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Institute
- Physik (3566) (remove)
We study the light scalar mesons a_0(980) and kappa using N_f = 2+1+1 flavor lattice QCD. In order to probe the internal structure of these scalar mesons, and in particular to identify, whether a sizeable tetraquark component is present, we use a large set of operators, including diquark-antidiquark, mesonic molecule and two-meson operators. The inclusion of disconnected diagrams, which are technically rather challenging, but which would allow us to extend our work to e.g. the f_0(980) meson, is introduced and discussed.
Electron beam-induced deposition with tungsten hexacarbonyl W(CO)6 as precursors leads to granular deposits with varying compositions of tungsten, carbon and oxygen. Depending on the deposition conditions, the deposits are insulating or metallic. We employ an evolutionary algorithm to predict the crystal structures starting from a series of chemical compositions that were determined experimentally. We show that this method leads to better structures than structural relaxation based on estimated initial structures. We approximate the expected amorphous structures by reasonably large unit cells that can accommodate local structural environments that resemble the true amorphous structure. Our predicted structures show an insulator-to-metal transition close to the experimental composition at which this transition is actually observed and they also allow comparison with experimental electron diffraction patterns.
A careful analysis of the magneto-transport properties of epitaxial nanostructured Nb thin films in the normal and the mixed state is performed. The nanopatterns were prepared by focused ion beam (FIB) milling. They provide a washboard-like pinning potential landscape for vortices in the mixed state and simultaneously cause a resistivity anisotropy in the normal state. Two matching magnetic fields for the vortex lattice with the underlying nanostructures have been observed. By applying these fields, the most likely pinning sites along which the flux lines move through the samples have been selected. By this, either the background isotropic pinning of the pristine film or the enhanced isotropic pinning originating from the nanoprocessing have been probed. Via an Arrhenius analysis of the resistivity data the pinning activation energies for three vortex lattice parameters have been quantified. The changes in the electrical transport and the pinning properties have been correlated with the results of the microstructural and topographical characterization of the FIB-patterned samples. Accordingly, along with the surface processing, FIB milling has been found to alter the material composition and the degree of disorder in as-grown films. The obtained results provide further insight into the pinning mechanisms at work in FIB-nanopatterned superconductors, e.g. for fluxonic applications.
In dieser Arbeit wurden Verfahren zur Identifikation hirnelektrischer Aktivität mit Zellularen Nichtlinearen Netzwerken (CNN), im Besonderen Reaktions-Diffusions-Netzwerken, entwickelt und untersucht. Mit Hilfe der eingeführten Methoden wurden Langzeitaufzeichnungen hirnelektrischer Aktivität bei Epilepsie analysiert und mittels eines automatisierten Verfahrens ermittelt, inwieweit sich mögliche Voranfallszustände vom anfallsfreien Zustand im statistischen Sinne trennen lassen.
Zunächst wurde ein Überblick über CNN gegeben und deren Beschreibung durch Systeme gekoppelter Differentialgleichungen dargestellt. Weiterhin wurden die Möglichkeiten der Informationsverarbeitung mit CNN durch Ausnutzung von Gleichgewichtszuständen oder der vollständigen raum-zeitlichen Dynamik der Netzwerke diskutiert. Zusätzlich wurde die Klasse der Reaktions-Diffusions-Netzwerke (RD-CNN) eingeführt. Für die Repräsentation der hierbei benötigten weitgehend allgemeinen nichtlinearen Zellkopplungsvorschriften wurden polynomiale Gewichtsfunktionen vorgeschlagen. Mit einer Darstellung der Theorie der Lokalen Aktivität wurden notwendige Bedingungen für emergentes Verhalten in RD-CNN angegeben. Die statistische Bewertung von Vorhersagemodellen wurde aus theoretischer Sicht beleuchtet. Mit der Receiver Operating Characteristic (ROC) wurde eine Analysemethode zur Beurteilung der Vorhersagekraft des zeitlichen Verlaufs von Kenngrößen bezüglich bevorstehender epileptischer Anfälle vorgestellt.
Als nächstes wurden Überlegungen zur numerischen Simulation von CNN und deren flexible und erweiterbare programmtechnische Umsetzung entwickelt. Die daraus resultierende und im Rahmen dieser Arbeit entstandene objektorientierte Simulationsumgebung FORCE++ wurde konzeptionell und im Hinblick auf die Softwarearchitektur vorgestellt.
Die Verfahren zur numerischen Simulation wurden auf die Problemstellung der Systemidentifikation mit CNN angewandt. Dazu wurden Netzwerke derart bestimmt, dass deren Zellausgangswerte entsprechende Signalwerte des beobachteten, zu identifizierenden Systems approximieren.
Da die Parameter der zu bestimmenden CNN im vorliegenden Fall der Untersuchung hirnelektrischer Aktivität nicht bekannt sind und nicht direkt abgeleitet werden können, wurden überwachte Lernverfahren zur Bestimmung der Netzwerke eingesetzt. Hierbei wurden Lernverfahren verschiedener Klassen für die Identifikation mit CNN mit polynomialen Gewichtsfunktionen untersucht. Die Leistungsfähigkeit des vorgestellten Identifikationsverfahrens wurde anhand bekannter Systeme einer genauen Betrachtung unterzogen. Dabei wurde festgestellt, dass die betrachteten Systeme mit hoher Genauigkeit durch CNN repräsentiert werden konnten. Exemplarisch wurde das Parametergebiet lokaler Aktivität für ein RD-CNN berechnet und durch numerische Simulationen die Ausbildung von Mustern innerhalb des Netzwerkes nachgewiesen.
Nach einem einleitenden Überblick über die medizinischen Hintergründe von Epilepsie und der Erfassung hirnelektrischer Aktivität wurde eine vergleichende Übersicht über den Stand veröffentlichter Studien zur Vorhersage epileptischer Anfälle gegeben. Für die Anwendung des hier vorgestellten Identifikationsverfahrens zur Analyse hirnelektrischer Aktivität wurde zunächst die Genauigkeit der Approximation kurzer, als quasi-stationär betrachteter Abschnitte, von EEG-Signalen untersucht. Durch gezielte Erhöhung der Komplexität herangezogener Netzwerke konnte hier die Genauigkeit der Repräsentation von EEG-Signalverläufen deutlich verbessert werden. Dabei wurde zudem die Verallgemeinerungsfähigkeit der ermittelten Netzwerke untersucht, wobei festgestellt wurde, dass auch solche Signalwerte mit guter Genauigkeit approximiert werden, die nicht im Identifikationsverfahren durch die überwachte Parameteroptimierung berücksichtigt waren. Um speziell den Einfluss der Information aus der Korrelation benachbarter Elektrodensignale zu untersuchen, wurde ein Verfahren zur multivariaten Prädiktion mit Discrete Time CNN (DT-CNN) entwickelt.
Hierbei werden durch ein CNN Signalwerte der betrachteten Elektrode aus vergangenen, korrelierten Signalwerten von Nachbarelektroden geschätzt. Für diese Aufgabenstellung konnte eine Methode zur Bestimmung der Netzwerkparameter im optimalen Sinn, alleine aus den statistischen Eigenschaften der Elektrodensignale angegeben werden. Dadurch gelang eine erhebliche Reduzierung der Rechenkomplexität, die eine umfangreiche Untersuchung intrakranieller Langzeitableitungen ermöglichte.
Zur Analyse von Langzeitaufzeichnungen mit dem RD-CNN Identifikationsverfahren, wurden die numerischen Berechnungen zur Simulation von CNN mit FORCE++ auf einem durchsatz-orientierten Hochleistungs-Rechnernetzwerk durchgeführt. Mit den so gewonnen Ergebnissen konnten vergleichende Analysen vorgenommen werden. Zudem wurden Untersuchungen zum Vorliegen lokaler Aktivität in den ermittelten RD-CNN durchgeführt.
Die bei den beschriebenen Verfahren extrahierten Kenngrößen hirnelektrischer Aktivität wurden durch ein automatisiertes Verfahren auf ihre Vorhersagekraft für epileptische Anfälle bewertet. Dabei wurde untersucht, inwieweit der anfallsfreie Zustand und ein angenommener Voranfallszustand durch die jeweils betrachtete Kenngröße im statistischen Sinn diskriminiert werden kann. Durch parallele Analysen mit Anfallszeitsurrogaten wurden hierzu ergänzende Signifikanztests durchgeführt.
Nach Auswertung von mehrtägigen Hirnstromsignalen verschiedener Patienten konnte festgestellt werden, dass mit den in dieser Arbeit entwickelten Verfahren Kenngrößen hirnelektrischer Aktivität bestimmt werden konnten, welche offenbar die Identifikation potentieller Voranfallszustände ermöglichen.
Auch wenn für eine breite medizinische Anwendung die Spezifität und Sensitivität noch weiter verbessert werden muss, so können doch die erzielten Ergebnisse einen wesentlichen Schritt hin zu einer implantierbaren, CNN-basierten Plattform zur Erkennung und Verhinderung epileptischer Anfälle darstellen. Die Berechnungen für das Identifikationsverfahren mit RD-CNN könnten dabei durch zukünftige, spezialisierte schaltungstechnische Realisierungen für mehrschichtige CNN mit polynomialen Gewichtsfunktionen eine erhebliche Beschleunigung erfahren.
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.
8th International Conference on Nuclear Physics at Storage Rings Stori11, October 9-14, 2011 Laboratori Nazionale di Frascati, Italy.
Storage rings offer the possibility of measuring proton- and alpha-induced reactions in inverse kinematics. The combination of this approachwith a radioactive beamfacility allows, in principle, the determination of the respective cross sections for radioactive isotopes. Such data are highly desired for a better understanding of astrophysical nucleosynthesis processes like the p-process. A pioneering experiment has been performed at the Experimental Storage Ring (ESR) at GSI using a stable 96Ru beam at 9-11 AMeV and a hydrogen target. Monte-Carlo simulations of the experiment were made using the Geant4 code. In these simulations, the experimental setup is described in detail and all reaction channels can be investigated. Based on the Geant4 simulations, a prediction of the shape of different spectral components can be performed. A comparison of simulated predictions with the experimental results shows a good agreement and allows the extraction of the cross section.
The development of a non- destructive measurement method for ion beam parameters has been treated in various projects. Although results are promising, the high complexity of beam dynamics has made it impossible to implement a real time process control up to now. In this paper we will propose analysing methods based on the dynamics of Cellular Nonlinear Networks (CNN) that can be implemented on pixel parallel CNN based architectures and yield satisfying results even at low resolutions.
After five years of running at RHIC, and on the eve of the LHC heavy-ion program, we highlight the status of femtoscopic measurements. We emphasize the role interferometry plays in addressing fundamental questions about the state of matter created in such collisions, and present an enumerated list of measurements, analyses and calculations that are needed to advance the field in the coming years.
Abrasion-ablation models and the empirical EPAX parametrization of projectile fragmentation are described. Their cross section predictions are compared to recent data of the fragmentation of secondary beams of neutron-rich, unstable 19,20,21O isotopes at beam energies near 600 MeV/nucleon as well as data for stable 17,18O beams.
The TATA Box Binding Protein (TBP) is a 20 kD protein that is essential and universally conserved in eucarya and archaea. Especially among archaea, organisms can be found that live below 0°C as well as organisms that grow above 100°C. The archaeal TBPs show a high sequence identity and a similar structure consisting of α-helices and β-sheets that are arranged in a saddle-shape 2-symmetric fold. In previous studies, we have characterized the thermal stability of thermophilic and mesophilic archaeal TBPs by infrared spectroscopy and showed the correlation between the transition temperature (Tm) and the optimal growth temperature (OGT) of the respective donor organism. In this study, a “new” mutant TBP has been constructed, produced, purified and analyzed for a deeper understanding of the molecular mechanisms of thermoadaptation. The β-sheet part of the mutant consists of the TBP from Methanothermobacter thermoautotrophicus (OGT 65°C, MtTBP65) whose α-helices have been exchanged by those of Methanosarcina mazei (OGT 37°C, MmTBP37). The Hybrid-TBP irreversibly aggregates after thermal unfolding just like MmTBP37 and MtTBP65, but the Tm lies between that of MmTBP37 and MtTBP65 indicating that the interaction between the α-helical and β-sheet part of the TBP is crucial for the thermal stability. The temperature stability is probably encoded in the variable α-helices that interact with the highly conserved and DNA binding β-sheets.
The CBM experiment will investigate heavy-ion collisions at beam energies from 8 to 45 AGeV at the future accelerator facility FAIR. The goal of the experiment is to study the QCD phase diagram in the vincinity of the QCD critical point. To do so, CBM aims at measuring rare probes among them open charm. In order to identify those rare and short lived particles despite the rich combinatorial background generated in heavy ion collisions, a micro vertex detector (MVD) providing an unprecedented combination of high rate capability and radiation hardness, very light material budget and excellent granularity is required. In this work, we will discuss the concept of this detector and summarize the status of the R&D.
CMOS sensors are the most promising candidates for the Micro-Vertex-Detector (MVD) of the CBM experiment at GSI, as they provide an unprecedented compromise between spatial resolution, low material budget, adequate radiation tolerance and readout speed. To study the integration of these sensors into a detector module, a so-called MVD-demonstrator has been developed. The demonstrator and its in-beam performance will be presented and discussed in this work.
We have developed a versatile software package for the simulation of di-electron production in pp and dp collisions at moderate beam kinetic energies (1-2GeV). Particular attention has been paid to incorporate different descriptions of the Dalitz decay Δ rightarrow Ne + e - via a common interface. In addition, suitable parameterizations for the virtual bremsstrahlung process NN rightarrow NNe + e - based on one-boson exchange models have been implemented. Such simulation tools with high flexibility of the framework are important for the interpretation of the di-electron data taken with the HADES spectrometer and demonstrates the wide applicability within the field of nuclear and hadronic physics.
Proteomic profiles of myocardial tissue in two different etiologies of heart failure were investigated using high performance liquid chromatography (HPLC)/Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Right atrial appendages from 10 patients with hemodynamically significant isolated aortic valve disease and from 10 patients with isolated symptomatic coronary heart disease were collected during elective cardiac surgery. As presented in an earlier study by our group (Baykut et al., 2006), both disease forms showed clearly different pattern distribution characteristics. Interesting enough, the classification patterns could be used for correctly sorting unknown test samples in their correct categories. However, in order to fully exploit and also validate these findings there is a definite need for unambiguous identification of the differences between different etiologies at molecular level. In this study, samples representative for the aortic valve disease and coronary heart disease were prepared, tryptically digested, and analyzed using an FT-ICR MS that allowed collision-induced dissociation (CID) of selected classifier masses. By using the fragment spectra, proteins were identified by database searches. For comparison and further validation, classifier masses were also fragmented and analyzed using HPLC-/Matrix-assisted laser desorption ionization (MALDI) time-of-flight/time-of-flight (TOF/TOF) mass spectrometry. Desmin and lumican precursor were examples of proteins found in aortic samples at higher abundances than in coronary samples. Similarly, adenylate kinase isoenzyme was found in coronary samples at a higher abundance. The described methodology could also be feasible in search for specific biomarkers in plasma or serum for diagnostic purposes.
We study electron transport through a single-molecule magnet (SMM) and the interplay of its anisotropic spin with quantized vibrational distortions of the molecule. Based on numerical renormalization group calculations we show that, despite the longitudinal anisotropy barrier and small transverse anisotropy, vibrational fluctuations can induce quantum spin-tunneling (QST) and a QST-Kondo effect. The interplay of spin scattering, QST and molecular vibrations can strongly enhance the Kondo effect and induce an anomalous magnetic field dependence of vibrational Kondo side-bands.
Directed deposition of silicon nanowires using neopentasilane as precursor and gold as catalyst
(2012)
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.
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.
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.
Zellulare Nichtlineare Netzwerke bzw. Zellulare Neuronale Netzwerke, sogenannte CNN, wurden 1988 von L.O. Chua und L.Yang eingeführt und seither intensiv untersucht. Diese sind als Simulations-Software und als schaltungstechnische Realisierungen, in Hardware, verfügbar.
Als analog arbeitende Hardware Schaltungen können diese Netzwerke erhebliche Rechenleistungen erzielen.
Durch ihren Aufbau ermöglichen sie eine parallele Daten- und Signalverarbeitung.
Eine Einführung in CNN wird gegeben und das EyeRIS 1.1 Systems des Unternehmens ANAFOCUS Ltd. vorgestellt.
Das EyeRIS 1.1 System ist mit einem analog arbeitenden Focal Plane Prozessor (FPP) und einem digitalen Prozessor ausgestattet, wobei der Focal Plane Prozessor auch als Kamera zur Aufnahme von Bildern und Bildsequenzen benutzt werden kann.
Dies ermöglicht es, analoge CNN-Algorithmen zusammen mit digitalen Algorithmen auf einem System zu implementieren und so die Vorteile beider Ansätze zu nutzen. Der Datenaustausch zwischen dem analogen und digitalem Teil des EyeRIS 1.1 Systems geschieht mittels digital/analog und analog/digital Wandlung. Es werden Algorithmen auf dem EyeRIS 1.1 System untersucht und mit Ergebnissen die mittels Simulationen erzeugt wurden verglichen.
In Voruntersuchungen werden die Darstellungsgenauigkeit von Werten im analogen Teil des EyeRIS 1.1 Systems und die Verarbeitungsgeschwindigkeiten des EyeRIS 1.1 Systems untersucht.
Im Weiteren wird besonderes Augenmerk auf medizinische und technische Anwendungsgebiete gelegt werden.
Im medizinischen Anwendungsbereich wird die Implementierung von Algorithmen zur Vorhersage epileptischer Anfälle untersucht.
Hierfür wird ein evolutionär motiviertes Optimierungsverfahren entwicklet und auf dem EyeRIS 1.1-System implementiert.
Hierbei werden Simulationen durchgeführt und mit Ergebnissen, die mittels Verwendung des EyeRIS 1.1 Systems erlangt wurden, verglichen.
Ein zweites Verfahren geht die Signalanalyse für die Vorhersage auf dem EyeRIS 1.1-System mittels Mustererkennung an.
Das Mustererkennungsverfahren wird eingehend beschrieben sowie die hierbei zu beachtenden Randbedingungen erläutert.
Die Ergebnisse zeigen, daß Algorithmen zur Vorhersage von epileptischen Anfällen auf schaltungstechnichen Realisierungen von CNN implementiert werden können.
Im technischen Bereich wird die Anwendbarkeit auf die Problemstellung der Bildverarbeitung gelegt und die Möglichkeit von CNN basierten Algorithmen zur Erkennung von Prozessparametern bei Laserschweißverfahren untersucht. Ein solcher Prozessparameter ist das sogenannte Key-Hole, welches in Bildsequenzen von Laserschweißprozessen als ein Maß für die zu erwartende Qualität einer Schweißnaht herangezogen werden kann. Ein CNN basierter Algorithmus für die Erkennung solcher Key-Holes wird in dieser Arbeit vorgestellt und untersucht.
Für die Überwachung eines Laserschweißverfahrens wird der entwickelte Algorithmius und seine Funktionsweise beschrieben.
Dieser wird in Teilalgorithmen auf die analog bzw. digital arbeitenden Komponenten des EyeRIS 1.1 Systems verteilt.
Die Teilalgorithmen und die möglichen Aufteilungen und deren Laufzeitverhalten werden beschrieben und untersucht.
Die Ergebnisse der Untersuchung zeigen, daß eine Prozessüberwachung mittels CNN möglich ist und heben die Vorteile hervor, welche die Bildaufnahme und -verarbeitung mittels analoger CNN-Hardware bietet.
Eine Untersuchung des Laufzeitverhaltens auf Grafikkarten Prozessoren (GPU's) wird im Anhang vorgestellt.
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. ...
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.
In den letzten Jahren haben die Forschungsaktivitäten im Bereich Thermoelektrik stetig zugenommen. Das neu erweckte Interesse an der Thermoelektrik ist zurückzuführen auf neue nanostrukturierte Materialien, Quantenschicht-Strukturen und Nanodrähte, welche
eine wesentliche Steigerung der thermoelektrischen Effektivität Z im Vergleich zum Massivmaterial versprechen. Für Nanodrähte ist die größte Steigerung der thermoelektrischen Effektivität zu erwarten. Zur Bestätigung der Theorie bedarf es neuer Messmethoden zur Bestimmung des Seebeck-Koeffizienten S, der elektrischen Leitfähigkeit σ und der Wärmeleitfähigkeit λ, um hieraus eine Steigerung der thermoelektrischen Effektivität Z = (Sexp2)σ/λ experimentell zu bestätigen.
Der Schwerpunkt der Doktorarbeit lag in der Untersuchung thermoelektrischer Eigenschaften von Nanodrähten. Hierzu wurden neueMessmethoden zur Bestimmung der elektrischen und thermischen Leitfähigkeit von Nanodrähten entwickelt.
Die elektrische und thermische Leitfähigkeit von Pt-Nanodrähten wurden mit dem in dieser Arbeit entwickelten λ-Chip gemessen. Die elektrische Leitfähigkeit der Pt-Nanodrähte ist im Vergleich zum Massivmaterial entsprechend der klassischen Size-Effekt-Theorie reduziert. Ebenso wurde eine Abnahme der Wärmeleitfähigkeit beobachtet. Die Ergebnisse stimmen mit den im Rahmen der klassischen Size-Effekt-Theorie zu erwartenden Resultaten gut überein, jedoch bedarf die Reduzierung der Lorenz-Zahl noch einer theoretischen Erklärung.
Im Weiteren wurde die elektrische Leitfähigkeit von BixTe1-x und BixSb1-x-Nanodrähten mit dem λ-Chip bestimmt. Hierzu wurden zunächst unterschiedliche Kontaktmaterialien getestet, um die Diffusion des Kontaktmaterials in den Nanodraht auszuschließen. Als bewährtes Kontaktmaterial stellte sich ein Schichtsystem aus Titan und Gold heraus. Die Ti-Schicht wirkt hierbei als Diffusionsbarriere und Haftvermittler-Schicht. Die Wärmeleitfähigkeit der Bi-haltigen Nanodrähte konnte mit dem λ-Chip nicht gemessen werden, da die Unterätzung der Nanodrähte mittels reaktivem Ionenätzen die Nanodrähte angriff. Als Alternative können die Nanodrähte auf dem λ-Chip mit einem fokusierten Ionenstrahl unterätzt werden. Der Aufwand hierzu ist jedoch relativ hoch und diese Alternative wurde deshalb nicht weiter verfolgt. Als weitere Alternative wurde der Z-Chip entwickelt. Hierbei werden die Nanodrähte auf den fertigen Chip aufgebracht und mittels Elektronenstrahl-induzierter Deposition an den elektrischen Kontakten fixiert. Der Chip ermöglicht die Messung der elektrische Leitfähigkeit in 4-Punkt-Anordnung, der Wärmeleitfähigkeit und des Seebeck-Koeffizienten an
einem einzelnen Nanodraht. Somit ist die Bestimmung der thermoelektrischen Effektivität an einem Nanodraht möglich. DesWeiteren wurden die theoretischen Grundlagen zur Bestimmung der Wärmekapazität an einzelnen Nanodrähten mit dem Z-Chip präsentiert. Zum Zeitpunkt der Durchführung dieser Arbeit fehlte jedoch das notwendige Equipment zur Ausführung der Wärmekapazitätsmessung an einzelnen Nanodrähten.
Des Weiteren wurde die Cross-Plane Methode zur Bestimmung der Wärmeleitfähigkeit an eingebetteten Nanodrähten entwickelt. Analog der Messmethode, welche für die Einzeldrahtmessungen verwendet wird, handelt es sich hierbei um eine stationäre „Joule-Heating“ Methode. Die Temperaturdifferenz wird aus der Widerstandsänderung einer auf die eingebetteten Nanodrähte aufgebrachten Heizschicht bestimmt.Mit derMethode wurde die Wärmeleitfähigkeit von BixTe1-x-Nanodrähten ermittelt.
Die elektrische Leitfähigkeit wurde von BixTe1-x-Nanodrähten unterschiedlicher Zusammensetzung und Herstellungsparameter mit dem λ- und dem Z-Chip bestimmt. Die gemessenen Nanodrähte zeigen sowohl intrinsisches wie extrinsisches Leitungsverhalten verbunden mit einer, im Vergleich zum Volumenmaterial, reduzierten Temperaturabhängigkeit der elektrischen Leitfähigkeit infolge von Oberflächen- und Korngrenzenstreuung der Ladungsträger. Die elektrischen Leitfähigkeitsmessungen stimmen mit Beobachtungen anderer Gruppen gut überein.
Die Wärmeleitfähigkeit konnte an einem einzelnen BixTe1-x-Nanodraht und an eingebetteten BixTe1-x-Nanodrähten gemessen werden. Die Wärmeleitfähigkeit ist gegenüber dem Massivmaterial reduziert. Die Ergebnisse sind in guter Übereinstimmung mit bisher publizierten Ergebnissen von Bismuttellurid-Nanodrähten.
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.
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.
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 simulator 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.
The design, construction, and commissioning of the ALICE Time-Projection Chamber (TPC) is described. It is the main device for pattern recognition, tracking, and identification of charged particles in the ALICE experiment at the CERN LHC. The TPC is cylindrical in shape with a volume close to 90 m3 and is operated in a 0.5 T solenoidal magnetic field parallel to its axis.
In this paper we describe in detail the design considerations for this detector for operation in the extreme multiplicity environment of central Pb–Pb collisions at LHC energy. The implementation of the resulting requirements into hardware (field cage, read-out chambers, electronics), infrastructure (gas and cooling system, laser-calibration system), and software led to many technical innovations which are described along with a presentation of all the major components of the detector, as currently realized. We also report on the performance achieved after completion of the first round of stand-alone calibration runs and demonstrate results close to those specified in the TPC Technical Design Report.
This paper introduces a new methodology for the fabrication of strain-sensor elements for MEMS and NEMS applications based on the tunneling effect in nano-granular metals. The strain-sensor elements are prepared by the maskless lithography technique of focused electron-beam-induced deposition (FEBID) employing the precursor trimethylmethylcyclopentadienyl platinum [MeCpPt(Me)3]. We use a cantilever-based deflection technique to determine the sensitivity (gauge factor) of the sensor element. We find that its sensitivity depends on the electrical conductivity and can be continuously tuned, either by the thickness of the deposit or by electron-beam irradiation leading to a distinct maximum in the sensitivity. This maximum finds a theoretical rationale in recent advances in the understanding of electronic charge transport in nano-granular metals.
This thesis has light mesons and their vacuum interactions as its topic. In particular, the work examines the question where the scalar antiquark-quark states are found in the physical spectrum -- in the energy region below or above 1 GeV. Contrary to the naive expectation, the mentioned states are found in the region above 1 GeV. This has consequences for the building of order parameters for the chiral symmetry breaking of Quantum Chromodynamics (QCD).
P-type ATPases are membrane proteins acting as ion pumps that drive an active transport of cations across the membrane against a concentration gradient. The required energy for the ion transport is provided by binding and hydrolysis of ATP. A reaction mechanism of ion transport and energy transduction is assumed to be common for all P-type ATPases and generally described by the Post-Albers cycle. Transient currents and charge translocation of P-type ATPases were extensively investigated by electrical measurements that apply voltage jumps to initiate the reaction cycle. In this study, we simulate an applied voltage across the membrane by an electric field and perform electrostatic calculations in order to verify the experimentally-driven hypothesis that the energy transduction mechanism is regulated by specific structural elements. Side chain conformational and ionization changes induced by the electric field are evaluated for each transmembrane helix and the selectivity in response is qualitatively analyzed for the Ca2+-ATPase as well as for structural models of the Na+/K+-ATPase. Helix M5 responds with more conformer changes as compared to the other transmembrane helices what is even more emphasized when the stalk region is included. Thus our simulations support experimental results and indicate a crucial role for the highly conserved transmembrane helix M5 in the energy transduction mechanism of P-type ATPases.
Towards a THz Bloch laser
(2011)
The realisation of tunable THz laser sources working at room temperature would give
rise to further applications in this range of the electromagnetic spectrum. The THz
Bloch laser could therefore become the basis for a technological breakthrough. Beside
this practical relevance, the physics of the gain mechanism has been investigated
theoretically for a long time and the experimental implementation of a self-starting
laser still has not been achieved.
At the beginning of this thesis the basic principles of Bloch oscillations and the
related Bloch gain are described. The need of a superlattice structure to make Bloch
oscillations possible in a semiconductor material is discussed. In this context, the effect
of negative differential resistance and its influence on the field distribution due to Gunn
domains is explained. The latter lead to an inhomogeneous field which may suppress
the Bloch gain mechanism. The Krömer criterion is introduced and the concept of
field-pinning layers to improve the field homogeneity is deduced. Finally, the design of
the laser material is shown and different types of laser waveguides are compared.
In chapter 3 detailed recipes for the processing of samples are given. Different types of
contacts (ohmic and Schottky), the wafer bonding process required for double-metal
lasers and the application of different photoresists for different purposes are described.
An explanation of the formation of waveguides due to dry etching, wet etching
and ion implantation follows. Dry etching is an established technique in the field
of microstructure processing but the challenge of etching about 20 μm has led to
problems. The high etching depth also makes wet etching difficult but this method
could be improved due to a hard bake of the photoresist. The protection of critical
areas on the surface of the samples with photoresist during ion implantation was
increased by optimising the spin coating process. However, a full implantation of the
active layer between the waveguides was not achieved which was the reason for the
development of the hybrid technology. Here a prior wet etching of about 10 μm is
performed and the rest of the material is implanted.
The experimental setup is shown in chapter 4. An alternative method for the electrical
contacting with the help of a copper bar is introduced. This improves the current
distribution and the risk of an electrical breakdown during the measurements could
therefore be lowered. Devices for THz beam guidance and spectroscopic measurements
are shown and the method of biasing the samples with pulses below 100 ns and
determining the effective voltage applied to the sample is depicted. These short pulses
are required to prevent the samples heating up drastically due to high power.
Chapter 5 contains the current-voltage characterisation of several structures including
I-V-samples, Bloch laser samples and a quantum cascade laser. Different contacts
(ohmic and Schottky) and different techniques for the formation of the ridges have
been used in the processing of these samples (performed at the University of Frankfurt
in all cases) and their influence on the I-V-dependence is discussed. The properties of
the THz emission of the quantum cascade laser are in good agreement with published
results from lasers processed with the same material. Another important result of
this chapter is that the Bloch laser samples show unstable behaviour compared to the
quantum cascade structure even with short pulses (of about 10 ns) where the risk of an
electrical breakdown or the building of filaments is low. THz radiation emitted from
one of the Bloch laser samples could not be observed.
Two aspects that may have prevented the Bloch laser to emit are discussed in
chapter 6. The saturation of the gain for higher amplitudes of the THz wave is
investigated in single mode and multiple mode operation (the latter could occur due
to the Bloch gain being expected to be broadband). In both cases it is shown that
the saturation effect would limit the output power only to values clearly above the
detection limit. In the subsequent section the distribution of the electric field is
simulated with SILVACO software. Structures with transit layer lengths above the
Krömer criterion are compared with structures which include field-pinning layers. It is
shown that the latter are useful to avoid propagating Gunn domains as they build up
in similar structures without field-pinning layers. Nevertheless, the electric field inside
the superlattice regions is not stable. Beside spatial inhomogeneities also temporal
variations of the field magnitude are observed. The lack of a suitable field distribution
is expected to be the main reason for the samples not to work.
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 first measurement of the fluctuation of the kaon-to-proton ratio in relativistic heavy-ion collisions is presented. This thesis details the analysis procedure for identifying kaons and protons using the NA49 experiment at CERN-SPS and discusses the results in the context of the current state of the field.
Feedforward inhibition and synaptic scaling are important adaptive processes that control the total input a neuron can receive from its afferents. While often studied in isolation, the two have been reported to co-occur in various brain regions. The functional implications of their interactions remain unclear, however. Based on a probabilistic modeling approach, we show here that fast feedforward inhibition and synaptic scaling interact synergistically during unsupervised learning. In technical terms, we model the input to a neural circuit using a normalized mixture model with Poisson noise. We demonstrate analytically and numerically that, in the presence of lateral inhibition introducing competition between different neurons, Hebbian plasticity and synaptic scaling approximate the optimal maximum likelihood solutions for this model. Our results suggest that, beyond its conventional use as a mechanism to remove undesired pattern variations, input normalization can make typical neural interaction and learning rules optimal on the stimulus subspace defined through feedforward inhibition. Furthermore, learning within this subspace is more efficient in practice, as it helps avoid locally optimal solutions. Our results suggest a close connection between feedforward inhibition and synaptic scaling which may have important functional implications for general cortical processing.
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.
Untersuchung von Korrelationseffekten in der Doppelphotoemission von normal- und supraleitendem Blei
(2012)
Im Rahmen dieser Arbeit wurde für die erstmalige Untersuchung der Doppelphotoemission von supraleitenden Materialien eine neue Messapparatur aufgebaut. Mit ihr lassen sich auf eine neue Weise Korrelationseffekte zwischen zwei Elektronen untersuchen, denn beide werden für jedes Reaktionsereignis mit ihrem vollständigen Impulsvektor aufgezeichnet. Die Apparatur kann daher für einen direkten Nachweis der Cooperpaarung in Supraleitern verwendet werden. Dazu wurden ein speziell für diesen Zweck angepasstes Spektrometer, Vakuumsystem und Probenhalter konstruiert. Ein mehrfach verbessertes Vakuumsystem sorgte dafür, dass eine Bleioberfläche über einen Zeitraum von mindestens 15 Stunden nach einer Reinigung gemessen werden konnte. Das Spektrometer erlaubte die koinzidente Messung von Elektronen über einen großen Raumwinkelbereich mit ausschließlich elektrischen Feldern. Dadurch war es auch im supraleitenden Zustand möglich, die Trajektorien der Elektronen zu berechnen. Die Energieauflösung für jedes Elektron lag zwischen 1/30 und 1/50, je nach untersuchtem Emissionswinkel. Ein eigens entwickelter Probenhalter erlaubte es, eine nur von einer Seite thermisch abgeschirmte Probe auf eine Temperatur von 4,5 K zu kühlen. Die Experimente wurden an einer Beamline des Berliner Synchrotrons BESSY durchgeführt.
Von entscheidender Bedeutung für die Auswertung der Daten ist die Qualität der Pulserkennungsroutine. Sie bestimmt die Totzeit der Messapparatur, das heisst wie nahe zwei Elektronen zeitlich und räumlich beieinander liegen dürfen, um noch detektiert zu werden. Sie beeinflusst somit die Beobachtung erheblich. In den als digitalisierte Pulse aufgenommen Rohdaten besteht die Schwierigkeit darin, zwei übereinander liegende Signale als solche zu erkennen und die richtige Zeit beider Signale zu bestimmten. Dies wurde erheblich verbessert, indem ein in Vorabeiten simulierter Doppelpulsalgorithmus modifiziert und erstmalig verwendet wurde. In der Folge konnte die Totzeit deutlich verringert und daher bis zu 20% mehr Doppelereignisse gefunden werden. Darüber hinaus ließen sich Fehler bei der Zeiterkennung nahe aufeinander folgender Pulse korrigieren. Ein in diesem Zusammenhang entwickeltes Programm erzeugte durch die Addition von gemessenen Einzelpulsen künstliche Doppelereignisse mit beliebiger Abstandsverteilung und erlaubte so erstmals eine exakte Simulation der Detektortotzeit mit verschiedenen Pulserkennungsalgorithmen.
Neben den Koinzidenzereignissen wurden auch die Ergebnisse der gewöhnlichen Photoemission untersucht und mit Bandstrukturrechnungen verglichen. Aufgrund der Messmethode wurde keine Vorauswahl bezüglich des Emissionswinkels oder der kinetischen Energie getroffen. Die Ergebnisse der Fermiflächen stimmen innerhalb der erreichten Auflösung mit den theoretischen Vorhersagen überein. Ebenso konnten die Strukturen in den Parallelimpulsspektren der Elektronen, die aus lokalisierten Energieniveaus emittiert wurden, mit der Interferenz der ausgehenden Wellenfunktionen erklärt werden. Eine Simulation dieses Effekts lieferte trotz der vergleichsweise sehr niedrigen Elektronenenergien eine gute Übereinstimmung der wesentlichen Merkmale.
Es wurden Doppelphotoemissionspektren von Blei bei verschiedenen Photonenenergien im Bereich von 21,22 eV bis 40 aufgenommen. Dabei konnten verschiedene Emissionskanäle identifiziert werden. Das Korrelationsloch ist ein sehr grundlegender Effekt, der aufgrund der Coulombabstoßung und des Pauli-Prinzips auftritt und daher bei allen Metallen vorkommt. Betrachtet man das Korrelationsloch im Impulsraum, so führt es dazu, dass zwei gleichzeitig emittierte Elektronen keine ähnlichen Impulsvektoren besitzen dürfen. Durch die verbesserten Pulserkennungsalgorithmen war es möglich, das Korrelationsloch zu untersuchen und über einen weiten Energiebereich zu vermessen. Es zeigte sich wie erwartet als Verarmungszone in der Impulsverteilung eines Elektrons um den Impuls eines zweiten. Ein solcher Effekt ist mit einem einzelnen Detektor sehr schwer zu messen, da die Totzeit die gleiche Auswirkung auf die Spektren hat. Durch eine Simulation konnte ihr Einfluss in jedem Spektrum herausgefunden und so beide Effekte voneinander getrennt werden. Sie stehen damit für einen Vergleich mit einer noch zu entwickelnden theoretischen Vorhersage zur Verfügung.
Aufgrund der bei Blei sehr nahe an der Fermikante liegenden, lokalisierten Energieniveaus konnte der Augerzerfall aus dem Valenzband identifiziert und untersucht werden. Korrelationseffekte zwischen den beiden Elektronen spielten aufgrund des sehr breiten Valenzbandes wie erwartet eine untergeordnete Rolle. Dies ließ sich nachweisen, indem die Energieverteilung durch eine Selbstfaltung der Valenzbandzustandsdichte beschrieben wurde und die Winkelverteilung der Augerelektronen keine Beeinflussung durch die Emissionsrichtung der Photoelektronen zeigte. Beide Beobachtungen deuten auf einen vollständig unabhängigen Emissionsprozess der beiden Elektronen hin. Überraschenderweise zeigte sich aber eine Energieverschiebung des Photoelektrons, abhängig von der kinetischen Energie des Augerelektrons. Dieser in der Gasphase als Post-Collision-Interaction bekannte Effekt sollte aufgrund der schnellen Abschirmung der im Festkörper zurückbleibenden Löcher nicht auftauchen. Die Ursache für die Energieverschiebung ist noch unbekannt.
Für die Identifizierung der Emission von Cooperpaaren wurden Messungen oberhalb und unterhalb der Sprungtemperatur bei verschiedenen Photonenenergien zwischen 20 eV und 40 eV durchgeführt. Verschiedene Spektren wurde nach der Signatur des Prozesses untersucht. Aufgrund der geringen Statistik konnte er nicht identifiziert werden. Demnach konnte auch die theoretische Vorhersage nicht widerlegt werden. Da dieses Experiment aus technischer Sicht äußerst herausfordernd ist, war die Untersuchung von Blei, als einfach zu präparierendes Material mit hoher Sprungtemperatur, naheliegend. Es stellte sich jedoch durch die Auswertung heraus, dass es im Hinblick auf die untersuchte Fragestellung einen wesentlichen Nachteil besitzt. Die Hauptintensität befindet sich im Gegensatz zu Kupfer für alle hier verwendeten Photonenenergien bei niedrigen Elektronenenergien, so dass nur wenige Ereignisse in dem für die Cooperpaaremission interessanten Energiefenster liegen.
Das HADES-Experiment (High Acceptance DiElectron Spectrometer) am SIS der GSI wurde zur Messung der e+e−-Paare dileptonischer Zerfälle der leichten Vektormesonen im Energiebereich von 1 − 2 AGeV entwickelt. Eine der Hauptanforderungen an das Spurverfolgungssystem des Spektrometers ist eine Ortsauflösung von 100 μm, die benötigt wird, um die geforderte Massenauflösung von 1 % im Bereich der !-Masse zu erzielen. Das Spurverfolgungssystem besteht aus vier Ebenen mit Vieldrahtdriftkammern (Multi-wire Drift Chambers (MDCs)) niedriger Massenbelegung (low-mass), die aus je 6 Auslesedrahtebenen bestehen. Die primäre Messgröße von Driftkammern ist die Driftzeit der entlang einer Teilchenspur generierten Elektronen der Primärionisation zum Auslesedraht. Um die gemessene Driftzeit in eine Ortskoordinate umrechnen zu können, ist eine genaue Kenntnis der Ort-Zeit-Korrelation der Driftzellen und eine präzise Kalibrierung der gemessenen Zeiten nötig. Im Rahmen dieser Arbeit wurde eine neue Methode zur Bestimmung der Kalibrierungsparameter der Driftzeiten mittels der Einkoppelung eines externen elektrischen Pulses auf die Drahtebenen der Driftkammern entwickelt und mit der herkömmlichen Methode der Kalibrierung verglichen. Zur Kalibrierung wurden elektrische Pulse durch die Hochspannungsverteilung der Driftkammern auf die Potentialdrahtebenen geleitet und somit Signale auf die Signaldrähte induziert. Die Ausbreitungsgeschwindigkeit der Signale auf den Drähten, die zur Berücksichtigung der Laufzeiten benötigt wird, wurde experimentell bestimmt. Die Genauigkeit der Methode zur Bestimmung der Kalibrierungsparameter übertrifft die Auflösung der Driftzeitmessung der Driftkammern. Die Kalibrierung der Driftzeiten benötigt nur wenig Rechen- und Zeitaufwand und stellt die Kalibrierungsparameter in gleich hoher Präzision für alle Driftzellen zur Verfügung. Die Qualität der Kalibrierung wird im Gegensatz zur herkömmlichen Methode prinzipbedingt nicht durch Fluktuationen der Startzeit des jeweiligen Stoßereignisses und die Flugzeit der dabei emittierten Teilchen zu den Driftzellen, sowie des elektronischen Rauschens beeinflusst. Die Qualität der Kalibrierung konnte gegenüber der herkömmlichen Methode signifikant verbessert werden. Die Auswirkung der Kalibrierungsparameter auf die Spurrekonstruktion wurde untersucht und für die beiden Kalibrierungsmethoden verglichen. Die Ergebnisse lassen keinen eindeutigen Schluss auf die Auswirkung der Fehler in der Kalibrierung auf die Qualität der Spurrekonstruktion zu, da die Ergebnisse der Spurrekonstruktion von anderen Effekten dominiert werden.
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
Ende der 70ger Jahre, fünf Jahre nach der Einführung des ersten kommerziellen, medizinischen Computertomographen wurde die Tomographie am Los Alamos Scientific Laboratory zum ersten Mal für die Diagnose von Teilchenstrahlen angewendet. Bei der Tomographie wird aus eindimensionalen Projektionen, sogenannten Profilen, welche in möglichst vielen Winkeln um ein Objekt herum aufgenommen werden, ein zweidimensionales Abbild der Dichteverteilung (Slice oder Scheibe) approximiert. Dies ist möglich durch das bereits 1917 von Johann Radon eingeführte Fourier-Scheiben-Theorem. In der Theorie kann die zwei-dimensionale Dichteverteilung exakt ermittelt werden, wenn Projektionen mit einer unendlich feinen Auflösung über unendlich viele Winkel um ein Objekt herum in die Rekonstruktion einbezogen werden. Durch die Rekonstruktion vieler Scheiben kann ein drei-dimensionales Abbild der Dichteverteilung in einem Objekt, in diesem Fall einem Ionenstrahl, berechnet werden, sofern dieses nicht optisch dicht ist.
Die Profile in der nicht-invasiven Strahldiagnose entstehen durch CCD-Kameraaufnahmen von strahlinduzierter Fluoreszenz, welche durch den Einlass von Restgas hervorgerufen wird. Es sind aber auch Profile, welche aus anderen Methoden gewonnen werden (z.B. Gittermessungen) denkbar. An Orten mit hoher Energie ist jedoch eine nicht-invasive Form der Profilaufnahme sowohl für die Qualität des Strahls, wie auch den Schutz der Messgeräte unabdingbar.
In den letzten 40 Jahren wurden im Bereich der Strahltomographie viele wichtige Fortschritte erzielt:
1. Anfangs standen nur sehr wenige Profile zur Verfügung, so dass die Methode der gefilterten Rückprojektion(FBP), welche sich direkt aus dem Fourier-Scheiben-Theorem ableitet und welches auch in der Medizin verwendet wird, nicht angewendet werden kann. Um dieses Problem zu lösen wurden iterative Methoden wie die Algebraische Rekonstruktion (ART) und die Methode der Maximalen Entropie (MEM) für die Strahltomographie erschlossen, so dass auch mit sehr geringer Profilanzahl eine Rücktransformation möglich wurde.
2. Neben der Ortsraumtomographie wurde die Phasenraumtomografie entwickelt, so dass mittlerweile eine Rekonstruktion des sechs-dimensionalen Phasenraumes möglich ist, mit welchem ein Ionenstrahl in seiner Gesamtheit beschrieben werden kann.
3. Die Projektionen wurden lange Zeit durch Aufnahmen von mehreren festen Anschlüssen aus gewonnen (Multi-Port-Technik). Auf diese Weise ist die Anzahl der möglichen Projektionen sehr begrenzt. So entwickelte man später eine Methode welche den Strahl mit Hilfe von Quadrupolen dreht (Quad-Scan-Technik), so dass auf diese Weise von einem Anschluss aus viele Projektionen gemessen werden konnten, so dass sogar die FBP angewendet werden konnte.
4. Die meisten Bestrebungen zielten darauf ab, die Tomographie für eine nicht-invasive Emittanzmessmethode zu nutzen, welches bis heute aufgrund der großen und noch immer zunehmenden Energien in modernen Beschleunigern ein wichtiges Problem ist. Um die Tomographie zur Emittanzmessung zu verwenden, führt man eine Rekonstruktion des Phasenraumes durch. Das Problem ist, dass hierfür das a priori Wissen über die Strahltransportmatrix in die Tomographie mit einfließt, die berechnete Strahltransportmatrix
jedoch nicht mit dem tatsächlichen Strahltransport übereinstimmt, da dieser bei hohen Energien durch auftretende Raumladung nicht-linear verändert wird. Hierzu wurden gute Fortschritte in der Abschätzung der tatsächlichen Transportmatrix gemacht um die Phasenraumtomographie trotzdem mit hinreichend gutem Ergebnis durchführen zu können.
Trotz all dieser Fortschritte und Entwicklungen ist die Tomographie bis heute keine weitverbreitete Methode in der Strahldiagnose. Der Grund ist, dass das Einrichten einer Tomografie eine komplexe Abfolge etlicher Entscheidungen und weitgestreutes Wissen aus vielen unterschiedlichen Bereichen erfordert, dieser nicht zu unterschätzende Mehraufwand jedoch auch durch einen signifikanten Nutzen gerechtfertigt sein muss. Der große Nutzen der Tomographie für die Strahldiagnose und Untersuchung der Strahldynamik ist bis heute allerdings weitgehend unerkannt und weiterhin reduziert auf die Entwicklung einer nicht-invasiven Methode für die Emittanzbestimmung. Ein zweites Hindernis stellte bisher auch die Diskrepanz zwischen Genauigkeit und Platzaufwand dar (hohe Genauigkeit durch viele Projektionen mit Quad-Scan-Technik auf mehreren Metern oder niedrige Genauigkeit durch wenig Projektionen mit Multi-Port-Technik auf weniger als einem Meter). Die Tomografie kann großen Nutzen leisten für die Online-Überwachung wichtiger Maschineneparameter im Strahlbetrieb (Monitoring) als auch für detaillierte Analysen zur Strahldynamik (Modellierung) weit über die Implementierung einer nicht-invasiven Emittanzmessmethode hinaus.
Um dies zu gewährleisten Bedarf es Zweierlei. Zum einen muss die Diskrepanz zwischen Genauigkeit und Platzaufwand aufgehoben werden. Hierzu wurde im Rahmen dieser Arbeit eine rotierbare Vakuumkammer entwickelt die nach dem Vorbild medizinischer Tomographen in mehr als 5000 Winkelschritten um den Strahl herum fahren kann, dabei ein Vakuum von mindestens 10-7mbar aufrecht erhält und einen Platzbedarf von weniger als 400 mm in der Strahlstrecke einnimmt. Zum anderen muss die Implementierung der Tomografie durch eine Angabe von schematischen Schritten und Entscheidungen vereinfacht werden. Eine Strahltomographie muss immer auf ihren jeweiligen Zweck hin implementiert werden, da Einzelelemente der Tomografie wie beispielsweise Messvorrichtung und dadurch die Profilanzahl, zu verwendender Tomographiealgorithmus, zu bestimmende Parameter sich je nach Einsatz unterscheiden können. Jedoch können die dazu nötigen Entscheidungen in ein Schema eingeordnet werden, welches die Implementierung der Tomographie vereinfacht und beschleunigt. Hierzu wurde in dieser Arbeit eine Diagnosepipeline und ein Entscheidungsschema eingeführt, sowie die Implementierung nach diesem Schema am Beispiel einer Strahltomographie für die Frankfurter Neutronenquelle (FRANZ) demonstriert und die entsprechenden Fragen und Entscheidungen diskutiert. Es wird gezeigt, wie sich aus den Messdaten über die Aufbereitung der Daten durch die Tomografie die erforderlichen Standardstrahlparameter für ein Monitoring gewinnen lassen. Zusätzlich wird ein Ebenen-Modell eingeführt, über welches nicht-Standardparameter oder neu modellierte Strahlparameter für detaillierte Analysen der Strahldynamik über die Standardparameter hinaus entwickelt werden können. Diese Arbeit soll ein grundlegendes Konzept für die routinemäßige Implementierung der Tomographie in der Strahldiagnose zur Verfügung stellen. Für die Verwendung zum Monitoring im Strahlbetrieb muss die Bestimmung von Standardparametern noch wesentlich im Zeitaufwand verbessert werden. Die Verwendung der Phasenraumtomographie benötigt noch eine Idee um den arcustangensförmigen Verlauf der berechneten Phasenraumrotationswinkel mit der Forderung der FBP nach äquidistanten Projektionswinkeln verträglicher zu machen.