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In der modernen Festkörperphysik spielen elektronisch stark korrelierte Systeme mit ihrem komplexen Vielteilchenverhalten eine zentrale Rolle. Insbesondere das Wechselspiel zwischen thermischen und Quantenfluktuationen in den Ladungs- und Spinfreiheitsgraden führt zur Entstehung verschiedenster neuartiger Grundzustände.
Die vorliegende Dissertation „Ultrasonic and Magnetic Investigations in frustrated Lowdimensional Spin Systems“ beschäftigt sich mit den besonderen physikalischen Eigenschaften niedrig dimensionaler Spinsysteme. Diese Materialklasse, die auch zu den stark korrelierten Systemen zählt, wird seit vielen Jahren intensiv sowohl experimentell als auch theoretisch untersucht. Auf theoretischer Seite sind die niedrigdimensionalen Spinsysteme besonders interessant, da sie als Modellsysteme die exakte Beschreibung des Grundzustandes und des Anregungsspektrums ermöglichen. Von experimenteller Seite ist es in den letzten Jahrzehnten gelungen, verschiedenste Materialklassen niedrigdimensionaler Spinsysteme zu synthetisieren.
In der vorliegenden Arbeit werden die grundlegenden Theorien und physikalischen Konzepte niedrigdimensionaler Spinsysteme diskutiert. Insbesondere auch die Spin-Phonon-Wechselwirkung dieser Materialien, die für die hier beobachteten elastischen Anomalien verantwortlich ist. Weiterhin wird auch das elastische Verhalten bei magnetischen Phasenübergängen beschrieben.
Da die Ultraschallexperimente einen Schwerpunkt dieser Arbeit bilden, wird der Versuchsaufbau zur phasenempfindlichen Detektion von Schallgeschwindigkeit und Ultraschalldämfung ausführlich beschrieben. Diese Messmethode ist ideal zur Untersuchung der Spin-Phonon Wechselwirkung geeignet.
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.
In this thesis I use effective models to investigate the properties of QCD-like theories at nonzero temperature and baryon chemical potential. First I construct a PNJL model using a lattice spin model with nearestneighbor interactions for the gauge sector and four-fermion interactions for the quarks in (pseudo)real representations of the gauge group. Calculating the phase diagram in the plane of temperature and quark chemical potential in QCD with adjoint quarks, it is qualitatively confirmed that the critical temperature of the chiral phase transition is much higher than the deconfinement transition temperature. At a chemical potential equal to half of the diquark mass in the vacuum, a diquark Bose–Einstein condensation (BEC) phase transition occurs. In the two-color case, a Ginzburg–Landau expansion is used to study the tetracritical behavior around the intersection point of the deconfinement and BEC transition lines which are both of second order. A compact expression for the expectation value of the Polyakov loop in an arbitrary representation of the gauge group is obtained for any number of colors, which allows us to study Casimir scaling at both nonzero temperature and chemical potential. Subsequently I study the thermodynamics of two-color QCD (QC2D) at high temperature and/or density using ZQCD, a dimensionally reduced superrenormalizable effective theory, formulated in terms of a coarse grained Wilson line. In the absence of quarks, the theory is required to respect the Z2 center symmetry, while the effects of quarks of arbitrary masses and chemical potentials are introduced via soft Z2 breaking operators. Perturbative matching of the effective theory parameters to the full theory is carried out explicitly, and it is argued how the new theory can be used to explore the phase diagram of two-color QCD.
Thermal expansion measurements provide a sensitive tool for exploring a material's thermodynamic
properties in condensed matter physics as they provide useful information
on the electronic, magnetic and lattice properties of a material. In this thesis, thermal
expansion measurements have been carried out both at ambient-pressure and under hydrostatic
pressure conditions. From the materials point of view, the spin-liquid candidate
Kappa-(BEDT-TTF) 2 Cu 2(CN)3 has been studied extensively as a function of temperature and
magnetic field. Azurite, Cu 3 (CO 3) 2 (OH) 2 - a realization of a one-dimensional distorted
Heisenberg chain is also studied both at ambient and hydrostatic pressure to demonstrate
the proper functioning of the newly built setup "thermal expansion under pressure". ...
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. ...
Interacting ultracold gases in optical lattices: non-equilibrium dynamics and effects of disorder
(2012)
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. ...
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.
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
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.