strength, and show that their magnetic and optical properties can be assessed to some degree within DFT, despite the correlated nature of these systems. Next, extending our analysis to the inclusion of correlations in the framework of LDA+DMFT, we discuss the electronic structure of the iron pnictide LiFeAs which we find to be well described by Fermi liquid theory with regard to many of its properties, yet we see distinct changes in its Fermi surface upon inclusion of correlations. We continue the study of low-energy properties and specifically Fermi surfaces on two more iron pnictides, LaFePO and LiFeP, and predict a topology change of their Fermi surfaces due to the effect of correlations, with possible implications for their superconducting properties. In our last study, we close the circle by presenting LDA+DMFT calculations on an organic molecular crystal on the verge of a Mott metal-insulator transition; there, we find the spectral and optical properties to display signatures of strong electronic correlations beyond Fermi liquid theory.

- Electron-tunneling studies on CeCoIn5 heavy-fermion thin films and microstructures (2012)
- Investigation of low-temperature electronic properties of MBE grown CeCoIn5 and CeIn3 thin films by means of electron tunneling and quantum electron interference effects.