Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05 Archiv

The detection of correlation and response functions plays a crucial role in the experimental characterization of quantum many-body systems. In this thesis, we present novel techniques for the measurement of such functions at the single-particle level. Specifically, we show the single-atom- and single-site-resolved detection of an ultracold quantum gas in an optical lattice. The quantum gas is described by the Bose-Hubbard model, which features a zero temperature phase transition from a superfluid to a Mott-insulating state, a paradigm example of a quantum phase transition. We used the aforementioned detection techniques to study correlation and response properties across the superfluid-Mott-insulator transition. The single-atom sensitivity of our method is achieved by fluorescence detection of individual atoms with a high signal-to-noise ratio. A high-resolution objective collects the fluorescence light and yields in situ `snapshots' of the quantum gas that allow for a single-site-resolved reconstruction of the atomic distribution. This allowed us to measure two-site and non-local correlation-functions across the superfluid-Mott-insulator transition. Non-local correlation functions are based on the information of an extended region of the system and play an important role for the characterization of low-dimensional quantum phases. While non-local correlation functions were so far only theoretical tools, our results show that they are actually experimentally accessible. Furthermore, we used a new thermometry scheme, based on the counting of individual thermal excitations, to measure the response of the system to lattice modulation. Using this method, we studied the excitation spectrum of the system across the two-dimensional superfluid-Mott-insulator transition. In particular, we detected a `Higgs' amplitude mode in the strongly-interacting superfluid close to the transition point where the system is described by an effectively Lorentz-invariant low-energy theory. Our experimental results helped to resolve a debate about the observability of Higgs modes in two-dimensional systems.

In this thesis we present a combination of methodological work with very wide focus and some specific astrophysical applications. We advance the knowledge on the Galactic interstellar medium by studying new ways of inferring select properties related to its magnetic field. We derive the statistical tools needed in a rigorous way from probabilistic considerations. One quantity describing the statistical properties of magnetic fields is their helicity. We apply a recently developed technique to detect magnetic helicity from astronomical observations to data from the Milky Way. No indications of helicity are found. Using a series of simulations in the Galactic setting, we are able to show that the technique fails to detect helicity in cases in which the underlying electron density varies too strongly. Thus, we are able to conclude that either this is the case in the Milky Way or the Galactic magnetic field is non-helical. We further develop a technique, needed among other things to enable the correct application of the helicity test, to reconstruct continuous signals from noisy data for which the noise level is unknown. To do this, we make use of the statistical correlation structure of the signal which we reconstruct from the same data set in a self-consistent way. Fluctuations in the data that are inconsistent with this correlation structure are then assigned to the data's error budget. The development of this technique is partly motivated by the goal of creating an all-sky map of the Galactic contribution to the astronomical Faraday rotation effect, which probes both the Galactic magnetic field and the density of free thermal electrons. Since the quantity that is observed is the Faraday rotation of a radio source, influenced by all magnetic fields between the source and the observer, extragalactic contributions need to be filtered out. We use the technique for reconstructions in the presence of an uncertain degree of noisiness to assign the extragalactic contributions - as well as some other observational effects - to the error budget of the data and thus single out the Galactic contribution. The resulting map is the most detailed and precise map of its kind and the only one in which the extragalactic contributions have been filtered out. Finally, we develop a method to reconstruct log-normal signal fields, i.e. strictly positive signal fields for which the strengths of the fluctuations vary over several orders of magnitude. This is done with a view to reconstructions of emission maps due to different processes in the Galactic interstellar medium.

Diese Dissertation untersucht den Urspung der kosmischen Strahlung mit Beobachtungen der MAGIC Teleskope im Bereich der hoch energeticher Gammastrahlenastronomie. Gammastrahlung wird bei der Wechselwirkung relativistischer Teilchen erzeugt. Im Gegensatz zur geladenen kosmichen Strahlung, werden Gammastrahlen nicht von interstellaren Magnetfeldern beinflusst. Daher erlaubt die Ankunftsrichtung von Gammastrahlen die Bestimmung ihres Ursprungs. Ein Teil dieser Arbeit widmet sich der Verbesserung der Analyse von MAGIC-daten. Im besonderen wurde ein neuer Algorithmus zur Hintergrundbestimmung entwickelt, wodurch die systematischen Unsicherheiten deutlich verbessert werden konnten. Zudem wurde die Reflekivitaet und Fokusierung beider MAGIC Teleskope anhand des Vergleichs zwischen echten und simulierten Muonereignissen bestimmt. Die heutige Meinung ist, dass die Ueberreste von Supernovae, die expandierende Schockwellen der Sternimplosionen, der Ursprung der galaktischen kosmischen Strahlung sind. Obwohl hoch energetische Gammastrahlung von vielen dieser Objekte beobachtet wurde, erlaubt die schwierige Unterscheidung von leptonisch und hadronsich produzierter Gammastrahlung in den meisten Faellen keine klaren Schlussfolgerungen ueber die Anwesenheit relativistischer Hadronen und damit kosmischer Strahlung. Da Gammastrahlung aus hadronischen Wechselwirkungen, nahezu ausschliesslich in inelastischen Proton-Proton Kollisionen erzeugt wird, ist ihre Produktion umso effektiver, desto hoeher die Dichte des mediums ist. Die Region W51 beherbergt den 30000 Jahre alten Supernovaueberrest W51C, welcher teilweise mit der grossen Molekuelwolke W51B kollidiert. MAGIC hat ausgedehnte Gammastrahlung von dieser Region mit hoher statistischer Signifikanz (11 sigma) gemessen. Es konnte gezeigt werden, dass das Zentrum der Emission in dem Bereich hoher Dichte liegt, wo der Supernovaueberrest mit der Molekuelwolke kollidiert. Das Energiespektrum wurde im Bereich von 75 GeV bis 5.5 TeV gemessen und folgt einem Potenzgesetz. Die moegliche Kontamination dieser Emission durch einen nahegelegenen potentiellen Pulsarwindnebel zeigt keine Energieabhaengigkeit und wurde als ~20\% der Gesamtemission bestimmt. Die Modellierung der nicht thermischen Mutliwellenlaengenemission deutet stark auf einen hadronischen Ursprung der Gammastrahlung hin. Diese Beschreibung impliziert, dass in etwa 16% der kinetsiche Energie der Schockwelle von W51C zur Produktion kosmischer Strahlung genutzt wurden. Damit is W51C eine der wenigen bekannten Supernovaueberreste wo eine Beschleunigung von Protonen der komsischer Strahlung, zumindestens bis 50 TeV, direkt beobachtet wird. HESS J1857+026 ist eine nicht identifizierte TeV-Quelle, die moeglicherweise den Pulsarwindnebel des, von der Gammastrahlung umschlossenen, hochenergetischen Pulsares PSR J1856+0245 darstellt. Eine augedehnte Emission wurde von MAGIC mit einer statistischen Signifikanz von mehr als 12 sigma gemessen. Das berechnete Spektrum verbindet die vorherigen Daten von Fermi/LAT und HESS, wobei es mit beiden Messungen ueberlappt. Anhand der MAGIC und Fermi/LAT Daten wurde ein Abweichung von einem einfachen Potenzgesetz bei ca. 100~GeV festgestellt. Bei hoeheren Energien werden zwei Emissionsregionen aufgeloest. Ueberhalb von einem TeV koennen zwei voneinander getrennte, einzeln signifikante Regionen festgestellt werden. Diese Dissertation zeigt die ersten morphologischen Untersuchungen, die mit den MAGIC Teleskopen durchgefuehrt wurden. Es wurde gezeigt, dass die Faehigkeit Strukturen in galaktischen Quellen aufloesen zu koennen, wichtige Informationen ueber die Physik der Teilchenbeschleunigung in astrophysikalischen Objekten liefert.

The production of a quantum gas with strong long - range dipolar interactions is a major scientific goal in the research field of ultracold gases. In their ro - vibrational ground state Li -K dimers possess a large permanent dipole moment, which could possibly be exploited for the realization of such a quantum gas. A production of these molecules can be achieved by the association of Li and K at a Feshbach resonance, followed by a coherent state transfer. In this thesis, detailed theoretical an experimental preparations to achieve state transfer by means of Stimulated Raman Adiabatic Passage (STIRAP) are described. The theoretical preparations focus on the selection of an electronically excited molecular state that is suitable for STIRAP transfer. In this context, molecular transition dipole moments for both transitions involved in STIRAP transfer are predicted for the first time. This is achieved by the calculation of Franck -Condon factors and a determination of the state in which the 6Li - 40K Feshbach molecules are produced. The calculations show that state transfer by use of a single STIRAP sequence is experimentally very well feasible. Further, the optical wavelengths that are needed to address the selected states are calculated. The high accuracy of the data will allow to carry out the molecular spectroscopy in a fast and efficient manner. Further, only a comparatively narrow wavelength tuneability of the spectroscopy lasers is needed. The most suitable Feshbach resonance for the production of 6Li - 40K molecules at experimentally manageable magnetic field strengths is occurring at 155G. Experimentally, this resonance is investigated by means of cross - dimensional relaxation. The application of the technique at various magnetic field strengths in the vicinity of the 155G Feshbach resonance allows a determination of the resonance position and width with so far unreached precision. This reveals the production of molecules on the atomic side of the resonance, thereby establishing the first observation of a many body effect in the crossover regime of a narrow Feshbach resonance. Further, mass dependent factors, with which the equilibration of an induced anisotropic temperature of the trapped particle samples can be described, are experimentally determined for the first time. The type of resonance as well as the measured molecular lifetimes are found to be very well suited for STIRAP transfer. A Raman laser system is designed based on the transition wavelengths and durations of state transfer which are predicted. As the wavelengths of the Raman lasers differ widely but coherence of the light fields is needed, the technical realization of a laser system is challenging. As a part of the laser system, the construction and characterization of a reference optical resonator are presented. Laser frequency stabilization with a linewidth of approximately 500Hz and an Allan deviation below 10−12 for timespans up to several ten seconds are demonstrated. Further, the stabilization of a frequency comb to this reference laser is demonstrated. For the laser spectroscopy of electronically excited Li -K states an interferometric laser frequency stabilization will be used. The device is a commercial design, for which a calibration procedure that enhances the precision by several orders of magnitude is worked out within this thesis. The calibration scheme includes the precise measurement of the stabilization’s wavelength dependent frequency deviations by means of a frequency comb. By the implementation of several calibration steps a remaining frequency deviation of less than 5.7MHz (rms 1.6MHz) in the whole relevant wavelength range 750 - 795 nm is achieved. Only the exceptional precision of the fully calibrated device permits the usage for the Li -K spectroscopy, while the demonstrated wide tuning capability facilitates the completion of the latter in a fast and convenient manner.

The end of the twentieth century saw a revolution in our knowledge of planetary systems. The detection of the first extrasolar planet in 1992 marked the beginning of a modern era and changed our idea of planets and planetary systems. The discoveries continue rapidly and reveal an extraordinary diversity of planetary systems and physical properties of the exoplanets, raising new questions in the field of planetary science. So far, more than 800 extrasolar planets have been detected, spanning a wide range of masses from a few Earth masses to a few tens of Jupiter masses. This Ph.D. Thesis is devoted to the confirmation via radial velocity follow-up of the candidate planets detected by the WFCAM Transit Survey (WTS), which is an on-going photometric monitoring campaign using the Wide Field Camera on the United Kingdom Infrared Telescope at Mauna Kea (Hawaii, USA). The WTS and the present work were supported by the RoPACS (Rocky Planets Around Cool Stars) group, a Marie Curie Initial Training Network funded by the Seventh Framework Programme of the European Commission. Since the WTS was primarily designed to find planets transiting M-dwarf stars, the observations are obtained in the J-band (1.25 micron). This wavelength is near to the peak of the spectral energy distribution of a typical M-dwarf. Simulations show that operating in the J-band reduces the effects of stellar variability, which became important at optical wavelengths in cool stars. The J-band light curves that show a periodic drop and pass all the selection criteria, progress to the candidate confirmation phase. After a transit depth consistency check performed with i'-band observations, intermediate resolution spectra enable to rule out false-positive eclipsing binaries scenarios. Finally, high-resolution spectroscopic follow-up is performed to confirm, by the radial velocity method, the planetary nature of the stellar companion detected by the WTS. The spectra employed in this phase were observed with the High Resolution Spectrograph (HRS) housed in the basement of the 9.2-m Hobby-Eberly Telescope (HET) in Texas, USA. The pipeline for the reduction and analysis of the HET spectra has been created. Debug, optimization and test of the whole procedure were performed observing several target stars with different apparent magnitude and spectral type. These observations allowed to estimate the precision on the velocity measures for different targets. Errorbars of 10 m/s are expected for solar type stars of magnitude up to mV=10 and SNR of the observed spectra >150. Spectra with a SNR of 30 can be measured for faint (mV=14) M stars, leading to a final radial velocity uncertainty of about 60 m/s. Furthermore, a technical problem occurring under given instrumental configurations could be identified and fixed, removing a possible source of systematic from any later observation. Finally, the zero-point offset with respect to the HARPS data was computed allowing the comparison of the HET measures with those related to any other instruments involved in radial velocity follow-up. The radial velocities computed from the HET high-resolution spectra allowed to confirm the detection of the first two extrasolar planet performed by the WTS. WTS1 b is a 4 MJ planet orbiting in 3.35 days a late F-star with possibly slightly sub-solar metallicity. With a radius of 1.49 RJ, it is the third largest planet of the known extrasolar planets in the mass range 3-5 MJ. Its unusual large radius can not be explained within the standard evolution models, even considering the strong radiation that the planet receives from the parent star. Ohmic heating could be a possible mechanism able to bring energy in the deeper layers of WTS1 b and hence explaining its radius anomaly. WTS2 b is instead a 1 MJ planet orbiting an early K-star in about 1 day only. The measure of its secondary eclipses in the Ks-band will allow to study a highly irradiated planet around a cool star, cooler than many of the currently known very hot-Jupiters host star. This will provide an insight to the effect of the stellar spectrum on the composition and structure of hot-Jupiter atmospheres. Beyond the RoPACS program, the pipeline has been employed in the radial velocity follow-up of the white dwarf NLTT 5306, confirming the presence of a brown dwarf companion of 56 MJ orbiting its host star in 102 minutes, the shortest period ever observed in such systems. The discoveries of WTS1 b and WTS2 b demonstrate the capability of WTS to find planets, even if it operates in a back-up mode during dead time on a queue-schedule telescope and despite of the somewhat randomised observing strategy. Moreover, the two new discovered planets are hot-Jupiters orbiting an F-star and a K-star. Both are hotter than an M-dwarf, the main target sample of the WTS. As described in Kovacs et al. (2012, MNRAS submitted), no planets around M-dwarf stars monitored by the WTS (mV

The introduction of a so-called dark sector in cosmology resolved many inconsistencies between cosmological theory and observation, but it also triggered many new questions. Dark Matter (DM) explained gravitational effects beyond what is accounted for by observed luminous matter and Dark Energy (DE) accounted for the observed accelerated expansion of the universe. The most sought after discoveries in the field would give insight into the nature of these dark components. Dark Matter is considered to be the better established of the two, but the understanding of its nature may still lay far in the future. This thesis is concerned with explaining and eliminating the discrepancies between the current theoretical model, the standard model of cosmology, containing the cosmological constant Λ as the driver of accelerated expansion and Cold Dark Matter (CDM) as main source of gravitational effects, and available observational evidence pertaining to the dark sector. In particular, we focus on the small, galaxy-sized scales and below, where N-body simulations of cosmological structure in the ΛCDM universe predict much more structure and therefore much more power in the matter power spectrum than what is found by a range of different observations. This discrepancy in small scale power manifests itself for example through the well known "dwarf-galaxy problem'" (e.g. Klypin, 1999), the density profiles and concentrations of individual haloes (Donato, 2009) as well as the properties of voids (Tikhonov, 2009). A physical process that would suppress the fluctuations in the dark matter density field might be able to account for these discrepancies. Free-streaming dark matter particles dampen the overdensities on small scales of the initial linear matter density field. This corresponds to a suppression of power in the linear matter power spectrum and can be modeled relatively straightforwardly for an early decoupled thermal relic dark matter particle. Such a particle would be neutrino-like, but heavier; an example being the gravitino in the scenario, where it is the Lightest Supersymmetric Particle and it decouples much before neutrinos, but while still relativistic. Such a particle is not classified as Hot Dark Matter, like neutrinos, because it only affects small scales as opposed to causing a suppression at all scales. However, its free-streaming prevents the smallest structures from gravitationally collapsing and does therefore not correspond to Cold Dark Matter. The effect of this Warm Dark Matter (WDM) may be observable in the statistical properties of cosmological Large Scale Structure. The suppression of the linear matter density field at high redshifts in the WDM scenario can be calculated by solving the Boltzmann equations. A fit to the resulting linear matter power spectrum, which describes the statistical properties of this density field in the simple thermal relic scenario is provided by Viel (2004). This linear matter power spectrum must then be corrected for late-time non-linear collapse. This is rather difficult already in the standard cosmological scenario, because exact solutions the the evolution of the perturbed density field in the nonlinear regime cannot be found. The widely used approaches are to the 'halofit' method of Smith (2002), which is essentially a physically motivated fit to the results of numerical simulations or using the even more physical, but slightly less accurate halo model. However, both of these non-linear methods were developed assuming only CDM and are therefore not necessarily appropriate for the WDM case. In this thesis, we modify the halo model (see also Smith, 2011) in order to better accommodate the effects of the smoothed WDM density field. Firstly, we treat the dark matter density field as made up of two components: a smooth, linear component and a non-linear component, both with power at all scales. Secondly, we introduce a cut-off mass scale, below which no haloes are found. Thirdly, we suppress the mass function also above the cut-off scale and finally, we suppress the centres of halo density profiles by convolving them with a Gaussian function, whose width depends on the WDM relic thermal velocity. The latter effect is shown to not be significant in the WDM scenario for the calculation of the non-linear matter power spectrum at the scales relevant to the present and near future capabilities of astronomical surveys in particular the Euclid weak lensing survey. In order to determine the validity of the different non-linear WDM models, we run cosmological simulations with WDM (see also Viel, 2012) using the cutting edge Lagrangian code Gadget-2 (Springel, 2005). We provide a fitting function that can be easily applied to approximate the non-linear WDM power spectrum at redshifts z = 0.5 - 3.0 at a range of scales relevant to the weak lensing power spectrum. We examine the simple thermal relic scenario for different WDM masses and check our results against resolution issues by varying the size and number of simulation particles. We finally briefly discuss the possibility that the effects of WDM on the matter power spectrum might resemble the analogous, but weaker and larger scale effects of the free-streaming of massive neutrinos. We consider this with the goal of re-examining the Sloan Digital Sky Survey data (as in Thomas, 2010). We find that the effects of the neutrinos might just differ enough from the effects of WDM to prevent the degeneracy of the relevant parameters, namely the sum of neutrino masses and the mass of the WDM particle.

Diese Doktorarbeit stellt eine Messung der Verzweigungsverhältnisse und CP Parameter der Zerfallskanäle B0->pi+pi-, B0->K+pi- und B0->K+K- vor. Der gesamte Belle Datensatz mit 772 Millionen BB Paaren wird untersucht. Die BB Paare wurden am KEKB e+e- Speicherring bei einer Schwerpunktsenergie, die der Masse der Y(4S) Resonanz entspricht, erzeugt. Die Analyse liefert folgende Ergebnisse für die Verzweigungsverhältnisse B(B0->pi+pi-) = (5,63 +- 0,16(stat) +- 0,16(syst)) x 10^-6 , B(B0->K+pi-) = (18,71 +- 0,25(stat) +- 0,37(syst)) x 10^-6 , B(B0->K+K-) < 14 x 10^-8 at 90% CL. Für die CP-Asymmetrien ergeben sich folgende Werte ACP (B0->pi+pi-) = 0,33 +- 0,06 (stat) +- 0,03 (syst), SCP (B0->K+pi-) = -0,64 +- 0,08 (stat) +- 0,03 (syst), ACP (B0->K+K-) = -0,061 +- 0,014 (stat) +- 0,008 (syst), wobei die Parameter ACP und SCP jeweils die direkte und die durch Mischung induzierte CP Verletzung messen. Unsere Ergebnisse schließen eine Winkel phi_2 des Unitaritätsdreiecks zwischen 23,8 und 66,8 Grad mit einer Signifikanz von 1 Sigma aus. Eine Modell unabhängige Suche nach neuer Physik beim Zerfall von B0->K+pi- zeigt eine kleine Abweichung vom Standardmodel von -0,289 +- 0,139(stat) +- 0,064(syst) mit einer Signifikanz von 1,9 Sigma.

Polymere mit Halbleiter-Eigenschaften haben ein großes Anwendungspotential in der organischen Photovoltaik, da sich ihre optischen und elektronischen Eigenschaften über die molekulare Struktur gezielt ändern lassen. Durch die Synthese von Copolymeren mit besonders kleiner optischer Bandlücke (low-bandgap Copolymere) konnte die Absorption von Sonnenlicht weiter in den infraroten Spektralbereich ausgedehnt und somit die Konversion von Sonnenlicht in elektrische Energie deutlich verbessert werden. Diese neuartigen Donor-Akzeptor Materialien basieren auf einer alternierenden Anordnung von elektronen-reichen und -armen Blöcken, die durch elektronische Kopplung neue Energieniveaus mit kleinerer optischer Bandlücke bilden. Ziel dieser Arbeit ist die eingehende Untersuchung der photophysikalischen Eigenschaften dieser weitgehend unerforschten Moleküle. Die ersten drei Kapitel bieten dem Leser eine Einführung in das Forschungsgebiet und in die theoretische Beschreibung konjugierter Polymere, sowie einen Überblick über den aktuellen technischen Stand organischer Photovoltaik. Kapitel 4 gibt eine Zusammenfassung der verwendeten experimentellen und theoretischen Methoden. Der erste Teil der Untersuchung von Donor-Akzeptor Materialien gilt den Photoanregungen und der korrekten Zuordnung ihrer spektralen Signaturen (Kap. 5). Diese ermöglicht eine Zuordnung der spektralen Signaturen zu stark gebundenen, elektrisch neutralen Exzitonen, bzw. leichter zu trennenden Ladungsträgerpaaren mit kleinerer Bindungsenergie, sogenannten Polaronenpaaren. Aufgrund der schwachen elektrischen Abschirmung von Ladungen in organischen Materialen liegen die meisten Photoanregungen als Exzitonen vor. In dieser Hinsicht zeigen spektroskopische Messungen auf Femtosekunden-Zeitskala erstmals den andersartigen Charakter von Donor-Akzeptor Materialien und demonstrieren den großen Einfluss ihrer Struktur auf die Art der erzeugten Photoanregungen. Sie zeigen, dass bei Photoanregungen dieser neuartigen Materialien neben Exzitonen auch ein beträchtlicher Anteil an Polaronenpaaren entsteht. Diese Donor-Akzeptor Materialien weisen einen Polaronenpaar-Anteil von bis zu 24% aller Photoanregungen auf, was dem Dreifachen der Effizienz vergleichbarer Homopolymere entspricht (Kap. 6). Weitere Untersuchungen zeigen außerdem eine erhöhte Erzeugungsrate bei kürzeren Anregungswellenlängen. Dies kann auf eine Korrelation mit einem ausgeprägten Elektronentransfer der involvierten Wellenfunktion zurückgeführt werden, welcher in theoretischen Simulationen deutlich wird (Kap. 7). Zusammenfassend geben die in dieser Arbeit dargestellten Ergebnisse einen detaillierten Einblick in die optischen und elektronischen Eigenschaften von Donor-Akzeptor Copolymeren und den starken Einfluss der molekularen Struktur auf die ersten Schritte der photovoltaischen Stromerzeugung. Zusammenhänge zweier Schlüsselfaktoren für die Effizienzsteigerung zukünftiger organischer Solarzellen mit Materialparametern werden deutlich. Dies sind die Erzeugungseffizienz und die Lebensdauer von Polaronenpaaren und deren Abhängigkeit von der Elektronegativität und der Abstand von Akzeptor- zu benachbarten Donorsegmenten. Weiterhin konnte eine ausgeprägte Polaronenpaar Erzeugung über das ganze Absorptionsspektrum nachgewiesen werden. Diese Erkenntnisse bieten eine große Hilfestellung bei der weiteren Optimierung von Polymeren für Photovoltaik. Außerdem heben sie den wichtigen Beitrag der Ultrakurzzeit Spektroskopie zum grundlegenden Verständnis der Polaronenpaarerzeugung hervor. Mit diesen Mitteln könnte eine Verringerung des Spannungsverlustes möglich werden, der zur Ladungsträgertrennung in organischen Materialien nötig ist.