Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 03/05

Fri, 29 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13320/ https://edoc.ub.uni-muenchen.de/13320/1/Weier_Henning.pdf Weier, Henning ddc:530, ddc:500, Fakultät für Physik

Attosecond (as) physics has become a wide spreaded and still growing research field over the last decades. It allows for probing and controlling core- and outer shell electron dynamics with never before achieved temporal precision. High harmonic generation in gases in combination with advanced extreme ultraviolet (XUV ) optical components enable the generation of isolated attosecond pulses as required for absolute time measurements. But until recently, single attosecond pulse generation has been restricted to the energy range below 100 eV due to the availability of sources and attosecond optics. Multilayer mirrors are the up to date widest tunable optical components in the XUV and key components in attosecond physics from the outset. In this thesis, the design, fabrication and measurement of periodic and aperiodic XUV multilayer mirrors and their application in the generation and shaping of isolated attosecond pulses is presented. Two- and three material coatings based on a combination of molybdenum, silicon, boron carbide, lanthanum and scandium covering the complete spectral range between 30 and 200 eV are developed and characterized. Excellent agreement between reflectivity simulations and experiments is based on the highly stable ion beam sputter deposition technique. It allows for atomically smooth deposition and the realization of aperiodic multilayer structures with high precision and reproducibility. XUV reflectivity simulation of lanthanum containing multilayer coatings are based on an improved measured set of optical constants, introduced in this thesis. This work enabled the generation of the shortest ever measured isolated light pulses so far, the creation of the first isolated attosecond pulses above 100 eV , the first demonstration of absolute control of the “attochirp” by means of multilayer mirrors and the formation of spectrally cleaned attosecond pulses, in a spectral region which lacks appropriate filter materials, for a never before achieved combination of spectral and temporal resolution at 125 eV . Here presented concepts are in principle not restricted to specific energies or experimental set-ups and may be extended in the near future to enter completely new regimes of ultrashort physics.

Tue, 26 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13357/ https://edoc.ub.uni-muenchen.de/13357/1/Figueiro_Spinelli_Patricia.pdf Figueiro Spinelli, Patricia ddc:530, ddc:500, Fakultät für Physik

Tue, 26 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13435/ https://edoc.ub.uni-muenchen.de/13435/1/Bernhardt_Birgitta.pdf Bernhardt, Birgitta ddc:530, ddc:500, Fakultät für Physik 0

Diese Arbeit untersucht mithilfe eines axisymmetrischen numerischen Modells die Prozesse, die zur Intensivierung tropischer Zyklone führen. Das Modell ist hydrostatisch, die Mo-dellgleichungen sind in Sigmakoordinaten auf einer f-Ebene formuliert. Es besteht aus drei Schichten: einer für die Grenzschicht und zwei für die freie Troposphäre. Insbesondere wird der Einfluss des Coriolisparameters f auf die Intensität und Größe von tropischen Zyklonen untersucht. In der ersten von zwei Experimentreihen zeigt sich, dass sich die stärksten Stürme bei mittleren Werten von f entwickeln. Ebenso gibt es einen optimalen Wert von f im mittleren Bereich, bei dem die größten Stürme entstehen. Diese Ergebnisse scheinen zunächst mit klassischen Laborexperimenten von Turner und Lilly übereinzustimmen. Eine mögliche Analogie dieser Laborexperimente zu tropischen Zyklonen wird eingehend untersucht. Dabei zeigt sich, dass diese Analogie unter anderem aufgrund des in der Grenzschicht stattfindenden Intensivierungsprozesses begrenzt ist. Zum weiteren Verständnis wird eine zweite Experimentreihe durchgeführt. Die modellierten Stürme werden hierbei durch ein vorgeschriebenes Profil der diabatischen Erwärmungsrate angetrieben. Andere Feuchtprozesse werden ausgeschlossen. Es ergibt sich nun kein optimaler Wert von f für die Intensität der Stürme. Die Beziehung zwischen der Stärke des Antriebs und der Stärke der Rotation ist somit ein wichtiger zusätzlicher limitierender Faktor bei tropischen Zyklonen. Dennoch gibt es einen optimalen Breitengrad für die Größe der Zyklone, vergleichbar mit dem in der ersten Experimentreihe. Außerdem wird die Sensitivität des Modells bezüglich der horizontalen Auflösung, des Eddy-Diffusions- und Reibungskoeffizienten und der Windgeschwindigkeitsabhängigkeit des Bodenflusses von Enthalpie untersucht. Die Intensität nimmt geringfügig mit größerer horizontaler Auflösung zu, die Größe des Sturms bleibt nahezu unverändert. In Übereinstimmung mit anderen Ergebnissen in der Literatur ist die Intensität stark abhängig vom horizontalen Eddy-Diffusionskoeffizienten. Erhöht man den Reibungskoeffizienten und lässt den Wärmeaustauschkoeffizienten konstant, bewirkt dies eine erhöhte Feuchtekonvergenz und damit einen früheren Beginn der schnellen Intensivierung. Die Intensität am Ende der Simulation nimmt, im Unterschied zu neuesten Ergebnissen von Montgomery et al., jedoch ab. Kappt man die Windgeschwindigkeitsabhängigkeit des Bodenflusses von Enthalpie bei kleinen Werten von 10 m/s, so simuliert das Modell dennoch Stürme mit Intensitäten, die Hurrikanstärke übersteigen. Dies zeigt, dass der in weiten Kreisen akzeptierte 'Verdunstungs-Wind-Rückkopplungsmechanismus' nicht wesentlich für die Intensivierung tropischer Zyklone ist.

Mon, 25 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/14365/ https://edoc.ub.uni-muenchen.de/14365/1/Batteiger_Valentin.pdf Batteiger, Valentin ddc:530, ddc:500, Fakultät für Physik

In this thesis we study microscopic aspects of Calabi-Yau black holes in string theory. We compute the absorption cross-section of the space-time massless scalars by the world-volume of D2-branes, wrapped on the S2 of an AdS_2 x S^2 x CY_3 geometry of a four-dimensional D4-D0 Calabi-Yau black hole. The D2-brane can also have a generic D0 probe-brane charge. However, we restrict ourselves to D2-branes with small D0-charge so that the perturbation theory is applicable. According to the proposed AdS_2/QM correspondence the candidate for the dual theory is the quantum mechanics of a set of probe D0-branes in the AdS2 geometry. For small but non-zero probe D0-charge we find the quantum mechanical absorption cross-section seen by an asymptotic anti-de Sitter observer. We repeat the calculations for vanishing probe D0-charge as well and discuss our result by comparing with the classical absorption cross-section. In other project, for a given four-dimensional Calabi-Yau black hole with generic D6-D4-D2-D0 charges we identify a set of supersymmetric branes, which are static or stationary in the global coordinates, of the corresponding eleven-dimensional near horizon geometry. The set of these BPS states, which include the branes partially or fully wrap the horizon, should play a role in understanding the partition function of black holes with D6-charge.

Fri, 22 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13265/ https://edoc.ub.uni-muenchen.de/13265/1/Demozzi_Vittoria.pdf Demozzi, Vittoria ddc:530, ddc:500, Fakultät für Physik

We are experiencing a unique epoch in the history of galaxy cluster studies. We have now open windows across the whole electromagnetic spectrum which offer us complementary approaches for cluster detection and analyses. Almost forty years after its theoretical prediction, first large radio telescopes started to scan the sky looking for massive clusters as "shadows" in the cosmic microwave background imprinted there by their hot gas content via the Sunyaev-Zel'dovich effect (SZE). In X-rays this hot plasma can be observed also directly. Optical and infrared telescopes give us a view on the galaxy population of clusters and through gravitational lensing also on its dominant, invisible component - the dark matter. The advent of multi-wavelength cluster surveys brings also the necessity to compare and cross-calibrate each cluster detection approach. This is the main aim of this work carried out in the framework of the XMM-emph{Newton}-Blanco Cosmology Survey project (XMM-BCS). This project is a coordinated multi-wavelength survey in a 14~deg$^2$ test region covered in the optical band by the Blanco Cosmology Survey, in the mid-infrared by the emph{Spitzer} Space Telescope and in X-rays by XMM-emph{Newton}. This area is also part of the sky scanned by both SZE survey instruments: the South Pole Telescope (SPT) and the Atacama Cosmology Telescope (ACT). In the first part of the thesis I describe the analysis of the initial 6~deg$^2$ core of the X-ray survey field. From the detected extended sources a cluster catalog comprising 46 objects is constructed. These cluster candidates are confirmed as significant galaxy overdensities in the optical data, their photometric redshifts are measured and for a subsample confirmed with spectroscopic measurements. I provide physical parameters of the clusters derived from X-ray luminosity and carry out a first comparison with optical studies. The cluster catalog will be useful for direct cross-comparison with optical/mid-infrared catalogs, for the investigation of the survey selection functions, stacking analysis of the SZE signal and for cosmological analyses after combing with clusters detected in the extension of the survey. The extension of the survey to 14~deg$^2$ is a first scientific utilization of the novel XMM-emph{Newton} mosaic mode observations. I have developed a data analysis pipeline for this operation mode and report on the discovery of two galaxy clusters, SPT-CL~J2332-5358 and SPT-CL~J2342-5411, in X-rays. The clusters were also independently detected through their SZE signal by the SPT and in the optical band in the BCS data. They are thus the first clusters detected under survey conditions by all major cluster search approaches. This work also demonstrates the potential of the mosaic mode observations to effectively cover large sky areas and detect massive clusters out to redshifts $sim1$ even with shallow exposures. The last part of the thesis provides an example of a multi-wavelength analysis of two high-redshift ($z>1$) systems in the framework of the XMM-emph{Newton} Distant Cluster Project. With the detection and studies of these high redshift systems we are for the first time able to see the assembly phase of the galaxy population of the clusters, which in nearby systems is totally passive, but at these high redshifts still show signatures of star formation.

Thu, 21 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13348/ https://edoc.ub.uni-muenchen.de/13348/1/Donnert_Julius.pdf Donnert, Julius ddc:530, ddc:500, Fakultät für Physik

In der vorliegenden Doktorarbeit werden die elektronischen Eigenschaften von elektrostatisch definierten Quantenpunktsystemen in GaAs/AlGaAs Heterostrukturen untersucht. Dazu werden gepulste Hochfrequenzexperimente zur Untersuchung der Dynamik von einzelnen Elektronen in Doppelquantenpunkten durchgeführt. Der im Laufe dieser Arbeit entwickelte Messaufbau ermöglicht zeitaufgelöste Messungen im sub-Nanosekunden Bereich. Verschiedene Kopplungsregime der Quantenpunkte an die Zuleitungen wurden untersucht. Durch das Anwenden eines Ratengleichungsmodells wurden aus den durchgeführten Messungen Relaxationsraten und Tunnelraten von Elektronen bestimmt. Des weiteren wurden die Rückkopplungen eines Quantenpunktkontakts, der als Ladungsdetektor betrieben wird, auf einen Doppelquantenpunkt untersucht. Ein Quantenpunktkontakt, über den eine Spannung angelegt wird, emittiert Ladungsträger, die eine Überschussenergie tragen. Diese Energie kann von einem Elektron im Doppelquantenpunkt absorbiert werden und Übergänge zwischen den Quantenpunkten (oder den Quantenpunkten und den Zuleitungen) induzieren. Solche Übergänge finden zwar häufig statt, können jedoch nur unter bestimmten Voraussetzungen im Doppelquantenpunkt detektiert werden. Es wurden zwei Mechanismen zum Austausch von Energie identifiziert, nämlich akustischen Phononen und indirekte Coulomb-Wechselwirkung.

Wed, 20 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/14012/ https://edoc.ub.uni-muenchen.de/14012/1/Wirth_Adrian.pdf Wirth, Adrian ddc:530, ddc:500, Fakultät für Physik

The scope of this work is to gain insight and a deeper understanding of exploring and controlling molecular devices like proteins and rotors by fine tuned manipulation via mechanical or electrical energies. I focus on three main topics. First, I investigate vectorial forces as a tool to explore the energy landscape of protein complexes. Second, I apply this method to a biologically important force transduction complex, the integrin-talin complex. Third, I use Terahertz electric fields to manipulate the energy landscape of a molecular rotor on a gold surface and drive their effective rotation bidirectionally. Force is by nature a vector and depends on its three parameters: magnitude, direction and attachment point. Here, the impact of different force protocols varying these parameters is shown for an antibody-antigen complex and the ribonuclease-inhibitor complex barnase-barstar. Antibodies are essential for our adaptive immune system in their function to bind specific antigens. Here, the binding of an antibody to a peptide is probed with varying attachment points. Different attachment points clearly change the dissociation pathways. The barriers identified using experimental atomic force microscopy (AFM) and molecular dynamics (MD) simulations are in excellent agreement. I determine the molecular interactions of two main barriers for each setup. This results in a common outer barrier of the complex and different inner barriers probed by AFM. The ribonuclease barnase and its inhibitor barstar form an evolutionary optimized complex. Different force protocols are shown to determine the hierarchy of relative stability within a protein complex. For the barnase-barstar complex, the internal fold of the barstar is identified to be less stable than the barnase-barstar binding interaction. High velocities probe the lability or barriers of the system while low velocities probe the stability or energy wells of this system. Forces impact biological life on totally different length scales which range from whole organisms to individual proteins. Integrins are the major cell adhesion receptors binding to the extracellular matrix and talin. Talin activates the integrins and creates the initial connection to the actin cytoskeleton of the cell. Here, I have chosen to investigate the integrin-talin complex as a biologically important force transduction complex. The force dependence of the system is probed by constant force MD simulations. The two main results include the activation of the complex and its force response. I demonstrate, that the binding of talin to integrin does not disrupt the integrin's transmembrane helix interactions sterically. Since, this disruption is necessary for integrin activation, a modified activation mechanism requiring a small force application is proposed. The response of the integrin-talin complex normal and parallel to the cell membrane is analyzed. The complete dissociation pathways generated for both directions identify a force-induced formation of a stabilizing beta strand between integrin and talin only for normal forces. Furthermore, the complex tries to rotate such that the external force aligns with the more force resistant axis of the complex. In nature, molecular rotors are essential building blocks of many molecular machines and brownian motors like the F1-ATPase or the flagellum of a bacterium. The direction of rotation often steers different processes in clockwise and counterclockwise directions. Rotation on the nanoscopic level in artificial devices is still very limited and requires a deeper understanding. In my last project, I study the switching and driving of a molecular diethylsulfid rotor on a gold (111) surface by Terahertz electric fields. The response of the rotational energy landscape to static and oscillation electric fields is analyzed. Varying the Terahertz driving frequency, the rotation direction and frequency are controlled. A theoretical framework is presented to describe the behavior of the molecular rotor. This can be seen as the first step into the direction of man-made controllable nano-devices driven and controlled by energy from the electric wall-socket.

Fri, 15 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13230/ https://edoc.ub.uni-muenchen.de/13230/1/Lee_Chien-Hsiu.pdf Lee, Chien-Hsiu ddc:530, ddc:500, Fakultät für Physik

Fri, 15 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13381/ https://edoc.ub.uni-muenchen.de/13381/1/Schlotterer_Oliver.pdf Schlotterer, Oliver ddc:530, ddc:500, Fakultät für Physik

Fri, 15 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13469/ https://edoc.ub.uni-muenchen.de/13469/1/Haertl_Daniel.pdf Härtl, Daniel ddc:530, ddc:500, Fakultät für Physik

In this dissertation we use gauge/gravity duality to investigate various phenomena of strongly coupled field theories. Of special interest are quantum phase transitions, quantum critical points, transport phenomena of charges and the thermalization process of strongly coupled medium. The systems studied in this thesis might be used as models for describing condensed matter physics in a superfluid phase near the quantum critical point and the physics of quark-gluon plasma (QGP), a deconfinement phase of QCD, which has been recently created at the Relativistic Heavy Ion Collider (RHIC). Moreover, we follow the line of considering different gravity setups whose dual field descriptions show interesting phenomena of systems in thermal equilibrium, slightly out-of-equilibrium and far-from-equilibrium. We first focus on systems in equilibrium and construct holographic superfluids at finite baryon and isospin charge densities. For that we use two different approaches, the bottom-up with an U(2) Einstein-Yang-Mills theory with back-reaction and the top-down approach with a D3=D7 brane setup with two coincident D7-brane probes. In both cases we observe phase transitions from a normal to a superfluid phase at finite and also at zero temperature. In our setup, the gravity duals of superfluids are Anti-de Sitter black holes which develop vector-hair. Studying the order of phase transitions at zero temperature, in the D3=D7 brane setup we always find a second order phase transition, while in the Einstein-Yang-Mills theory, depending on the strength of the back-reaction, we obtain a continuous or first order transition. We then move to systems which are slightly out-of-equilibrium. Using the D3/D7 brane setup with Nc coincident D3-branes and Nf coincident D7-brane probes, we compute transport coefficients associated with massive N = 2 supersymmetric hypermultiplet fields propagating through an N = 4 SU(Nc) super Yang-Mills plasma inthe limit of Nf

In this thesis ultrafast processes occuring in photolabile protecting groups of the ortho-nitrobenzyl type have been studied. These protecting groups enable the spatially and temporarily defined release of chemically or biologically active molecules. Due to these properties they find widespread applications, besides technical ones mainly in biochemical research. Molecules of the o-nitrobenzyl type represent the major part of photolabile protecting groups used today. Therefore, in-depth knowledge of the reaction and release mechanism is desired to improve and adapt the properties of these molecules to their applications. Despite this relevance experiments on the ultrafast behaviour of these molecules are scarce. Here, femtosecond pump probe experiments with UV/VIS, IR and Raman probing have been employed to elucidate the early processes occuring during de-protection. Model systems as well as a "real" protecting group have been investigated. Thereby, a general picture of the ultrafast processes occuring in the photoreaction of these molecules could be obtained. Excitation with uv-light to an upper singlet state leads to ultrafast internal conversion to the S1 state. Three competing processes, internal conversion, inter system crossing and the photoreaction, contribute to the decay of this state on the time scale of 1 ps. For the reaction path the formation of the first intermediate could be traced. It results from a hydrogen transfer from the substituent in o-position to the nitro group and exhibits aci-nitro like structure. In addition to the direct formation from the S1 state this intermediate is also formed via a second channel on the nanosecond time scale. This channel involves a triplet state. The "local" triplet state undergoes a hydrogen transfer reaction resulting in a triplet phased bi-radical. The recombination of this biradical then yields the aci-nitro tautomer. Once formed the aci-nitro tautomer transforms into the final nitroso product and the released functionality in a yield of 100 %. Furtheron, the similarity between the reaction mechanisms of all molecules investigated leads to the conclusion that it is universal for the group of o-nitrobenzenes. Additionally, for oNBA the influence of vibrational excitation on the stability of a ground state intermediate could be traced. The ultrafast formation of the first intermediate is about one order of magnitude faster than typical cooling times for these molecules. Therefore, the dynamics of vibrational excitation and its impact on the subsequent reaction could be studied. The vibrational excitation results in bi-phasic kinetics: a fast phase during vibrational excitation of the molecule and a slower one for the thermalized molecule.

Wed, 13 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13354/ https://edoc.ub.uni-muenchen.de/13354/1/Regner_Nadja.pdf Regner, Nadja ddc:530, ddc:500, Fakultät für Physik

Mon, 11 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/14824/ https://edoc.ub.uni-muenchen.de/14824/1/Melbinger_Anna.pdf Melbinger, Anna Tatjana ddc:530, ddc:500, Fakultät

The scheme of short-pulse pumped optical parametric chirped-pulse amplification (OPCPA) offers a promising route towards a completely new regime of ultra-high power few-cycle pulse generation, which reaches well beyond the limits of the conventional laser technology. In this approach, the gain bandwidth limitations of conventional laser amplification are circumvented by using thin OPA crystals in a non-collinear pump-signal geometry (NOPA), while the high gain and pulse energies are ensured by the intense pumping and large crystals sizes. The Petawatt-Field-Synthesizer (PFS) project at the Max Planck Institute of Quantum Optics (Garching, Germany), aims at delivering waveform-controlled few-cycle laser pulses with PW-scale peak power based on few-ps pumped OPCPA. This work focuses on the development of a frontend light source for the PFS system to deliver optically synchronized seed pulses for the OPCPA beam-line and the pump laser. Methods of generating the broadband near-infrared seed pulses for the OPCPA chain by spectral broadening using few-cycle pulses, and idler generation using NOPA are presented. Concepts of stretching both seed pulses, for the pump and the OPCPA, in time and their recompression after amplification are discussed. A detailed experimental and theoretical investigation of timing jitter between the pump and seed pulses in our system is presented. The experimental demonstration of shortpulse-pumped non-collinear OPCPA in a DKDP crystal is presented showing an ultrabroad gain bandwidth in the visible-near infrared, which supports sub-two optical cycle pulse duration.

Wed, 6 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13270/ https://edoc.ub.uni-muenchen.de/13270/1/krischek_roland.pdf Krischek, Roland Patrik ddc:530, ddc:500, Fakultät für Physik

For over 150 years it is known that particles in a temperature gradient conduct a directed movement. This is called the Soret effect or thermophoresis. Still the underlying physical principles of thermophoresis in aqueous solutions are not totally understood. In the first part of this thesis new experiments on the thermophoresis of small single-stranded DNA oligonucleotides try to elucidate the fundamental principles of the Soret effect and the results seem to support a thermodynamic description of the thermophoretic movement. With this approach the experimental results for DNA could be predicted without free fitting parameters. Assuming this theory the thermophoretic movement mainly depends on the strength of ionic shielding and on the hydration sphere of the particle. This direct influence of the water-particle interface implicates that thermophoresis is very sensitive to even slight changes of particles. Applied to biomolecules like DNA or proteins the Soret effect allows for a precise analysis of the molecule under investigation. Any binding reaction, for example, will at least result in a change of the hydration sphere of the molecule and thus, binding reactions are readily accessible with thermophoresis. This is demonstrated in the second part of this work. The experiments range from DNA aptamers binding to nucleotides or proteins over protein-protein interactions to single ion binding. Especially low molecular weight binders like small molecules or ions are notoriously difficult to measure with standard interaction analysis tools. Interestingly, in thermophoresis measurements the signal to noise ratio does not significantly depend on the molar weight ratio as it is the case for other interaction analysis techniques. High affinities in the nanomolar regime are equally well measured as low affinities in the high micromolar range. The thermophoretic method also allows monitoring interactions of biomolecules directly in biological liquids like cell lysate or blood serum. To overcome potential influences of the typically used fluorescent label on the interaction strength, intrinsic protein fluorescence is also suitable for monitoring the thermophoretic movement of proteins. This approach allows a complete label-free measurement of protein interactions directly in solution without any labeling or surface functionalizing procedure. Third, also structural changes of molecules could be analyzed with thermophoresis. This is demonstrated in the last part of this thesis with measurements on the thermal stability of nucleic acids. Most conformational changes affect at least the hydration sphere of a molecule and thus lead to a measurable readout in the thermophoresis signal. Again, the thermophoretic method shows a high sensitivity for small changes in the molecule structure and thus, allows for revealing intermediate states upon the unfolding of nucleic acids.

We analyse the effects of thermal quasiparticles in leptogenesis using hard-thermal-loop-resummed propagators in the imaginary time formalism of thermal field theory. We perform our analysis in a leptogenesis toy model with three right-handed heavy neutrinos N_1, N_2 and N_3. We consider decays and inverse decays and work in the hierarchical limit where the mass of N_2 is assumed to be much larger than the mass of N_1, that is M_2 >> M_1. We neglect flavour effects and assume that the temperatures are much smaller than M_2 and M_3. We pay special attention to the influence of fermionic quasiparticles. We allow for the leptons to be either decoupled from each other, except for the interactions with neutrinos, or to be in chemical equilibrium by some strong interaction, for example via gauge bosons. In two additional cases, we approximate the full hard-thermal-loop lepton propagators with zero-temperature propagators, where we replace the zero-temperature mass by the thermal mass of the leptons m_l(T) in one case and the asymptotic mass of the positive-helicity mode sqrt{2} m_l(T) in the other case. We calculate all relevant decay rates and CP-asymmetries and solve the corresponding Boltzmann equations we derived. We compare the final lepton asymmetry of the four thermal cases and the vacuum case for three different initial neutrino abundances; zero, thermal and dominant abundance. The final asymmetries of the thermal cases differ considerably from the vacuum case and from each other in the weak washout regime for zero abundance and in the intermediate regime for dominant abundance. In the strong washout regime, where no influences from thermal corrections are commonly expected, the final lepton asymmetry can be enhanced by a factor of two by hiding part of the lepton asymmetry in the quasi-sterile minus-mode in the case of strongly interacting lepton modes.

Wed, 22 Jun 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13158/ https://edoc.ub.uni-muenchen.de/13158/1/Hennig_Martin.pdf Hennig, Martin ddc:530, d

Fri, 17 Jun 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13157/ https://edoc.ub.uni-muenchen.de/13157/2/Hirschmann_Michaela.pdf Hirschmann, Michaela ddc:530, ddc:500, Fakultät für Physik

This doctoral thesis covers the question about the existence and evolution of magnetic fields in galaxies. The kinematic evolution of magnetic fields in isolated spiral galaxies as well as their self- consistent evolution in interacting spirals is studied with the help of high-resolution state-of-the-art numerical simulations. Also, implications of an interaction-driven amplification for the magnetic field evolution during the phase of hierarchical structure formation in the Universe are discussed.

In der vorliegenden Dissertation wurden auf alternativen Nanopartikelmorphologien basierende Hybridsysteme hinsichtlich ihrer optische Eigenschaften untersucht, um die bekannten Limitierungen und Unzulänglichkeiten der etablierten Nanopartikelsysteme und -formen, wie stäbchenförmige oder sphärische Nanopartikel, weitgehend zu beseitigen. Es werden sternförmige Goldnanopartikel und ihre besonderen plasmonischen Eigenschaften vorgestellt. Mit Methoden der Dunkelfeldspektroskopie, der Photoemissionelektronenmikroskopie und Rasterelektronenmikroskopie (SEM) werden die Nah- und Fernfeldeigenschaften einzelner Nanosterne mit ihrer Morphologie korreliert. So konnte der erstmalige experimentelle Nachweis der Lokalisierung von optisch angeregten Heißpunkten an den Spitzen der einzelnen Nanosterne geliefert werden. Durch geeignete Wahl der Polarisation und Wellenlänge werden Heißpunkte an mehreren oder ausschließlich an einzelnen Spitzen eines Nanosterns selektiv anregt und die detektierten Plasmonresonanzen nur aufgrund experimenteller Daten einzelnen Nanosternspitzen zugeordnet. Desweiteren werden Feldverstärkungsfaktoren im Bereich von 58-79 an den Nanosternspitzen zum ersten Mal direkt quantitativ bestimmt. Die hohen und für Moleküle leicht zugänglichen Heißpunkte an den Spitzen der Nanosterne werden verwendet, um die hervorragende Verstärkung der Ramanstreuung auf der Ebene einzelner Nanosterne zu demonstrieren. Die Ramanverstärkungsfaktoren von 10^7 für die untersuchten Nanosterne ohne Ausnutzung plasmonischer Kopplungseffekte, zeichnen die Nanosterne als wichtige Kandidaten für die Verwendung in komplexen dynamischen Umgebungen aus. Eine weitere Möglichkeit der Ausnutzung der an Spitzen lokalisierten Heißpunkte bieten Nanobipyramiden. Durch Manipulation mit einem Rasterkraftmikroskop lässt sich eine Positionierung der Heißpunkte bis zu 1 nm oberhalb der Oberfläche und an beliebigen Stellen auf einem Probensubstrat erreichen. Nanoresonatoren aus zwei Nanobipyramiden können in drei verschiedenen Adsorptionskonfigurationen hinsichtlich der zueinander weisenden Spitzen der zwei Nanopartikel mechanisch hergestellt werden. Durch die mechanische Änderung der Konfiguration des Nanoresonators kann die Position des im Partikelzwischenraum entstehenden Heißpunktes in der Höhe bewegt werden ohne wesentliche Änderung der spektralen Lage der gekoppelten Plasmonresonanz.

Wed, 15 Jun 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13883/ https://edoc.ub.uni-muenchen.de/13883/1/Rohr_Carsten.pdf Rohr, Carsten ddc:530, ddc:500, Fakultät für Physik

The first determination of the strong coupling constant alpha_s via the differential 2-jet-rate in pp collisions at the LHC (at a center-of-mass-energy of 7 TeV) is presented. Data gathered by the ATLAS experiment are fitted by next-to-leading order (NLO) perturbative QCD predictions from calculations with the program NLOJET++. As an observable, the jet-flip-parameter from 3 to 2 reconstructed jets is investigated, using the infrared and collinear safe kT jet algorithm in the exclusive reconstruction mode. The jet-flip-parameters from real data are compared to simulated data from Monte Carlo generators. For the determination of alpha_s, real data have been corrected for the jet-energy-scale, whereas the calculations from NLOJET++ have been corrected for the influence of hadronization effects as well as the impact of the Underlying Event by applying bin-by-bin corrections. The fit between real data and the calculations from NLOJET++ yields a value of alpha_s(M_Z)=0.120 +/-0.001(stat.) +/-0.005(syst.), which is in very good agreement with the current world average.

This work describes experiments with quantum-degenerate atomic mixtures at ultracold temperatures, where quantum statistics determine macroscopic system properties. The first heteronuclear molecules at ultracold temperatures are formed in a quantum degenerate two-species Fermi-Fermi mixture on the repulsive side of a narrow s-wave Feshbach resonance. Elastic collisions in this mixture are investigated with the method of cross-dimensional relaxation. Long-lived two-body bound states on the atomic side of the resonance are detected due to a many-body effect at the crossover of the narrow Feshbach resonance. In addition, atom scattering with fermionic 40K on a light field grating in the Bragg and Kapitza-Dirac regimes is realized for the first time. The versatile experimental platform, where the investigations are done, offers the possibility to perform studies on mixtures involving the bosonic species 87Rb and the two fermionic species 6Li and 40K. Within this work, mainly interactions between the two fermionic species are considered. A quantum-degenerate mixture of 6Li and 40K can be used to create heteronuclear bosonic molecules close to an interspecies s-wave Feshbach resonance. By an adiabatic magnetic field sweep, up to 4 × 10^4 molecules are produced with conversion efficiencies close to 50%. A direct and sensitive molecule detection method is developed to probe molecule properties. The lifetime of the molecules in an atom-molecule mixture exhibits a strong magnetic field dependence. Close to resonance, lifetimes of more than 100ms are observed what offers excellent starting conditions for further investigation and manipulation of the molecular cloud. The interspecies Feshbach resonance, which serves for the production of molecules, is further characterized. The method of cross-dimensional relaxation is applied for the first time to a Fermi-Fermi mixture. For this method, a non-equilibrium state is created, which rethermalizes by pure interspecies collisions due to the fermionic nature of the two species. The lighter atomic species, 6Li, relaxes faster in the mixture than the heavier one, 40K. This is verified by an analytical model, Monte-Carlo simulations, and measurements. With this technique, elastic scattering cross sections are measured over a wide range of magnetic field strengths across the Feshbach resonance. The position (B0 = 154.71(5)G) and the magnetic field width of the Feshbach resonance (Delta = 1.02(7)G) are determined. By comparison of the several measurements, long-lived bound states exist on the atomic side of the resonance due to a many-body effect in the crossover regime of the resonance. In addition, atomic scattering with ultracold 40K on a light field crystal is studied for the first time. The light grating is generated by two counter-propagating laser beams. Suitable pulse parameters for the realization of atom scattering in the Bragg and Kapitza-Dirac regime are found. The momentum spread of the cloud determines the efficiency of the scattering process, which is increased by lowering the temperature of the system.

Wed, 8 Jun 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13164/ https://edoc.ub.uni-muenchen.de/13164/1/Misgeld_Ingo.pdf Misgeld, Ingo ddc:530, ddc:500, Fa

Um Galaxien, Galaxienhaufen oder noch größere Strukturen im Universum detailliert zu simulieren, benötigt man eine korrekte Simulation des in diesen Objekten vorhandenen Gases. Eine Möglichkeit zur Simulation dieses Gases bietet das etablierte Verfahren ``Smoothed Particle Hydrodynamics (SPH)''. Diese Methode empfiehlt sich besonders wegen ihrer intrinsischen geometrischen Flexibilität und ihrer adaptiven Auflösung. Neuere Untersuchungen zeigten aber, dass SPH in Situationen, in denen große Dichtesprünge auftreten, ungenau wird. Hier kann es zu einem unphysikalisch verlangsamten Wachstum von hydrodynamischen Instabilitäten kommen. Diese Probleme von SPH können vor allem auf systematisch bedingte Ungenauigkeiten in der Dichtebestimmung dieser Methode zurückgeführt werden. Um diese Probleme zu vermeiden, haben wir eine neue ``Voronoi Particle Hydrodynamics'' (VPH) genannte Methode enwickelt, um die Hydrodynamik zu simulieren. Dabei wird die Dichte der Simulationsteilchen mit Hilfe eines zusätzlichen Gitters bestimmt. Dieses Gitter ist eine Voronoi Pflasterung, die auf auf den Positionen der Teilchen basiert. Mit Hilfe dieses Prinzips können hydrodynamische Instabilitäten korrekt simuliert werden. Situationen, in denen Scherströmungen entlang großer Dichtesprünge auftreten und zu hydrodynamische Instabilitäten führen, sind besonders ungünstig für SPH, da es hier zu großen Ungenauigkeiten kommen kann. Eine Anwendung, in der solche Situationen zu erwarten sind, ist der Einfall einer Galaxie in einen Galaxienhaufen. Dabei verliert die Galaxie aufgrund des anströmenden Galaxienhaufen-Gases zunehmend Gas an den Galaxienhaufen. Da SPH aufgrund seiner Dichtebestimmung diesen Prozess nicht korrekt simuliert, ermittelt SPH einen zu geringen Verlust von Gas. Wir konnten dies mit Hilfe unserer Simulationen belegen. Wir haben diese Resultate sowohl mit Simulationen von Galaxien, die in einen Galaxienhaufen fallen, als auch mit kosmologischen Simulationen von sich bildenden Galaxienhaufen überprüft. Dort bestätigte sich, dass in SPH der Gasverlust der einfallenen Galaxien zu gering ist. Desweiteren ist der Gasverlust in den AREPO Simulationen stets am höchsten, während VPH eine mittlere Stellung einnimmt. Wir konnten ingesamt zeigen, dass VPH in Situationen mit großem Dichtekontrast eine Verbesserung zu SPH darstellt. Auch wenn unsere Resultate keine vollständige Übereinstimmung mit dem Gitter-basierten AREPO Code zeigen, stellen sie doch eine wichtige Annährung zwischen Teilchen- und Gitter-basierten hydrodynamischen Verfahren dar. VPH empfiehlt sich vor allem als eine gegenüber SPH verbesserte Methode zur Simulation von hydrodynamischen Prozesssen in kosmologischen Problemen.

Wed, 1 Jun 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13403/ https://edoc.ub.uni-muenchen.de/13403/1/Harnisch_Florian.pdf Harnisch, Florian

This thesis reports on new experimental techniques for the study of strongly correlated states of ultracold atoms in optical lattices. We used a high numerical aperture imaging system to probe 87Rb atoms in a two-dimensional lattice with single-site resolution. Fluorescence imaging allows to detect single atoms with a large signal to noise ratio and to reconstruct the atom distribution on the lattice. We applied this new technique to a two-dimensional Mott insulator and directly observed number squeezing and the emerging shell structure. A comparison of the radial density and variance distributions to theory provides a precise in situ temperature and entropy measurement from single images. We find entropies around the critical value for quantum magnetism. In a second series of experiments, we demonstrated two-dimensional single-site spin control in the optical lattice. The differential light shift of a tightly focused laser beam shifts selected atoms into resonance with a microwave field driving a spin flip. In this way, we reach sub-diffraction limited spatial resolution well below the lattice spacing. Starting from a Mott insulator with unity filling we were able to create arbitrary spin patterns. We used this ability to prepare atom distributions to study one-dimensional single-particle tunneling dynamics in a lattice. By discriminating the dynamics of the ground state and of the first excited band, we find that our addressing scheme leaves most atoms in the vibrational ground state. Moreover, we studied coherent light scattering from the atoms in the optical lattice and found diffraction maxima in the far-field. We showed that an antiferromagnetic order leads to additional diffraction peaks which can be used to detect this order also when single-site resolution is not available. The new techniques described in this thesis open the path to a wide range of novel applications from quantum dynamics of spin impurities, entropy transport, implementation of novel cooling schemes, and engineering of quantum many-body phases to quantum information processing.

Tue, 17 May 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/14097/ https://edoc.ub.uni-muenchen.de/14097/1/Cremer_Jonas.pdf Cremer, Jonas ddc:530, ddc:500, Fakultät für Physik

Dwarf galaxies are related to important cosmological questions, and central to our understanding of the physics of galaxy formation. In this thesis, I present the results of cosmological, hydrodynamical simulations of the formation and evolution of dwarf galaxies. I compare the simulation results with observations, and interpret them in the context of a Lambda-CDM cosmology. In high resolution simulations of isolated dwarf galaxies, I show that a combination of supernova feedback and the cosmic UV background results in the formation of galaxies with properties similar to the Local Group dwarf spheroidals, and that both effects are strongly moderated by the depth of the gravitational potential. The simulations naturally reproduce the observed scaling relations between luminosity and mass-to-light ratio, and between total stellar mass and metallicities. The final objects have halo masses between 2.3 x 10^8 and 1.1 x 10^9 solar masses, mean velocity dispersions between 6.5 and 9.7 kms^-1, stellar masses ranging from 5 x 10^5 to 1.2 x 10^7 solar masses, median metallicities between [Fe/H]=-1.8 and -1.1, and half-light radii of the order of 200 to 300 pc, all comparable with Local Group dwarf spheroidals. The simulations also indicate that the dwarf spheroidal galaxies observed today lie near a mass threshold around 10^9 solar masses, in agreement with stellar kinematic data, where supernova feedback not only suffices to completely expel the interstellar medium and leave the residual gas-free, but where the combination of feedback, UV radiation and self-shielding establishes a dichotomy of age distributions similar to that observed in the Milky Way and M31 satellites. A second line of work has been the analysis of the dwarf galaxy population resulting from the Aquila simulation. By simultaneously including the formation of a Milky Way type galaxy along with ~500 dwarf-sized haloes in the mass range of ~10^8 - 10^10 solar masses, this simulation allows a study of the effect of the environment on dwarf galaxy evolution. I study the relative importance, and interplay, of the different mechanisms for gas loss, and compare the properties of the satellites with those of isolated dwarf galaxies. A third set of simulations focuses on the formation of dwarf galaxies in a representative sample of haloes extracted from the Millennium-II simulation. The six haloes in these simulations all have a z=0 mass of ~10^10 solar masses and show different mass assembly histories, which are reflected in different star formation histories. The galaxies reach final stellar masses in the range of 5 x 10^7 - 10^8 solar masses, consistent with other published simulations of galaxy formation in similar mass haloes. The resulting objects have structures and stellar populations consistent with dwarf elliptical and dwarf irregular galaxies. However, in a Lambda-CDM universe, 10^10 solar mass haloes must typically contain galaxies with much lower stellar mass than these simulations predict, if they are to match observed galaxy abundances. The dwarf galaxies formed in my own and all other current hydrodynamical simulations are more than an order of magnitude more luminous than expected for haloes of this mass. I discuss the significance and possible implications of this result for cosmological models, and for the assumptions about the physics of galaxy formation. Finally, I present preliminary results of a direct comparison between hydrodynamical simulations and semi-analytical models for the formation of dwarf galaxies. Current semi-analytical models, which are tuned to match the statistical properties of galaxies, do not agree with the predictions of hydrodynamical simulations for individual objects. Conversely, when tuned to accurately reproduce the simulations, semi-analytical models can give a more qualitative interpretation of the simulation results, in terms of equations of galaxy formation. The combination of the two methods allows an extrapolation from individual cases to cosmological volumes, not currently attainable with direct simulations alone.

This thesis reports on theoretical and experimental examination of two-photon direct frequency comb spectroscopy (DFCS) using atomic two-level systems. This method is a very promising tool to extend optical spectroscopy into the short wavelength region where only few cw laser sources exist. The high peak intensities of pulsed lasers facilitate efficient nonlinear conversion into frequency regions which are so far unexplored, for example by high harmonic generation (HHG). DFCS is based on theoretical work in the 1970s which showed that a pulse train of a mode-locked laser drives a two-photon transition as efficient as a cw laser of same average power. Thereby the effective line width is determined by the narrow width of a single comb mode rather than by the spectral width of a single pulse. In this way a frequency comb combines the spectral purity of a cw laser with the high peak intensity of a pulsed laser. To demonstrate the capability of a nonlinearly converted frequency comb for DFCS, the absolute frequency of a two-photon transition in $^{24}$Mg at 431~nm was measured in a first experiment. The accuracies of the values could be improved by more than two orders of magnitude with respect to previously reported values. Furthermore two crucial effects which affect the transition rates were analyzed quantitatively for the first time: the impact of a linear chirp and non-centered spectral envelope on the spectroscopy. In general the pulses of a nonlinear converted frequency comb are not bandwidth limited leading to a partially destructive pairwise addition of modes. To describe the impact of a linear chirp a theoretical model was developed and verified experimentally using two-photon spectroscopy on cesium. Moreover, theory and experiment have shown a Gaussian decrease in the transition rate with increasing detuning of the laser spectrum. %Another important aspect is the centering of the laser spectrum with respect to the atomic transition. Theory and experiment show here a Gaussian decrease in the transition rate with increasing detuning. Finally the progress of $1S - 3S$ spectroscopy in hydrogen is presented. This transition is a promising candidate for a test of bound state quantum electrodynamics. Using cw lasers the required wavelength of 205~nm is hard to generate, making this transition to an eligible system for DFCS. Beside the experimental achievements also the lessons learned from Cs and Mg spectroscopy and their consequences for the H spectroscopy are discussed. In the scope of this work a frequency quadrupled laser system was extended and improved, providing ps frequency combs of high average power and good beam profile at 820~nm, 410~nm and 205~nm. An overall output power of max. 100~mW is now available at 205~nm, the up to date highest power generated by SHG. Moreover for H spectroscopy a new spectrometer was designed and built.

Tue, 10 May 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13697/ https://edoc.ub.uni-muenchen.de/13697/1/Youssef_Simon.pdf Youssef, Simon ddc:530, ddc:500, Fakultät für Physik

Mon, 9 May 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13076/ https://edoc.ub.uni-muenchen.de/13076/1/Gross_Silke.pdf Groß, Silke Martha ddc:530, ddc:500, Fak

Mon, 18 Apr 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13587/ https://edoc.ub.uni-muenchen.de/13587/1/Kuehnlein_Christian.pdf Kühnlein, Christian ddc:530, ddc:500, Fakultät für Physik

In this thesis I present experiments investigating controlled coupling between mechanical oscillators and ultracold atoms. I report on three different coupling mechanisms. In a first experiment, the surface potential experienced by atoms close to the mechanical oscillator is employed to couple the oscillator motion to the center of mass (COM) motion of a trapped Bose-Einstein condensate (BEC). The magnetic trapping potential is modified by the surface potential arising from the oscillator surface which results in a reduced trap depth. Vibration of the oscillator leads to a modulation of the trap frequency and the minimum of the trapping potential. Observing the loss of atoms from the BEC allows us to read out the amplitude of the mechanical oscillator with the atoms. In a second experiment, we study the coupling of a mechanical membrane oscillator and thermal atoms trapped in a 1D optical lattice. The membrane is the end mirror of the lattice, and oscillation of the membrane couples to the COM mode of the atomic ensemble. Conversely, the center of mass motion of the atomic ensemble redistributes photons between the two running waves forming the 1D optical lattice, effectively modulating their power, and hence the radiation pressure acting onto the membrane. We observe the action of the oscillating membrane onto the atoms by detecting the resulting temperature increase of the atomic ensemble in absorption imaging. To observe the backaction of the atoms onto the mechanical oscillator, the mechanical damping is measured in experiments with and without atoms in the lattice, and we measure higher damping in the presence of atoms in agreement with the theoretical predictions. These experiments are the first demonstration of backaction of an atomic system onto a mechanical oscillator. We investigate a third coupling mechanism, where the motion of a mechanical oscillator is coupled to the collective spin of a BEC. The tip of a mechanical oscillator is functionalized with a magnet, which transduces the oscillators' motion into oscillations of the magnetic field. This drives spin-flip transitions of trapped atoms to untrapped motional states. The coupling strength is not limited by the square root of the mass ratio of atoms and oscillator as in the other coupling schemes discussed in this thesis. We investigate this coupling scheme theoretically, and discuss the realization of a nanometer-sized mechanical oscillator with a magnetic island. I report on the status of the fabrication, and propose a simplified fabrication method.

Thu, 7 Apr 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13084/ https://edoc.ub.uni-muenchen.de/13084/1/Thoumany_Pierre.pdf Thoumany, Pierre ddc:530, ddc:500, Fakultät für Physik

Mon, 4 Apr 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13044/ https://edoc.ub.uni-muenchen.de/13044/1/Megerle_Uwe.pdf Megerle, Uwe

This thesis presents an entirely novel approach for frequency comb generation based on nonlinear frequency conversion in micrometer sized optical resonators. Here, the comb generation process can be directly described in frequency domain as energy conserving interactions between four photons (four-photon mixing). This process is a result of extremely high light intensities that build up in microresonators with long photon storage times. The thesis is composed of four main parts that answer fundamental questions in the context of microresonator-based frequency comb generation as well as providing insights in the control and possible applications of this type of comb generators.

Fri, 1 Apr 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13081/ https://edoc.ub.uni-muenchen.de/13081/1/Das_Payel.pdf Das, Payel ddc:530, ddc:500, Fakultät für Physik

Thu, 31 Mar 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13057/ https://edoc.ub.uni-muenchen.de/13057/1/Magerl_Elisabeth.pdf Magerl, Elisabeth ddc:530, ddc:500, Fakultät für Physik

The first topic of this thesis is the study of many-body effects in an one-dimensional strongly correlated electronic system - the Kondo lattice model. This system is tackled numerically by means of the density matrix renormalization group, since analytic method, i.e., perturbation the- ory fail due to competing coupling constants. The Kondo lattice model consists of a conduction band of electrons which couple via a spin exchange coupling to a localized spin lattice. We study the spectral properties of the one-dimensional Kondo lattice model as a function of the exchange coupling, the band filling, and the quasimomentum in the ferromagnetic and paramagnetic phases. We compute the dispersion relation of the quasiparticles, their lifetimes, and the Z factor. The exact ground state and the quasiparticle-dispersion relation of the Kondo lattice model with one conduction electron are well known. The quasiparticle could be identified as the spin polaron. Our calculations of the dispersion relation for partial band fillings give a result similar to the one-electron case, which suggests that the quasiparticle in both cases is the spin polaron. We find that the quasiparticle lifetime differs by orders of magnitude between the ferromagnetic and paramagnetic phases and depends strongly on the quasimomentum. Further- more, we study the effects of the Coulomb interaction on the phase diagram, the static magnetic susceptibility and electron spin relaxation. We show that onsite Coulomb interaction supports ferromagnetic order and nearest neighbor Coulomb interaction drives, depending on the elec- tron filling, either a paramagnetic or ferromagnetic order. Furthermore, we calculate electron quasiparticle life times, which can be related to electron spin relaxation and decoherence times, and explain their dependence on the strength of interactions and the electron filling in order to find the sweet spot of parameters where the relaxation time is maximized. We find that effective exchange processes between the electrons dominate the spin relaxation and decoherence rate. In the second topic of this thesis, we numerically calculate the electron transport through carbon nanotube based quantum dot devices. We use a master equation’s approach in first order of the tunneling rate to the leads and an extended constant interaction model to model the carbon nanotube system. This work has been done in collaboration with two experimental groups and we compare their respective experimentally obtained data to our numerical calculations. In both collaborations striking similarity between the numerical data and the experimental data is found. In the first collaboration transport through a carbon nanotube peapod, i.e, a carbon nanotube filled with fullerenes, has been measured. We identify a small hybridization between a fullerene molecule and the surrounding carbon nanotube to be of crucial importance for the understanding of the transport data. In the second collaboration, electron transport through a carbon nanotube rope, i.e., a bundle of carbon nanotubes has been measured. Also here, hybridization between the different nanotubes plays a crucial role. Furthermore, an external magnetic field is applied, which enables the identification of specific spin states of the compound quantum dot system. This might be important for future applications of such devices in spin-dependent electronics.

Tue, 22 Mar 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/12933/ https://edoc.ub.uni-muenchen.de/12933/1/Hansen_Camilla_Juul.pdf Hansen, Camilla Juul ddc:530, ddc:500

Fri, 18 Mar 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/12898/ https://edoc.ub.uni-muenchen.de/12898/1/Petkova_Margarita.pdf Petkova, Margarita ddc:530, ddc:500, Fakultät für Phy

In der vorliegenden Arbeit werden isolierte, einzeln ortsaufgelöste molekulare Ionen mit einer Femtosekundenspektroskopie auf der Basis von Einzelreaktionsereignissen untersucht. Für die zur simultanen Speicherung von atomaren und molekularen Ionen notwendige Radiofrequenzfalle wurde eine transportable Vakuumapparatur konzipiert und realisiert sowie die zugehörigen Lasersysteme aufgebaut und eingerichtet. Um die Ultrahochvakuumbedinungen bei 2e-10 mbar auch bei häufiger Molekülpräparation gewährleisten zu können, wurde ein modularer Aufbau gewählt, bei dem Präparations- und Expermentierbereich durch differentielle Pumpstrecken voneinander getrennt sind. Durch diese hindurch führt ein 48 cm langer Quadrupolionenleiter, in welchem Ionen zwischen den Kammern transferiert werden können. Entlang des Ionenleiters ermöglichen ringförmige Gleichspannungselektroden eine dreidimensionale Speicherung der Ionen. Im Rahmen dieser Arbeit wurde mit atomaren 24Mg+ und molekularen 24MgH+ Ionen gearbeitet. Erstere werden durch Photoionisation von Magnesiumatomen aus einem thermischen Strahl erzeugt und ihre Bewegungsenergie durch Laserkühlung soweit reduziert, dass sie in etwa 20 μm Abstand voneinander in einer kristallinen Struktur erstarren. Magnesiumhydridionen werden nach Einleiten von Wasserstoffgas in einer photochemischen Reaktion mit 24Mg+ generiert und – von verbleibenden atomaren Ionen sympathetisch gekühlt – auf Gitterplätze des Kristalls integriert. Bei der Laserkühlung von 24Mg+ ausgesendete Fluoreszenzphotonen ermöglichen die optische Detektion der Ionen mit derzeit bis zu 1 μm Ortsauflösung. Die nicht fluoreszierenden molekularen Ionen werden indirekt als vermeintlich unbesetzte Stellen der Kristallstruktur sichtbar. Neben der Demonstration des Erfolges unseres Fallenkonzepts sowie dessen Charakterisierung bildet der verlustfreie, kontrollierte Transport von atomaren und molekularen Ionen aus dem Präparations- in den Experimentierbereich, eine wichtige Errungenschaft, welche zu einem kontinuierlichen Nachladen von Ionen mit einer Rate von über 100 Hz ausgebaut werden kann. Diese Arbeit präsentiert eine Machbarkeitsstudie zur Kombination von Präzisionsmethoden zweier Forschungsgebiete. Dazu wurde die Fallenapparatur mit einem weiteren Vakuumsystem, in dem ultraviolette Femtosekundenpulse erzeugt werden können, über ein System von differenziellen Pumpstrecken verbunden. Als Resultat werden 5 fs zeitaufgelöste Pump-Probe Experimente vorgestellt, die die Oszillation eines Vibrationswellenpaketes von individuellen 24MgH+ Molekülionen zeigen. Dabei wird die Bewegung des Wellenpaketes auf die Dissoziationswahrscheinlichkeit in einem bestimmten Zerfallskanal abgebildet. Einzelne Reaktionsereignisse konnten eindeutig nachgewiesen und daraus das zeitabhängige Verhalten extrahiert werden. Diese Resultate untermauern das Potenzial der von uns angestrebten Kombination der exzellenten Kontrolle über externe und interne Freiheitsgrade gespeicherter Ionen mit der extremen Zeitauflösung von modernen Kurzpulslasern. Weitere Arbeiten können die Vorteile beider Gebiete nutzen um bisher unzugängliche Experimente zu realisieren. Die besonderen Eigenschaften der präsentierten Apparatur sollten es beispielsweise erlauben, einzelne isolierte molekulare Ionen mit hoher räumlicher Präzision und wohl kontrollierten Anfangsbedingungen für zukünftige Strukturuntersuchungen mittels derzeit entstehender, intensiver Kurzpuls-Röntgenquellen an freien Elektronenlasern bereitzustellen.