21 search hits
-
RF acceleration of intense laser generated proton bunches
(2012)
-
Ali Almomani
-
Dynamical effects and disorder in ultracold bosonic matter
(2012)
-
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.
-
Verification of Monte Carlo transport codes by activation experiments
(2012)
-
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.
-
Mechanisms of nanofractal structure formation and post-growth evolution
(2011)
-
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.
-
Density functional theory and dynamical mean field theory: applications to correlated electron materials
(2012)
-
Johannes Ferber
- 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)
-
Oleksandr Foyevtsov
- Investigation of low-temperature electronic properties of MBE grown CeCoIn5 and CeIn3 thin films by means of electron tunneling and quantum electron interference effects.
-
Investigation of the microscopic behavior of Mott insulators by means of the density functional theory and many-body methods
(2012)
-
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.
-
Emergent inert adjoint scalar field in SU(2) Yang-Mills thermodynamics due to coarse-grained topological fluctuation
(2011)
-
Ulrich Herbst
Ralf Hofmann
- 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.
-
J/psi Production in √s=7 TeV pp Collisions
(2012)
-
Frederick Kramer
- Quarkonia are very promising probes to study the quark-gluon plasma. The essential baseline
for measurements in heavy-ion collisions is high-precision data from proton-proton interactions.
However, the basic mechanisms of quarkonium hadroproduction are still being debated. The
most common models, the Color-Singlet Model, the non-relativistic QCD approach and the
Color-Evaporation Model, are able to describe most of the available cross-section data, despite
of their conceptual differences. New measures, such as the polarization, and data at a new
energy regime are crucial to test the competing models. Another issue is an eventual interplay
between the production process of a quarkonium state and the surrounding pp event. Current
Monte Carlo event generators treat the hard scattering independently from the rest of the
so-called underlying event. The investigation of possible correlations with the pp event might
be very valuable for a detailed understanding of the production processes.
ALICE ist the dedicated heavy-ion experiment at the LHC. Its design has been optimized for
high-precision measurements in very high track densities and down to low transverse momenta.
ALICE is composed of various different detectors at forward and at central rapidities. The most
important detectors for this study are the Inner Tracking System and the Time Projection
Chamber, allowing to reconstruct and identify electron candidate tracks within eta < 0.9. The
Transition Radiation Detector has not been utilized at this stage of the analysis; however, it
will strongly improve the particle identification and provide a dedicated trigger in the upcoming
beam periods. ...
-
Coulomb dissociation of 31Cl and 32Ar - constraining the rp process
(2012)
-
Christoph Langer
- The subject of this thesis aimed at a better understanding of the spectacular X-ray
burst. The most likely astrophysical site is a very dense neutron star, which accretes
H/He-rich matter from a close companion. While falling towards the neutron star, the
matter is heated up and a thermonuclear runaway is ignited. The exact description of
this process is dominated by the properties of a few proton-rich radioactive isotopes,
which have a low interaction probability, hence a high abundance.
The topic of this thesis was therefore an investigation of the short-lived, proton-rich
isotopes 31Cl and 32Ar. The Coulomb dissociation method is the modern technique of
choice. Excitations with energies up to 20 MeV can be induced by the Lorentz contracted
Coulomb field of a lead target. At the GSI Helmholtzzentrum für Schwerionenforschung
GmbH in Darmstadt, Germany, a Ar beam was accelerated to an energy of 825 AMeV
and fragmented in a beryllium target. The fragment separator was used to select the
desired isotopes with a remaining energy of 650 AMeV. They were subsequently directed
onto a 208 Pb target in the ALAND/LAND setup. The measurement was performed in
inverse kinematics. All reaction products were detected and inclusive and exclusive measurements of the respective Coulomb dissociation cross sections were possible.
During the analysis of the experiment, it was possible to extract the energy-differential
excitation spectrum of 31Cl, and to constrain astrophysically important parameters for
the time-reversed 30S(p,γ)31Cl reaction. A single resonance at 0.443(37) MeV dominates
the stellar reaction rate, which was also deduced and compared to previous calculations.
The integrated Coulomb dissociation cross section of this resonance was determined to
15(6) mb. The astrophysically important one- and two-proton emission channels were
analyzed for 32Ar and energy-differential excitation spectra could be derived. The integrated Coulomb dissociation cross section for two proton emission were determined
with two different techniques. The inclusive measurement yields a cross section of
214(29stat)(20sys) mb, whereas the exclusive reconstruction results in a cross section
of 226(14stat)(23sys) mb. Both results are in very good agreement. The Coulomb dissociation cross section for the one-proton emission channel is extracted solely from the
exclusive measurement and is 54(8stat)(6sys) mb.
Furthermore, the development of the Low Energy Neutron detector Array (LENA) for
the upcoming R3B setup is described. The detector will be utilized in charge-exchange
reactions to detect the low-energy recoil neutrons from (p,n)-type reactions. These reaction studies are of particular importance in the astrophysical context and can be used to
constrain half lifes under stellar conditions. In the frame of this work, prototypes of the detector were built and successfully commissioned in several international laboratories.
The analysis was supported by detailed simulations of the detection characteristics.
