Universitätspublikationen
983 search hits
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Strongly correlated ultracold bosons in an optical lattice
(2012)
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Yongqiang Li
- In this thesis, we have investigated strongly correlated bosonic gases in an
optical lattice, mostly based on a bosonic version of dynamical mean field theory
and its real-space extension. Emphasis is put on possible novel quantum
phenomena of these many-body systems and their corresponding underlying
physics, including quantum magnetism, pair-superfluidity, thermodynamics,
many-body cooling, new quantum phases in the presence of long-range interactions,
and excitational properties. Our motivation is to simulate manybody
phenomena relevant to strongly correlated materials with ultracold lattice
gases, which provide an excellent playground for investigating quantum
systems with an unprecedented level of precision and controllability. Due to
their high controllability, ultracold gases can be regarded as a quantum simula-
tor of many-body systems in solid-state physics, high energy astrophysics, and
quantum optics. In this thesis, specifically, we have explored possible novel
quantum phases, thermodynamic properties, many-body cooling schemes, and
the spectroscopy of strongly correlated many-body quantum systems. The
results presented in this thesis provide theoretical benchmarks for exploring
quantum magnetism in upcoming experiments, and an important step towards
studying quantum phenomena of ultracold gases in the presence of long-range
interactions.
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Spontaneous Symmetry Breaking and Nambu–Goldstone Bosons in Quantum Many-Body Systems
(2010)
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Tomáš Brauner
- Spontaneous symmetry breaking is a general principle that constitutes the underlying concept of a vast number of physical phenomena ranging from ferromagnetism and superconductivity in condensed matter physics to the Higgs mechanism in the standard model of elementary particles. I focus on manifestations of spontaneously broken symmetries in systems that are not Lorentz invariant, which include both nonrelativistic systems as well as relativistic systems at nonzero density, providing a self-contained review of the properties of spontaneously broken symmetries specific to such theories. Topics covered include: (i) Introduction to the mathematics of spontaneous symmetry breaking and the Goldstone theorem. (ii) Minimization of Higgs-type potentials for higher-dimensional representations. (iii) Counting rules for Nambu–Goldstone bosons and their dispersion relations. (iv) Construction of effective Lagrangians. Specific examples in both relativistic and nonrelativistic physics are worked out in detail.
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Ultrasonic and magnetic investigations in frustrated low-dimensional spin systems
(2012)
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Thanh Cong Pham
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Electroweak quantum chemistry: Parity violation in spectra of chiral molecules containing heavy atoms
(2012)
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Sophie Nahrwold
- The intriguing effects of electroweak induced parity violation (PV) in molecules have yet to be observed, but experiments on molecular PV promise to provide fascinating insights. They potentially offer a novel testing ground for the low energy sector of the standard model and, in addition, a successful measurement of PV differences between the two enantiomers of a chiral molecule could promote a deeper understanding of molecular chirality, by essentially establishing a new link between particle physics and biochemistry. A key challenge in the design of such experiments is the identification of suitable molecules, which in turn requires widely applicable computational schemes for the prediction of PV experimental signals. To this end, a quasirelativistic density functional theory approach to the calculation of PV effects in nuclear magnetic resonance (NMR) spectra of chiral molecules has been developed and implemented during the course of this thesis. It includes relativistic as well as electron--correlation effects and has been used extensively in the screening of molecules possibly suited for a first observation of molecular PV. Some relevant compound classes have been identified, but none of their selected representatives are predicted to exhibit PV NMR frequency shifts that can be detected under current experimental restrictions. In order to advance the design of molecules which exhibit particularly large PV signals in experiments, systematic effects on PV NMR frequency splittings such as scaling with nuclear charge, conformational dependence and the impact of atomic substitution around the NMR active nucleus have been studied. Previously predicted scaling laws were confirmed and it was determined that the environment of the NMR active nucleus, both in terms of conformation and atomic composition, can be tuned to increase PV frequency shifts by several orders of magnitude. In addition to molecules suited for NMR experiments, a fascinating chiral actinide compound was studied with regard to PV frequency shifts in vibrational spectra. This compound displays the largest such shift ever predicted for an existing molecule, which lies well within the attainable experimental resolution. The challenge now lies in making it compatible with current experimental setups.
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RF acceleration of intense laser generated proton bunches
(2012)
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Ali Almomani
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J/psi Production in √s=7 TeV pp Collisions
(2012)
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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. ...
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Electron-tunneling studies on CeCoIn5 heavy-fermion thin films and microstructures
(2012)
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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.
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Emergent inert adjoint scalar field in SU(2) Yang-Mills thermodynamics due to coarse-grained topological fluctuation
(2011)
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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.
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Gas system, gas quality monitor and detector control of the ALICE Transition Radiation Detector and studies for a pre-trigger data read-out system
(2012)
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Nora Pitz
- The main purpose of the Transition Radiation Detector (TRD) located in the central
barrel of ALICE (A Large Ion Collider Experiment) is electron identification
for separation from pions at momenta pt > 1 GeV/c, since in this momentum range
the measurements of the specific energy loss (dE/dx) of the Time Projection Chamber
(TPC) is no longer sufficient. Furthermore, it provides a fast trigger for high
transverse momentum charged particles (pt > 3 GeV/c) and makes a significant
contribution to the optimization of the tracking of reaction products in heavy-ion
collisions. Its whole setup comprises 18 supermodules out of which 13 are presently
operational and mounted cylindrically around the beam axis of the Large Hadron
Collider (LHC). A supermodule contains either 30 or 24 chambers, each consisting of
a radiator for transition radiation creation, a drift and an amplifying region followed
by the read-out electronics. In total, the TRD is an array of 522 chambers operated
with about 28 m3 of a Xe-CO2 [85-15%] gas mixture.
During the work of this thesis, the testing, commissioning, operation and maintenance
of detector parts, the gas system and its online quality monitor, improvements
on the detector control user-interface and studies about a new pre-trigger module
for data read-out have been accomplished.
The TRD gas system mixes, distributes and circulates the operational gas mixture
through the detector. Its overall optimization has been achieved by minimizing gas
leakage, surveying, controlling, maintaining and continuously improving it as well
as designing and carrying out upgrades.
Gas quality monitors of the type \GOOFIE" (Gas prOportional cOunter For drIfting
Electrons) can be used in gaseous detectors as on-line monitors of the electron
drift velocity, gain and gas properties. One of these devices has been implemented
within the TRD gas system, while another one surveys the gas of the TPC. Both
devices had to be adapted to the specific needs of the detectors, were under constant
surveillance and control, and needed to be further developed on both hardware and
software side.
To improve the operation of the TRD, modifications on its DCS software (Detector
Control System) used for monitoring, controlling, operating, regulating and configuring of hardware and computing devices have been carried out. The DCS is
designed to enable an operator to interact with equipment through user interfaces
that display the information from the system. The main focus of this work was laid
on the optimization of the usability and design of the user interface.
The front-end electronics of the TRD require an early start signal (\pre-trigger")
from the fast forward detectors or the Time-Of-Flight detector during the running
periods. The realization of a new hardware concept for the read-out of the TRD
pre-trigger system has been studied and first tests were performed. This new module
called PIMDDL (Pre-trigger Interface Module Detector Data Link) is meant to
acquire all data necessary to simulate and predict the full pre-trigger functionality,
and to verify its proper operation. Furthermore, it shall provide all functionalities of
the so-called Control Box Bottom as well as keep the functionalities of the already
existing PIM (Pre-trigger Interface Module) in order to combine and replace these
two modules in the future.
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Directed deposition of silicon nanowires using neopentasilane as precursor and gold as catalyst
(2012)
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Britta Kämpken
Verena Wulf
Norbert Auner
Marcel Winhold
Michael Huth
Daniel Rhinow
Andreas Terfort
- In this work the applicability of neopentasilane (Si(SiH3)4) as a precursor for the formation of silicon nanowires by using gold nanoparticles as a catalyst has been explored. The growth proceeds via the formation of liquid gold/silicon alloy droplets, which excrete the silicon nanowires upon continued decomposition of the precursor. This mechanism determines the diameter of the Si nanowires. Different sources for the gold nanoparticles have been tested: the spontaneous dewetting of gold films, thermally annealed gold films, deposition of preformed gold nanoparticles, and the use of “liquid bright gold”, a material historically used for the gilding of porcelain and glass. The latter does not only form gold nanoparticles when deposited as a thin film and thermally annealed, but can also be patterned by using UV irradiation, providing access to laterally structured layers of silicon nanowires.
