27 search hits
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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. ...
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Thermal expansion studies on low-dimensional frustrated quantum magnets: the case of Kappa-(BEDT-TTF) 2 Cu 2 (CN) 3 and azurite
(2012)
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Rudra Sekhar Manna
- 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". ...
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Study of QCD-like theories at nonzero temperatures and densities
(2012)
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Tian Zhang
- 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.
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Coulomb dissociation of 31Cl and 32Ar - constraining the rp process
(2012)
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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.
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Shedding light on reaction mechanisms : structure determination of reactive intermediates and investigation of protein structural dynamics using 2D-IR spectroscopy
(2012)
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Andreas T. Meßmer
- Detailed knowledge of reaction mechanisms is key to understanding chemical, biological, and biophysical processes. For many reasons, it is desirable to comprehend how a reaction proceeds and what influences the reaction rate and its products.
In biophysics, reaction mechanisms provide insight into enzyme and protein function, the reason why they are so efficient, and what determines their reaction rates. They also reveal the relationship between the function of a protein and its structure and dynamics.
In chemistry, reaction mechanisms are able to explain side products, solvent effects, and the stereochemistry of a product. They are also the basis for potentially optimizing reactions with respect to yield, enhancing the stereoselectivity, or for modifying reactions in order to obtain other related products.
A key step to investigate reaction mechanisms is the identification and characterization of intermediates, which may be reactive, short-lived, and therefore only weakly populated. Nowadays, the structures of those can in most cases only be hypothesized based on products, side products, and isolable intermediates, because intermediates with a life time of less than a few microseconds are not accessible with the commonly used techniques for structure determination such as X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy.
In this thesis, two-dimensional infrared (2D-IR) spectroscopy is shown to be a powerful complement to the existing techniques for structure determination in solution. 2D-IR spectroscopy uses a femtosecond laser setup to investigate interactions between vibrations - analogous to 2D-NMR, which investigates the interactions between spins. Its ultrafast time resolution makes 2D-IR spectroscopy particularly well suited for the two topics investigated in this thesis: Structure Determination of Reactive Intermediates and Conformational Dynamics of Proteins.
Structure Determination of Reactive Intermediates: The focus of this thesis is using polarization-dependent 2D-IR (P2D-IR) spectroscopy for structure determination of N-crotonyloxazolidinone (referred to as 1), a small organic compound with a chiral oxazolidinone, known as Evans auxiliary, and its reactive complexes with the Lewis acids SnCl4 and Mg(ClO4)2. Chiral oxazolidinones in combination with Lewis acids have frequently been used in stereoselective synthesis for over 30 years. Nevertheless, the detailed mechanisms are in many cases xvi ABSTRACT still mere hypotheses and have not yet been experimentally proven. By accurately measuring the angles between the transition dipole moments in the molecules using an optimized P2D-IR setup and comparing the results to DFT calculations, the conformation of 1 and the conformation and coordination of the main complexes with SnCl4 and Mg(ClO4)2 are unequivocally identified and analyzed in depth. Structural details, such as a slight twist in the solution structure of 1, are detected using P2D-IR spectroscopy; these cannot be inferred from NMR spectroscopy or DFT calculations. In addition to the main Lewis acid complexes, complexes in low concentration are detected and tentatively assigned to different conformations and complexation geometries. The knowledge of those structures is essential for rationalizing the observed stereoselectivities. Additionally, a method is introduced that enables structure determination of molecules in complex mixtures and even in the presence of molecules with similar spectral properties and in high concentration. This work sets the stage for future studies of other substrate-catalyst complexes and reaction intermediates for which the structure determination has not been possible to date.
Conformational Dynamics of Proteins: Exchange 2D-IR spectroscopy allows the investigation of fast dynamics without disturbing the equilibrium of the exchanging species. It is therefore well suited to investigate fast dynamics of proteins and to reveal the speed limit of those. The temperature dependence of the conformational dynamics between the myoglobin substates A1 and A3 in equilibrium is analyzed. The various substates of myoglobin can be detected with FTIR spectroscopy, if carbon monoxide is bound to the heme. From previous studies it is known that the exchange rates at room temperature are in the picosecond time range, well suited to be investigated by 2D-IR spectroscopy. In the temperature range between 0 °C and 40 °C only a weak temperature dependence of the exchange rate in the myoglobin mutant L29I is observed in the present study. The exchange rate approximately doubles from 15 ns-1 at 0 °C to 31 ns-1 at 40 °C. It turned out that the conformational dynamics correlates linearly with the solvent viscosity, which itself is temperature dependent. Comparing our results to measurements at cryogenic temperatures, the linear relation between exchange time constant for this process and the viscosity is shown for the temperature range between -100 °C and 40 °C (corresponding to a viscosity change of 14 orders of magnitude). Thus, it is proven that the dynamics of the conformational switching are mainly determined by solvent dynamics, i.e., the protein dynamics are slaved to the solvent dynamics. This is the first time slaving is observed for such fast processes (in the picosecond time range). The observation implies a long-range structural rearrangement between the myoglobin substates A1 and A3. In addition, the exchange for other mutants and wild type myoglobin is analyzed qualitatively and found to agree with the conclusions drawn from L29I myoglobin.
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Resonant pickups for non-destructive single-particle detection in heavy-ion storage rings and first experimental results
(2013)
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Mohammad Shahab Sanjari
