No description available.

Recent advancements in laser technology are quickly moving the frontiers of research: quantum dynamics can now be investigated in more detail, on new timescales, with an unprecedented level of control. These new possibilities offer a new ground for the theoretical study of fundamental processes; at the same time, a proper understanding of phenomena involved is necessary to explain measurements, and to indicate directions for further experiments. This Thesis deals with the theoretical investigation of particular cases of light-matter interaction, in atoms and in dielectrics. Regimes considered here have just become a subject of intensive investigation: they are acquiring more and more relevance as technological advancements make them experimentally accessible. In the first part of the Thesis I consider a process as fundamental as the single-photon ionization of atoms: my modeling will include an ultrashort pulse (full width half maximum ~ 100 as = 10^-16 s) exciting an electron to the continuum, and a strong few-cycle near-infrared laser field. This configuration is suitable to reproduce recent streaking experiments on atoms. I developed a numerical tool to simulate these dynamics in three dimensions: the process is quite elaborate and requires an adequate description of multi-electron atoms. With proper approximations I was able to calculate photoelectron spectra using just a few dipole matrix elements, which were obtained with the aid of our external collaborators, from refined atomic structure calculations. The results of our relatively simple tool are in very good agreement with more sophisticated numerical calculations. In addition to that, I discuss my contribution to the theoretical support of a fundamental experiment [I]: both simulations and measurements indicate a delay between two different channel of photoemission in neon. A careful investigation of the limit of validity of approximations employed reveals that the Coulomb-Volkov approximation is not suitable to describe fine details of the interaction with the laser pulse. I also report on our analysis of experimental data from angle-resolved attosecond streaking. The second part of the Thesis is devoted to the investigation of inter-band excitations in dielectrics; driving this process with a high degree of control is on the edge of current technology. The ultrafast creation of charge carriers in an insulator is intriguing: dielectric properties of the medium change drastically, revealing features of the peculiar electron dynamics in such a situation. I have simulated this process solving the time dependent Schroedinger equation for a single electron in a one-dimensional lattice and analyzed how the charge Q displaced during the interaction with the pulse depends on laser parameters. These calculations reproduce to a good extent the behavior observed in the experiment. Both the theory and the experiment point out a strong dependence of Q on laser parameters: this promises a high degree of control, and at the same time suggests the possibility of a solid-state device to characterize an optical pulse. I also study in detail the modification occurring in the electric response of the sample to the electric field. The purpose of this analysis is to identify some features directly related to dynamics of newly created charge carriers. During my investigation of electron dynamics during an excitation process, I have often faced the difficulty to identify quantities which might resemble eigenstates of the time-dependent Hamiltonian. Similar field-dressed states would describe the distortion due to the field, of eigenstates of the field-free Hamiltonian. A proper definition of field-dressed states would allow a correct interpretation of the wavefunction in terms of instantaneous excited population, which is otherwise impossible to define. / Neueste Fortschritte im Bereich der Lasertechnologie erweitern schnell die Grenzen der Forschung. Quantendynamiken koennen genauer den je untersucht werden, aus kuerzeren Zeitskalen und mit einer hoeheren Kontrollebene. Diese Entwicklung bietet neue Moeglichkeiten, fundamentale Prozesse theoretisch zu untersuchen; darueber hinaus ist ein Verstaendnis der zu Grunde liegenden physikalischen Vorgaenge erforderlich, um Messresultate zu erklaeren und moegliche Richtungen fuer kuenftige Experimente aufzuzeigen. Diese Doktorarbeit befasst sich mit der theoretischen Analyse bestimmter Licht-Materie-Wechselwirkungen in Atomen und Dielektrika. Die im Rahmen dieser Thesis untersuchten Bereiche sind aktuell Thema intensiver Forschung. Dank weiterer technologischer Entwicklungen, die Experimente in diesen Bereichen ausfuehrbar machen, gewinnen sie immer weiter an Relevanz.Neueste Fortschritte im Bereich der Lasertechnologie erweitern schnell die Grenzen der Forschung. Quantendynamiken koennen genauer den je untersucht werden, aus kuerzeren Zeitskalen und mit einer hoeheren Kontrollebene. Diese Entwicklung bietet neue Moeglichkeiten, fundamentale Prozesse theoretisch zu untersuchen; darueber hinaus ist ein Verstaendnis der zu Grunde liegenden physikalischen Vorgaenge erforderlich, um Messresultate zu erklaeren und moegliche Richtungen fuer kuenftige Experimente aufzuzeigen. Diese Doktorarbeit befasst sich mit der theoretischen Analyse bestimmter Licht-Materie-Wechselwirkungen in Atomen und Dielektrika. Die im Rahmen dieser Thesis untersuchten Bereiche sind aktuell Thema intensiver Forschung. Dank weiterer technologischer Entwicklungen, die Experimente in diesen Bereichen ausfuehrbar machen, gewinnen sie immer weiter an Relevanz. Im ersten Teil der Arbeit beschreibe ich den fundamentalen Prozess der atomaren Ionisation durch ein einzelnes Photon. Mein Model enthaelt einen ultrakurzen Lichtpuls mit einer Halbwertsbreite von ~ 100 as = 10^-16 s, der ein Elektron in das Kontinuum anregt, sowie einen starkes Laserfeld im nahen infraroten Spektralbereich mit wenigen Zyklen. Diese Konfiguration erlaubt die Nachbildung von neuesten Streaking Experimenten an Atomen. Ich habe ein numerisches Werkzeug entwickelt, um diese Dynamiken in drei Dimensionen zu simulieren. Der Prozess ist sehr komplex und bedarf einer hinreichenden Beschreibung von Atomen mit mehreren Elektronen. Unter Beruecksichtigung geeigneter Naeherungen war es mir moeglich, Photoelektronenspektren mit Hilfe nur weniger Dipolmatrixelemente zu berechnen, welche in Zusammenarbeit mit unseren externen Kollaborationspartner durch verfeinerte Atomstruktur-Berechnungen bestimmt wurden. Die Ergebnisse unseres verhaeltnismaeßig einfachen Vorgehens stimmen in einem hohen Grad mit fortgeschritteneren numerischen Methoden ueberein. Darueber hinaus diskutiere ich meinen Beitrag zur theoretischen Unterstuetzung eines grundlegenden Experiments. Sowohl Simulationen als auch Messungen weisen auf eine Verzoegerung zwischen zwei Photoemissionskanaelen in Neon hin. Eine sorgfaeltige Pruefung der Gueltigkeit der verwendeten Naeherungen verraet, dass die Coulomb-Volkov Naeherung nicht geeignet ist, um feine Einzelheiten in der Wechselwirkung mit dem Laserpuls zu beschreiben. Außerdem berichte ich ueber unsere Analyse der Messdaten der winkelaufgeloesten Attosekunden Streaking Experimente. Der zweite Teil der Thesis widmet sich der Untersuchung von Interband-Anregungen in Dielektrika. Die kontrollierte Lenkung dieser Ue bergaenge wurde erst mit aktuellster Technologie ermeoglicht. Die ultraschnelle Erzeugung von Ladungstraegern in einem Isolator ist bemerkenswert. Die dielektrischen Eigenschaften aendern sich dramatisch, was Rueckschluesse auf die Elektronendynamik waehrend dieser Anregung zulaesst. Ich habe diesen Prozess durch Loesung der zeitabhaengigen Schroedingergleichung fuer ein einzelnes Elektron in einem eindimensionalen Gitter simuliert und untersucht, wie sich die waehrend des Lichtpulses verlagerte Ladung mit den Laserparametern aendert. Diese Berechnungen reproduzieren in hohem Maße das im Experiment beobachtete Verhalten. Sowohl Theorie als auch Experiment weisen auf eine starke Abhaengigkeit der Ladung von den Laserparametern hin. Dies verspricht ein hohes Maß an Kontrolle und deutet auf eine moegliche Anwendung eines Festkoerperbauelements fuer die Charakterisierung eines optischen Pulses hin. Außerdem untersuche ich detailliert die Modifikationen der elektrischen Antwort des Samples auf ein externes elektrisches Feld. Das Ziel dieser Analyse ist die Identifikation einiger Eigenschaften die direkt mit der Dynamik der erzeugten Ladungstraeger zusammenhaengen. Waehrend der Untersuchung der Elektronendynamiken in einem Anregungsprozeß, stieß ich oft auf die Problematik, Groeßen zu ermitteln, die Eigenzustaenden des zeitabhaengigen Hamilton- Operators aehneln koennten. Aehnliche “Field-dressed States” wuerden die Verzerrung der Eigenzusteande des feldfreien Hamiltonoperators aufgrund des Felds beschreiben. Eine geeignete Definition der Field-dressed States wuerde eine korrekte Interpretation der Wellenfunktion in Abhaengigkeit der instantanen angeregten Besetzung ermoeglichen, welche sich auf anderem Wege nicht bestimmen laesst.

In dieser Arbeit wird eine Messung der Top-Quark Masse im vollhadronischen Top-Antitop Zerfallskanal präsentiert. Die Messung basiert auf Daten des ATLAS Detektors aus Proton-Proton Kollisionen, welche am LHC bei einer Schwerpunktsenergie von 7 TeV durchgeführt wurde. Der Datensatz wurde im Jahr 2011 aufgezeichnet und entspricht einer integrierten Luminosität von 4.7 fb^-1. Die Messung der Top-Quark Masse erfolgt mittels einer Template-Technik, wobei die sogenannte R_3/2 Variable als Top-Quark-Massen sensitive Größe benutzt wird. Die R_3/2 Variable errechnet sich hierbei aus dem Verhältnis der invarianten Massen des hadronisch zerfallenden Top-Quarks und W-Bosons. Die Zuordnung von Jets zu den jeweiligen Top-Antitop Zerfalls-Partonen ergibt sich anhand einer Wahrscheinlichkeitsvorhersage, welche die kinematischen Eigenschaften der Jets berücksichtigt. Der Multijet Untergrund des vollhadronischen Top-Antitop Signals wird mittels Daten abgeschätzt, wodurch eine präzise Vorhersage der kinematischen Verteilungen von Untergrund-Ereignissen ermöglicht wird. Die Messung der Top-Quark Masse ergibt m_top = 175.1 +- 1.4 (stat.) +- 1.8 (syst.) GeV/c^2, wobei die dominanten Beiträge zur systematischen Unsicherheit von der Jet-Energie-Skala sowie von der b-Jet Energie-Skala stammen. / A measurement of the top quark mass in the fully hadronic top-antitop decay channel is presented. The measurement uses data recorded with the ATLAS detector from proton-proton collisions provided by the LHC in 2011. The data were taken at a centre-of-mass energy of 7 TeV and correspond to an integrated luminosity of about 4.7 fb^-1.To measure the top quark mass a template technique is used, based on the so-called R_3/2 estimator which is built from the invariant mass ratio of the hadronically decaying top quarks and W bosons. A kinematic likelihood fit is performed to properly reconstruct the ttbar decay. The multijet background to the fully hadronic ttbar signal is modelled from data by means of an ABCD method, allowing for a precise prediction of kinematical distributions in background events. The top quark mass is measured to be m_top = 175.1 +- 1.4 (stat.) +- 1.8~(syst.) GeV/c^2, with the dominant sources of systematic uncertainty coming from the jet energy scale and the residual b-jet energy scale.

In the first part of this thesis, basic mathematical and physical concepts are introduced. The notion of a Lie algebroid is reviewed in detail and we explain the generalization of differential geometric structures when the tangent bundle is replaced by a Lie algebroid. In addition, Lie bi-algebroids and Courant algebroids are defined. This branch of mathematics finds its application in deformation quantization, which in string theory is the dynamics of open strings in the presence of a background B-field. We explain how the Moyal-Weyl star product arises for constant background fields and how this can be generalized to arbitrary backgrounds and non-associative products. Non-commutative or even non-associative spaces are expected to play a role also in closed string theory: Starting with a compactification on toroidal backgrounds with non-trivial H-flux, T-duality leads on the one hand to configurations with geometric f-flux, but on the other hand to spaces which are only locally geometric in case of Q-flux, or even non-commutative or non-associative in case of the R-flux. We describe the action of T-duality in detail and review the motivation and structure of non-geometric fluxes. It will turn out, that in the local description of non-geometric backgrounds, a bi-vector $\beta$ is more appropriate than the original B-field. Based on these foundations, we will describe our results in the second part. On the world-sheet level, we will analyse closed string theory with flat background and constant H-flux. The correct choice of left- and right-moving currents allows for a conformal field theory description of this background up to linear order in the H-flux. It is possible to define tachyon vertex operators and T-duality is implemented as a simple reflection of the right-moving sector. In analogy to the open string case, correlation functions allow to extract information on the algebra of observables on the target space. We observe a non-vanishing three-coordinate correlator and after the application of an odd number of T-dualities, we are able to extract a three-product which has a structure similar to the Moyal-Weyl product. We then focus on the target space and the local structure of the H-,f-,Q- and R-fluxes. An algebra based on vector fields is proposed, whose structure functions are given by the fluxes and Jacobi-identities allow for the computation of Bianchi-identities. Based on the latter, we give a proof for a special Courant algebroid structure on the generalized tangent bundle $TM \oplus T^*M$, where the fluxes are realized by the commutation relations of a basis of sections. As was reviewed in the first part of this work, in the description of non-geometric Q- and R-fluxes, the B-field gets replaced by a bi-vector $\beta$, which is supposed to serve as the dual object to B under T-duality. A natural question is about the existence of a differential geometric framework allowing the construction of actions manifestly invariant under coordinate- and gauge transformations, which couple the $\beta$-field to gravity. It turns out that we have to use the language of Lie algebroids to extend differential geometry from the tangent bundle of the target space to its cotangent bundle, equipped with a twisted version of the Koszul-Schouten bracket, to answer this question positively. This construction enables us to formulate covariant derivatives, torsion, curvature and gauge symmetries and culminates in an Einstein-Hilbert action for the metric and $\beta$-field. We observe that this action is related to standard bosonic low energy string theory by a field redefinition, which was discovered by Seiberg and Witten and which we described in detail in the first part. Furthermore it turns out, that the whole construction can be extended to higher order corrections in $\alpha'$ and to the type IIA superstring. We conclude by giving an outlook on future directions. After clarifying the relation of Lie algebroids to non-geometry, we speculate about the application of Lie algebroid constructions to supersymmetry and the extension to the case of Filippov three-algebroids, which could play a role in M-theory.

No description available.

In der vorliegenden Arbeit wird mit Hilfe der verallgemeinerten Eichtheorie/Gravitations-Dualität, welche stark gekoppelte Eichtheorien mit schwach gekrümmten gravitativen Theorien verbindet, stark korrelierte Quantenzustände der Materie untersucht. Der Schwerpunkt liegt dabei in Anwendungen auf Systeme der kondensierten Materie, insbesondere Hochtemperatur-Supraleitung und kritische Quantenzustände bei verschwindender Temperatur. Die Eichtheorie/Gravitations-Dualität entstammt der Stringtheorie und erlaubt eine Umsetzung des holographischen Prinzips. Aus diesem Grund wird eine kurze Einführung in die Konzepte der Stringtheorie und ihre Auswirkungen auf das holographische Prinzip gegeben. Für das tiefere Verständnis der effektiven Niederenergie-Feldtheorien wird zusätzlich die Supersymmetrie benötigt. Ausgestattet mit einem robusten Stringtheorie-Hintergrund wird die unterschiedliche Interpretation der Dirichlet- oder D-Branen, ausgedehnte Objekte auf denen offene Strings/Fäden enden können, diskutiert: Zum einen als massive solitonische Lösungen der Typ II Supergravitation und auf der anderen Seite, ihre Rolle als Quelle für supersymmetrische Yang-Mills Theorien. Die Verbindung dieser unterschiedlichen Betrachtungsweise der D-Branen liefert eine explizite Konstruktion der Eichtheorie/Gravitations-Dualität, genauer der AdS_5/CFT_4 Korrespondenz zwischen der N=4 supersymmetrischen SU(N_c) Yang-Mills Theorie in vier Dimensionen mit verschwindender beta-Funktion in allen Ordnungen, also eine echte konforme Theorie, und Type IIB Supergravitation in der zehn dimensionalen AdS_5 X S^5 Raumzeit. Darüber hinaus wird das Wörterbuch, das zwischen den Operatoren der konformen Feldtheorie und den gravitativen Feldern übersetzt, im Detail eingeführt. Genauer gesagt, die Zustandssumme der stark gekoppelten N=4 supersymmetrischen Yang-Mills Theorie im Grenzwert großer N_c, ist identisch mit der Zustandssumme der Supergravitation unter Berücksichtigung der zugehörigen Lösungen der Bewegungsgleichungen, ausgewertet am Rand des AdS-Raumes. Die Anwendung der perturbativen Quantenfeldtheorie und die Verbindungen zur quantenstatistischen Zustandssumme erlaubt die Erweiterung des holographischen Wörterbuchs auf Systeme mit endlichen Dichten und endlicher Temperatur. Aus diesem Grund werden alle Aspekte der Quantenfeldtheorie behandelt, die für die Anwendung der ``Linear-Response''-Theorie, der Berechnung von Korrelationsfunktionen und die Beschreibung von kritischen Phänomenen benötigt werden, wobei die Betonung auf allgemeine Zusammenhänge zwischen Thermodynamik, statistischer Physik bzw. statistischer Feldtheorie und Quantenfeldtheorie liegt. Des Weiteren wird der Renormierungsgruppen-Formalismus zur Beschreibung von effektiven Feldtheorien und kritischen Phänomene im Kontext der verallgemeinerten Eichtheorie/Gravitations-Dualität ausführlich dargelegt. Folgende Hauptthemen werden in dieser Arbeit behandelt: Die Untersuchung der optischen Eigenschaften von holographischen Metallen und ihre Beschreibung durch das Drude-Sommerfeld Modell, ein Versuch das Homes'sche Gesetz in Hochtemperatur-Supraleitern holographisch zu beschreiben indem verschiedene Diffusionskonstanten und zugehörige Zeitskalen berechnet werden, das mesonische Spektrum bei verschwindender Temperatur und schlussendlich holographische Quantenzustände bei endlichen Dichten. Entscheidend für die Anwendung dieses Rahmenprogramms auf stark korrelierte Systeme der kondensierten Materie ist die Renormierungsgruppenfluss-Interpretation der AdS_5/CFT_4 Korrespondenz und die daraus resultierenden emergenten, holographischen Duale, welche die meisten Beschränkungen der ursprünglichen Theorie aufheben. Diese sogenannten ``Bottom-Up'' Zugänge sind besonders geeignet für Anwendungen auf Fragestellungen in der Theorie der kondensierten Materie und der ``Linear-Response''-Theorie, mittels des holographischen Fluktuations-Dissipations-Theorem. Die Hauptergebnisse der vorliegenden Arbeit umfassen eine ausführliche Untersuchung der R-Ladungs-Diffusion und der Impulsdiffusion in holographischen s- und p-Wellen Supraleitern, welche durch die Einstein-Maxwell Theorie bzw. die Einstein-Yang-Mills Theorie beschrieben werden, und eine Vertiefung des Verständnisses der universellen Eigenschaften solcher Systeme. Als zweites wurde die Stabilität der kalten holographischen Quantenzustände der Materie untersucht, wobei eine zusätzliche Diffusions-Mode entdeckt wurde. Diese Mode kann als eine Art ``R-Spin-Diffusion'' aufgefasst werden, die der Spin-Diffusion in Systemen mit frei beweglichen ``itineranten'' Elektronen ähnelt, wobei die Entkopplung der Spin-Bahn Kopplung die Spin-Symmetrie in eine globale Symmetrie überführt. Das Fehlen der Instabilitäten und die Existenz einer ``Zero-Sound'' Mode, bekannt von Fermi-Flüssigkeiten, deuten eine Beschreibung der kalten holographischen Materie durch eine effektive hydrodynamische Theorie an. / In this dissertation strongly correlated quantum states of matter are explored with the help of the gauge/gravity duality, relating strongly coupled gauge theories to weakly curved gravitational theories. The main focus of the present work is on applications to condensed matter systems, in particular high temperature superconductors and quantum matter close to criticality at zero temperature. The gauge/gravity duality originates from string theory and is a particular realization of the holographic principle. Therefore, a brief overview of the conceptual ideas behind string theory and the ramifications of the holographic principle are given. Along the way, supersymmetry and supersymmetric field theories needed to understand the low energy effective field theories of superstring theory will be discussed. Armed with the string theory background, the double life of D-branes, extended object where open strings end, is explained as massive solitonic solutions to the type II supergravity equations of motion and their role in generating supersymmetric Yang-Mills theories. Connecting these two different pictures of D-branes will give an explicit construction of a gauge/gravity duality, the AdS_5/CFT_4 correspondence between N=4 supersymmetric SU(N_c) Yang-Mills theory in four dimensions with vanishing beta-function to all orders, describing a true CFT, and type IIB supergravity in ten-dimensional AdS_5 X S^5 spacetime. Furthermore, the precise dictionary relating operators of the conformal field theory to fields in the gravitational theory is established. More precisely, the partitions functions of the strongly coupled N=4 supersymmetric Yang-Mills theory in the large N_c limit is equal to the on-shell supergravity partitionevaluated at the boundary of the AdS space. Applying the knowledge of perturbative quantum field theory and its relation to the quantum partition function the dictionary may be extended to finite temperature and finite density states. Thus, all aspects of quantum field theory relevant for the application of linear response theory, the computation of correlation functions, and the description of critical phenomena are covered with emphasis on elucidating connections between thermodynamics, statistical physics, statistical field theory and quantum field theory. Furthermore, the renormalization group formalism in the context of effective field theories and critical phenomena will be developed explaining the critical exponents in terms of hyperscaling relations. The main topics covered in this thesis are: the analysis of optical properties of holographic metals and their relation to the Drude-Sommerfeld model, an attempt to understand Homes' law of high temperature superconductors holographically by computing different diffusion constants and related timescales, the mesonic spectrum at zero temperature and holographic quantum matter at finite density. Crucially for the application of this framework to strongly correlated condensed matter systems is the renormalization flow interpretation of the AdS_5/CFT_4 correspondence and the resulting emergent holographic duals relaxing most of the constraints of the original formulation. These so-called bottom up approaches are geared especially towards applications in condensed matter physics and to linear response theory, via the central operational prescription, the holographic fluctuation-dissipation theorem. The main results of the present work are an extensive analysis of the R-charge- and momentum diffusion in holographic s- and p-wave superconductors, described by Einstein-Maxwell theory and the Einstein-Yang-Mills model, respectively, and the lessons learned how to improve the understanding of universal features in such systems. Secondly, the stability of cold holographic quantum matter is investigated. So far, there are no instabilities detected in such systems. Instead, an interesting additional diffusion mode is discovered, which can be interpreted as an ``R-spin diffusion'', resembling spin diffusion in itinerant electronic systems where the spin decouples from the orbital momenta and becomes an internal global symmetry. The lack of instabilities and the existence of a zero sound and diffusion mode indicates that cold holographic matter is closely described by an effective hydrodynamic theory.

Self-assembled InGaAs quantum dots are nano-objects embedded in the solid-state matrix of GaAs. They act as natural potential traps for charge carriers and feature a number of quantized states due to the quantum confinement. When incorporated in a field effect structure the quantum dot states can be conveniently manipulated with an electric field and probed by resonant laser spectroscopy. In this thesis self-assembled quantum dots were investigated with an emphasis on the study of interactions between localized quantum dot states and charge or spin reservoirs in the environment. Experimentally the quantum dots were addressed in distinct regimes where the quantum dot spectrum was sensitive to individual charge fluctuations or mesoscopic reservoirs. The fundamental transition of a neutral quantum dot was found to exhibit a number of discontinuities in the usually linear dispersion of the exciton energy in external electrostatic fields. The discontinuities were identified to arise from charge fluctuations in the surrounding crystalline matrix in which impurity atoms can capture or release electrons. At characteristic conditions charging and discharging events lead to discrete changes of the electrostatic environment which in turn gives rise to an energy shift of the optical resonance condition. An electrostatic model was developed for a quantitative analysis of charging events and their signatures. On the basis of the model a comprehensive study of nearby quantum dots allowed to map out the relative spatial positions of quantum dots and impurities. In contrast to previous reports our results provide evidence for bulk impurities as the main source of charge fluctuations. By means of resonant laser spectroscopy in the energy dispersion of the neutral exciton a kink with a continuous energy shift has been observed which only occurs close to the regime where an electron is tunneling between the quantum dot and a 2D electron reservoir. The tunneling induces a weak coupling between the localized electron state of the quantum dot and the continuum of states in the reservoir. The tunnel coupling between the interacting states leads to hybridization into a new superposition state. In consequence the energy of the transition is renormalized which explains the kink in the energy dispersion. The hybridization model based on an Anderson-Fano approach quantitatively agrees with the experimental data and allows to extract the coupling strength between the reservoir and the localized state. In addition to the neutral exciton hybridization effects were also ob-served on the charged exciton. To study optical signatures of many-body effects sub-K laser spectroscopy was established and the setup performance was characterized with optical studies of a quantum dot in the Pauli-blockade regime. The electron bath temperature was determined using experimental and calculated electron spin populations as a function of magnetic field and temperature. The experiment provided quantitative access to all parameters except the electron bath temperature. With the optical Bloch equations the electron spin populations were modeled taking into account all relevant external parameters. An analysis of the evolution of the spin population in magnetic fields with the electron bath temperature as the only free fitting parameter was performed. An electron bath temperature of 380 mK was derived being slightly offset to the nominal base temperature of 250 mK. This proves the successful implementation of the sub-K laser spectroscopy setup.

The centers of galaxies are their own ultimate gravitational sinks. Massive black holes and star clusters as well as gas are especially likely to fall into the centers of galaxies by dynamical friction or dissipation. Many galactic centers harbor supermassive black holes (SMBH) and dense nuclear (star) clusters which possibly arrived there by these processes. Nuclear clusters can be formed in situ from gas, or from smaller star clusters which fall to the center. Since the Milky Way harbors both an SMBH and a nuclear cluster, both can be studied best in the Galactic Center (GC), which is the closest galactic nucleus to us. In Chapter 1, I introduce the different components of the Milky Way, and put these into the context of the GC. I then give an overview of relevant properties (e.g. star content and distribution) of the GC. Afterwards, I report the results of four different studies about the GC. In Chapter 2, I analyze the limitations of astrometry, one of the most useful methods for the study of the GC. Thanks to the high density of stars and its relatively small distance from us it is possible to measure the motions of thousands of stars in the GC with images, separated by few years only. I find two main limitations to this method: (1) for bright stars the not perfectly correctable distortion of the camera limits the accuracy, and (2) for the majority of the fainter stars, the main limitation is crowding from the other stars in the GC. The position uncertainty of faint stars is mainly caused by the seeing halos of bright stars. In the very center faint unresolvable stars are also important for the position uncertainty. In Chapter 3, I evaluate the evidence for an intermediate mass black hole in the small candidate cluster IRS13E within the GC. Intermediate mass black holes (IMBHs) have a mass between the two types of confirmed black hole: the stellar remnants and the supermassive black holes in the centers of galaxies. One possibility for their formation is the collision of stars in a dense young star cluster. Such a cluster could sink to the GC by dynamical friction. There it would consist of few bright stars like IRS13E. Firstly, I analyze the SEDs of the objects in IRS13E. The SEDs of most objects can be explained by pure dust emission. Thus, most objects in IRS13E are pure dust clumps and only three young stars. This reduces the significance of the 'cluster' IRS13E compared to the stellar background. Secondly, I obtain acceleration limits for these three stars. The non-detection of accelerations makes an IMBH an unlikely scenario in IRS13E. However, since its three stars form a comoving association, which is unlikely to form by chance, the nature of IRS13E is not yet settled. In the third study (Chapter 4) I measure and analyze the extinction curve toward the GC. The extinction is a contaminant for GC observations and therefore it is necessary to know the extinction toward the GC to determine the luminosity properties of its stars. I obtain the extinction curve by measuring the flux of the HII region in the GC in several infrared HII lines and in the unextincted radio continuum. I compare these ratios with the ratios expected from recombination physics and obtain extinctions at 22 different lines between 1 and 19 micron. For the K-band I derive A_Ks=2.62+/-0.11. The extinction curve follows a power law with a steep slope of -2.11+/-0.06 shortward of 2.8 micron. At longer wavelengths the extinction is grayer and there are absorption features from ices. The extinction curve is a tool to constrain the properties of cosmic dust between the sun and the GC. The extinction curve cannot be explained by dust grains consisting of carbonaceous and silicate grains only. In addition composite particles, which also contain ices are necessary to fit the extinction curve. In the final part of this thesis (Chapter 5) I look at the properties of most of the stars in the GC. These are the old stars that form the nuclear cluster of the Milky Way. I obtain the mass distribution and the light distribution of these stars. I find that the flattening of the stellar distribution increases outside 70''. This indicates that inside a nearly spherical nuclear cluster dominates and that the surrounding light belongs mostly to the nuclear disk. I dissect the light in two components and obtain for the nuclear cluster L_Ks=2.7*10^7 L_sun. I obtain proper motions for more than 10000 stars and radial velocities for more than 2400 stars. Using Jeans modeling I combine velocities and the radial profile to obtain within 100'' (4 pc) a mass of 6.02*10^6 M_sun and a total nuclear cluster mass of 12.88*10^6 M_sun. The Jeans modeling and various other evidence weakly favor a core in the extended mass compared to a cusp. The old star light shows a similar core. The mass to light ratio of the old stars of the nuclear cluster is consistent with the usual initial mass function in the Galaxy. This suggests that most stars in GC formed in the usual way, in a mode different from the origin of the youngest stars there.

No description available.

This thesis discusses several aspects of string field theory. The first issue is bosonic open-closed string field theory and its associated algebraic structure -- the quantum open-closed homotopy algebra. We describe the quantum open-closed homotopy algebra in the framework of homotopy involutive Lie bialgebras, as a morphism from the loop homotopy Lie algebra of closed string to the involutive Lie bialgebra on the Hochschild complex of open strings. The formulation of the classical/quantum open-closed homotopy algebra in terms of a morphism from the closed string algebra to the open string Hochschild complex reveals deformation properties of closed strings on open string field theory. In particular, we show that inequivalent classical open string field theories are parametrized by closed string backgrounds up to gauge transformations. At the quantum level the correspondence is obstructed, but for other realizations such as the topological string, a non-trivial correspondence persists. Furthermore, we proof the decomposition theorem for the loop homotopy Lie algebra of closed string field theory, which implies uniqueness of closed string field theory on a fixed conformal background. Second, the construction of string field theory can be rephrased in terms of operads. In particular, we show that the formulation of string field theory splits into two parts: The first part is based solely on the moduli space of world sheets and ensures that the perturbative string amplitudes are recovered via Feynman rules. The second part requires a choice of background and determines the real string field theory vertices. Each of these parts can be described equivalently as a morphism between appropriate cyclic and modular operads, at the classical and quantum level respectively. The algebraic structure of string field theory is then encoded in the composition of these two morphisms. Finally, we outline the construction of type II superstring field theory. Specific features of the superstring are the appearance of Ramond punctures and the picture changing operators. The sewing in the Ramond sector requires an additional constraint on the state space of the world sheet conformal field theory, such that the associated symplectic structure is non-degenerate, at least on-shell. Moreover, we formulate an appropriate minimal area metric problem for type II world sheets, which can be utilized to sketch the construction of a consistent set of geometric vertices. The algebraic structure of type II superstring field theory is that of a $\mathcal{N}=1$ loop homotopy Lie algebra at the quantum level, and that of a $\mathcal{N}=1$ homotopy Lie algebra at the classical level.