21 search hits
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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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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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Commissioning of the ALICE High-Level Trigger
(2012)
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Jochen Thäder
- A new era in experimental nuclear physics has begun with the start-up of the
Large Hadron Collider at CERN and its dedicated heavy-ion detector system
ALICE. Measuring the highest energy density ever produced in nucleus-nucleus
collisions, the detector has been designed to study the properties of the created
hot and dense medium, assumed to be a Quark-Gluon Plasma.
Comprised of 18 high granularity sub-detectors, ALICE delivers data from
a few million electronic channels of proton-proton and heavy-ion collisions.
The produced data volume can reach up to 26 GByte/s for central Pb–Pb
collisions at design luminosity of L = 1027 cm−2 s−1 , challenging not only the
data storage, but also the physics analysis. A High-Level Trigger (HLT) has
been built and commissioned to reduce that amount of data to a storable value
prior to archiving with the means of data filtering and compression without the
loss of physics information. Implemented as a large high performance compute
cluster, the HLT is able to perform a full reconstruction of all events at the time
of data-taking, which allows to trigger, based on the information of a complete
event. Rare physics probes, with high transverse momentum, can be identified
and selected to enhance the overall physics reach of the experiment.
The commissioning of the HLT is at the center of this thesis. Being deeply
embedded in the ALICE data path and, therefore, interfacing all other ALICE
subsystems, this commissioning imposed not only a major challenge, but also a
massive coordination effort, which was completed with the first proton-proton
collisions reconstructed by the HLT. Furthermore, this thesis is completed with
the study and implementation of on-line high transverse momentum triggers.
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Density functional theory and dynamical mean field theory: applications to correlated electron materials
(2012)
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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.
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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.
