25 search hits
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Verification of Monte Carlo transport codes by activation experiments
(2012)
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Vera Chetvertkova
- 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.
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Dynamical effects and disorder in ultracold bosonic matter
(2012)
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Ulf Bissbort
- 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.
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Mechanisms of nanofractal structure formation and post-growth evolution
(2011)
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Veronika V. Dick
- 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.
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Prototyp-Radiatoren eines Barrel-DIRC für das PANDA-Experiment
(2011)
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Roland Hohler
- Das Antiprotonen-Experiment PANDA an der zukünftigen Beschleunigeranlage FAIR
wird unter anderem Charmonium-Zustände mit einer bis dato unerreichten
Genauigkeit messen können. Um dieses Ziel zu erreichen, wird eine sehr gute
Teilchenidentifikationsfähigkeit verlangt. Eine gute Trennung zwischen Pionen
und Kaonen wird durch den Einsatz eines Cherenkovdetektors erreicht. Die
Leistungsfähigkeit eines DIRC hängt von dessen Radiatorgüte ab. Um die
Qualität der Radiatorstäbe spezifizieren zu können, wurde im Rahmen dieser
Doktorarbeit eine optische Messapparatur entwickelt. Dieser Aufbau erlaubt es
die Transmission sowie die Oberflächenrauheit der Stäbe zu messen. Es wurden
mehrere Radiatorstäbe aus synthetischem Quarzglas und Acrylglas untersucht. Die
Messgenauigkeit bei hochqualitativen Stäben liegt für die Transmissionsmessung
bei etwa 1 Promille und für die Rauheit bei 1-2 Angström. Die Messergebnisse bei
verschiedenen Wellenlängen zeigen eine gute Übereinstimmung mit der skalaren
Streutheorie, die den Zusammenhang zwischen Reflexionskoeffizienten und Rauheit
beschreibt. Bei einer Strahlzeit an der GSI mit einem 2 GeV Protonenstrahl wurde
ein erster Prototyp für den Barrel-DIRC mit einem Stab aus synthetischem
Quarzglas als Radiator getestet. Durch Variation des Einfallswinkels und der
Position des Protonenstrahls auf dem Radiator konnten Cherenkovringe eindeutig
nachgewiesen werden. Zudem wurde der Cherenkovwinkel und die
Einzelphotonauflösung in guter Übereinstimmung mit dem Erwartetem und der
Simulation bestimmt.
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Die Untersuchung der Ionisationsdynamik von Heliumdimeren in Stößen mit Alpha-Teilchen
(2011)
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Jasmin Titze
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Investigation of the microscopic behavior of Mott insulators by means of the density functional theory and many-body methods
(2012)
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Kateryna Foyevtsova
- The objective of this work is twofold. First, we explore
the performance of the density functional theory (DFT)
when it is applied to solids with strong electronic correlations, such
as transition metal compounds. Along this direction, particular effort is put
into the refinement and development of parameterization techniques
for deriving effective models on a basis of DFT calculations.
Second, within the framework of the DFT, we address
a number of questions related to the physics of Mott insulators,
such as magnetic frustration and electron-phonon coupling (Cs2CuCl4 and Cs2CuBr4), high-temperature superconductivity (BSCCO) and doping of Mott insulators (TiOCl).
In the frustrated antiferromagnets Cs2CuCl4 and Cs2CuBr4,
we investigate the interplay between strong electronic
correlations and magnetism on one hand and electron-lattice coupling
on the other as well as the effect of this interplay on the microscopic model parameters.
Another object of our investigations
is the oxygen-doped cuprate superconductor BSCCO,
where nano-scale electronic inhomogeneities have been
observed in scanning tunneling spectroscopy experiments.
By means of DFT and many-body calculations, we analyze the connection
between the structural and electronic inhomogeneities and the superconducting
properties of BSCCO.
We use the DFT and molecular dynamic simulations
to explain the microscopic origin of the persisting under doping
Mott insulating state in the layered compound TiOCl.
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The production of _j63 [eta] and _w63 [omega] mesons in 3.5 GeV p+p interaction in HADES
(2011)
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Khaled Teilab
- 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
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A numerical renormalization group approach to dissipative quantum impurity systems
(2011)
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David Roosen
- 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 proposal 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.
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Interacting ultracold gases in optical lattices: non-equilibrium dynamics and effects of disorder
(2012)
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Julia Wernsdorfer
- This dissertation aims at giving a theoretical description of various applications of ultracold
gases. A particular focus is cast upon the dynamical evolution of bosonic condensates
in non-equilibrium by means of the time-dependent Gutzwiller method. Ground state
properties of strongly interacting fermionic atoms in box and speckle disordered lattices
are investigated via real-space dynamical mean-field theory. ...
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Strongly correlated ultracold gases in disordered optical lattices
(2012)
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Denis Semmler
- Seit Anbeginn der Festkörperphysik ist die Frage, warum manche Materialien
metallisch sind, andere dagegen isolierend, von zentraler Bedeutung. Eine erste
Erklärung wurde durch die Bändertheorie [23, 44] gegeben. Die Elektronen
sind dem periodischen Potential der Rumpfatome ausgesetzt, wodurch ein
Energiespektrum bestehend aus Bändern erzeugt wird und die Füllung dieser
Bänder bestimmt die Leitungseigenschaften des Festkörpers. ...
