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We show the first results for parton distribution functions within the proton at the physical pion mass, employing the method of quasi-distributions. In particular, we present the matrix elements for the iso-vector combination of the unpolarized, helicity and transversity quasi-distributions, obtained with Nf = 2 twisted mass cloverimproved fermions and a proton boosted with momentum = 0.83 GeV. The momentum smearing technique has been applied to improve the overlap with the proton boosted state. Moreover, we present the renormalized helicity matrix elements in the RI’ scheme, following the non-perturbative renormalization prescription recently developed by our group.
In this proceeding we review our recent work using supervised learning with a deep convolutional neural network (CNN) to identify the QCD equation of state (EoS) employed in hydrodynamic modeling of heavy-ion collisions given only final-state particle spectra ρ(pT, Ф). We showed that there is a traceable encoder of the dynamical information from phase structure (EoS) that survives the evolution and exists in the final snapshot, which enables the trained CNN to act as an effective “EoS-meter” in detecting the nature of the QCD transition.
The standard implementation of the HRG model has been shown to be unable to describe all the available data on QCD matter. Here we show the balance of repulsive and attractive hadronic interactions on QCD thermodynamics through observables both calculated by lattice simulations and measured in experiment. Attractive interactions are mediated by resonance formation, which are here implemented through extra states predicted by the Quark Model, while repulsive interactions are modelled by means of Excluded Volume (EV) effects. Informations on flavour dependent effective sizes are extracted. It is found that EV effects are present in lattice QCD thermodynamics, and are essential for a comprehensive description of higher order fluctuations of conserved charges.
This paper traces the military role of Tibnīn and its rulers in the Latin East against the Muslims until 1187/ 583. Tibnīn played a key role in overcoming the Muslims in Tyre and controlled it in 1124. It also played a vital role in the conflict between Damascus and the Kingdom of Jerusalem. Tibnīn participated in defending Antioch, Banyas, Hebron and Transjordan several times. Furthermore, its soldiers and Knights joined the army of the Kingdom of Jerusalem to capture Ascalon in 1153, and joined the campaigns of Amaury I, King of Jerusalem, against Egypt from 1164 to1169. The military situation of Tibnīn under the rule of the royal house until its fall to the Muslims in 1187/ 583 will be studied as well
We will discuss the issue of Landau levels of quarks in lattice QCD in an external magnetic field. We will show that in the two-dimensional case the lowest Landau level can be identified unambiguously even if the strong interactions are turned on. Starting from this observation, we will then show how one can define a “plowest Landau level” in the four-dimensional case, and discuss how much of the observed effects of a magnetic field can be explained in terms of it. Our results can be used to test the validity of low-energy models of QCD that make use of the lowest-Landau-level approximation.
In this talk we discuss the effects of the hadronic rescattering on final state observables in high energy nuclear collisions. We do so by employing the UrQMD transport model for a realistic description of the hadronic decoupling process. The rescattering of hadrons modifies every hadronic bulk observable. For example apparent multiplicity of resonances is suppressed as compared to a chemical equilibrium freeze-out model. Stable and unstable particles change their momentum distribution by more than 30% through rescattering. The hadronic rescattering also leads to a substantial decorrelation of the conserved charge distributions. These findings show that it is all but trivial to conclude from the final state observables on the properties of the system at an earlier time where it may have been in or close to local equilibrium.
A full session was organized in memory of Helmut Oeschler during the 2017 edition of the Strangeness in Quark Matter Conference. It was heart-warming to discuss with the audience his main achievements and share anecdotes about this exceptionally praised and appreciated colleague, who was also a great friend for many at the conference. A brief summary of the session is provided with these proceedings.
Observations of long rang azimuthal correlations in small collision systems (p+p/A) have triggered an enormous excitement in the heavy-ion community. However, it is presently unclear to what extent the experimentally observed correlations should be attributed to initial state momentum correlations and/or the final state response to the initial state geometry. We discuss how a consistent theoretical description of the nonequilibrium dynamics is important to address both effects within a unified framework and present first results from weakly coupled non-equilibrium simulations in [1] to quantify the relative importance of initial state and final state effects based on theoretical calculations.
We compute hybrid static potentials in SU(3) lattice gauge theory. We present a method to automatically generate a large set of suitable creation operators with defined quantum numbers from elementary building blocks. We show preliminary results for several channels and discuss, which structures of the gluonic flux tube seem to be realized by the ground states in these channels.
We discuss the current developments by the European Twisted Mass Collaboration in extracting parton distribution functions from the quasi-PDF approach. We concentrate on the non-perturbative renormalization prescription recently developed by us, using the RI′ scheme. We show results for the renormalization functions of matrix elements needed for the computation of quasi-PDFs, including the conversion to the MS scheme, and for renormalized matrix elements. We discuss the systematic effects present in the Z-factors and the possible ways of addressing them in the future.
We report on the status of ongoing investigations aiming at locating the deconfinement critical point with standard Wilson fermions and Nf = 2 flavors towards the continuum limit (standard Columbia plot); locating the tricritical masses at imaginary chemical potential with unimproved staggered fermions at Nf = 2 (extended Columbia plot); identifying the order of the chiral phase transition at μ = 0 for Nf = 2 via extrapolation from non integer Nf (alternative Columbia plot).
Targeting for rare observables, the CBM experiment will operate at high interaction rates of up to 10 MHz, which is unprecedented in heavy-ion experiments so far. It requires a novel free-streaming readout system and a new concept of data processing. The huge data rates of the CBM experiment will be reduced online to the recordable rate before saving the data to the mass storage. Full collision reconstruction and selection will be performed online in a dedicated processor farm. In order to make an efficient event selection online a clean sample of particles has to be provided by the reconstruction package called First Level Event Selection (FLES).
The FLES reconstruction and selection package consists of several modules: track finding, track fitting, event building, short-lived particles finding, and event selection. Since detector measurements contain also time information, the event building is done at all stages of the reconstruction process. The input data are distributed within the FLES farm in a form of time-slices. A time-slice is reconstructed in parallel between processor cores. After all tracks of the whole time-slice are found and fitted, they are collected into clusters of tracks originated from common primary vertices, which then are fitted, thus identifying the interaction points. Secondary tracks are associated with primary vertices according to their estimated production time. After that short-lived particles are found and the full event building process is finished. The last stage of the FLES package is a selection of events according to the requested trigger signatures. The event reconstruction procedure and the results of its application to simulated collisions in the CBM detector setup are presented and discussed in detail.
Professor Walter Greiner, our mentor, colleague, and friend, passed away in the age of eighty. During his lifetime, the search for elements beyond uranium started and elements up to the so far heaviest one with atomic number 118 were discovered. In this talk I will present a short history from early searches for ‘trans-uraniums’ up to the production and safe identification of shell-stabilized ‘Super-Heavy Nuclei’ (SHN). The nuclear shell model reveals that these nuclei should be located in a region with closed shells for the protons at Z = 114, 120 or 126 and for the neutrons at N = 184. The outstanding aim of experimental investigations is the exploration of this region of spherical SHN. Systematic studies of heavy ion reactions for the synthesis of SHN revealed production cross-sections which reached values down to one picobarn and even below for the heaviest species. The systematics of measured cross-sections can be understood only on the basis of relatively high fission barriers as predicted for nuclei in and around the island of SHN. A key role in answering some of the open questions plays the synthesis of isotopes of element 120. Attempts aiming for synthesizing this element at the velocity filter SHIP will be reported.
We study tetraquark resonances with lattice QCD potentials computed for two static quarks and two dynamical quarks, the Born-Oppenheimer approximation and the emergent wave method of scattering theory. As a proof of concept we focus on systems with isospin I = 0, but consider different relative angular momenta l of the heavy b quarks. We compute the phase shifts and search for S and T matrix poles in the second Riemann sheet. We predict a new tetraquark resonance for l = 1, decaying into two B mesons, with quantum numbers I(JP) = 0(1−), mass MeV and decay width MeV.
Die vorliegende Dissertation befasst sich mit der Entwicklung und Erforschung eines konzeptionell neuartigen Injektionssystems zum Transport von Ionenstrahlen in toroidale Magnetfeldstrukturen. Die Forschungsarbeit ist dabei Teil des Figure-8 Speicherringprojekts (F8SR) des IAP, bei welchem es um die Erforschung der Physik und die Entwicklung eines niederenergetischen, supraleitenden, magnetostatischen Figure-8 Hochstromspeicherrings geht. Dieser neuartige Speicherring ermöglicht aufgrund des Einsatzes von fokussierenden solenoidalen und toroidalen Magnetfeldern das Speichern von Strahlströmen von bis zu einigen Ampere. Diese Arbeit baut auf früheren Forschungsarbeiten zu diesem Themenfeld auf, in welchen die Grundlagen und Ausgangsparameter für die experimentelle Untersuchung der Injektion gelegt und mit dem Aufbau des Injektionsexperiments begonnen wurde.
In dieser Dissertation wird den Fragen nachgegangen, ob ein magnetisches Konzept des Injektionssystems mittels eines „Scaled-Down“-Experiments experimentell umsetzbar ist und ob mit diesem die Injektion von Ionenstrahlen in toroidale Magnetfeldstrukturen realisiert werden kann. Ziel ist es dabei, ein Injektionssystem aufzubauen, durch welches sowohl ein seitlich injizierter Injektionsstrahl, welcher den in den Speicherring zu injizierenden Strahl darstellt, als auch ein gleichzeitig durch die toroidalen Magnetfelder driftender Ringstrahl, welcher den im Speicherring zirkulierenden Strahl darstellt, ohne Verluste transportiert werden können. Das Injektionssystem besteht dabei aus drei normalleitenden Magneten, wobei es sich um zwei baugleiche 30 Grad Toroide sowie einen Solenoid handelt. Die Toroide bilden den Transportkanal für den Ringstrahl, während der Injektionssolenoid senkrecht zwischen den beiden Toroiden endet und den Injektionskanal für den Injektionsstrahl darstellt.
Zunächst wurde das Injektionssystem mittels Strahltransportsimulationen untersucht und aufbauend auf den Ergebnissen die benötigen Vakuumkomponenten sowie der Injektionsmagnet ausgelegt, entwickelt und umgesetzt. Anschließend wurde mit dem fertigstellten Injektionsexperiment der Transport von zwei Ionenstrahlen durch das Injektionssystem experimentell erforscht. Dabei wurden die Strahlpfade mit einem in Entwicklung befindlichen Kameradetektorsystem aus verschiedenen Perspektiven aufgenommen und das Strahlverhalten in Abhängigkeit von unterschiedlichen Parametern phänomenologisch analysiert und diskutiert, mit den Ergebnissen der Simulationen verglichen sowie theoretisch bzgl. der RxB Drift und eines Gedankenmodells eingeordnet. Die technische Umsetzung, Inbetriebnahme und Durchführung verschiedener Vorabexperimente bzgl. weiterer Komponenten des Injektionsexperiments (bspw. Ionenquellen und Filterkanäle) ist ebenfalls Bestandteil dieser Arbeit.
Bei den experimentellen Untersuchungen mit Wasserstoff- und Heliumionenstrahlen konnte beobachtet werden, wie der Injektionsstrahl in den zweiten Toroid driftet und somit erfolgreich injiziert wird. Des Weiteren wurde eine Heliummessung durchgeführt, bei der sowohl der Injektionsstrahl als auch der Ringstrahl erfolgreich durch das Injektionssystem transportiert werden konnten. Auch die Auswirkungen des Injektionsmagneten auf den Ringstrahl konnten experimentell untersucht werden. Die verschiedenen Messungen wurden mittels des Gedankenmodells diskutiert und mit den Ergebnissen der Simulationen sowie untereinander verglichen.
Das abschließende Ergebnis dieser Arbeit ist, dass durch den Einsatz von solenoidalen und toroidalen Magnetfeldern der Injektionsstrahl vom Injektionsmagneten in den zweiten Toroid transportiert und dieser somit in die gekoppelte magnetische Konfiguration der Toroide eingelenkt werden kann. Der gleichzeitige verlustfreie Transport eines Ringstahls durch das Injektionssystem konnte dabei ebenfalls realisiert werden. Des Weiteren stimmen die Ergebnisse der Simulationen und Experimente sowie die theoretischen Überlegungen überein.
Das neuartige Injektionskonzept, welches als Schlüsselkomponente für die Umsetzung des Figure-8 Hochstromspeicherrings benötigt wird, wurde somit mittels Theorie, Simulation und Experiment überprüft und die Funktionalität bestätigt.
Zukünftige Forschungsfragen für welche der Figure-8 Hochstromspeicherring verwendet werden könnte, bspw. aus den Bereichen der experimentellen Astrophysik oder Fusionsforschung, wurden abschließend diskutiert.
Defossiliation of the energy system is crucial in the face of the impending risks of climate change. Electricity generation by burning fossil fuels is being displaced by renewable energy sources like hydro, wind and solar, driven by support schemes and falling costs from technological advances as well as manufacturing scale effects. The unavoidable shift from flexibly dispatchable generation to weather-dependent spatio-temporally varying generators transforms the generation and distribution of electricity into highly interdependent complex systems in multiple dimensions and disciplines:
In time, different scales, stretching from intra-day, diurnal, synoptic to seasonal oscillations of the weather interact with years and decades of planning and construction of capacity. In space, long-range correlations and local variations of weather systems as well as local bottlenecks in transmission networks affect solutions. The investment decisions about technological mix and spatial distribution of capacity follow economic principles, within restrictions which adapt in social feedback loops to public opinion and lobbyist influences.
In this work, a family of self-consistent models is developed which map physical steady-state operation, capacity investments and exogeneous restrictions of a European electricity system, in higher simultaneous spatial and temporal detail as well as scope than has previously been computationally tractable. Increasing the spatial detail of the renewable resources and co-optimizing the expansion of only a few transmission lines, reveals solutions to serve the European electricity demand at about today’s electricity cost with only 5% of its carbon-dioxide emissions; and importantly their electricity mix differs from the findings at low spatial resolution.
As important intermediate steps,
• new algorithms for the convex optimization of electricity system infrastructure are derived from graph-theoretic decompositions of network flows. Only these enable the investigation of model detail beyond previous computational limitations.
• a comprehensive European electricity network model down to individual substations at the transmission voltage levels is built by combining and completing data from freely available sources.
• a network reduction technique is developed to approximate the detailed model at a sequence of spatial resolutions to investigate the role of spatial scale, and identify a level of spatial resolution which captures all relevant detail, but is still computationally tractable.
• a method to trace the flow of power through the network, which is related to a vector diffusion process on a directed flow graph embedded in a network, is used to analyse the resulting technology mix and its interactions with the power network
The open-source nature of the model and restriction to freely available data encourages an accessible and transparent discussion about the future European electricity system, primarily based on renewable wind and solar resources.
We present a 360∘ (i.e., 4π steradian) general-relativistic ray-tracing and radiative transfer calculations of accreting supermassive black holes. We perform state-of-the-art three-dimensional general-relativistic magnetohydrodynamical simulations using the BHAC code, subsequently post-processing this data with the radiative transfer code RAPTOR. All relativistic and general-relativistic effects, such as Doppler boosting and gravitational redshift, as well as geometrical effects due to the local gravitational field and the observer’s changing position and state of motion, are therefore calculated self-consistently. Synthetic images at four astronomically-relevant observing frequencies are generated from the perspective of an observer with a full 360∘ view inside the accretion flow, who is advected with the flow as it evolves. As an example we calculated images based on recent best-fit models of observations of Sagittarius A*. These images are combined to generate a complete 360∘ Virtual Reality movie of the surrounding environment of the black hole and its event horizon. Our approach also enables the calculation of the local luminosity received at a given fluid element in the accretion flow, providing important applications in, e.g., radiation feedback calculations onto black hole accretion flows. In addition to scientific applications, the 360∘ Virtual Reality movies we present also represent a new medium through which to interactively communicate black hole physics to a wider audience, serving as a powerful educational tool.
In the course of this thesis we discuss a certain kind of supersolid, the lattice-supersolid, which can be realized using quantum gases in an optical lattice trap. The lattice-supersolid, which simultaneously possesses off-diagonal and diagonal long-range order in its density matrix and also breaks the discrete translational symmetry of an underlying lattice, is induced by self-ordering of the gas due to strong long-range van der Waals interactions. In the considered scenario, the interactions are facilitated by the excitation of atomic Rydberg states, which exhibit enhanced van der Waals forces.
In the first part of this thesis (chapters 1-3), we review the relevant basics of quantum gases, Rydberg physics and introduce the extended Bose-Hubbard model. We start with the relevant methods and devices of the vast toolbox available in common quantum gas experiments, as well as consider the main concepts behind superfluidity and supersolidity. This is followed by an introduction of some basic concepts of Rydberg atoms in quantum many-body systems, with a focus on the facilitation of long-range interactions and the implementation in a theoretical model. Thereafter a brief introduction is given, on the realization of the Bose-Hubbard model in optical lattice systems and its extension to include Rydberg states, which concludes the introductory part of this thesis.
In the following part (chapters 4-6), we introduce the theoretical tools used to derive the results presented in the final part. First, an introduction to a real-space extension of bosonic dynamical mean-field theory (RB-DMFT) for bosonic systems with long-range interactions in the Hartree approximation is given. This method is based on the non-perturbative self-consistent evaluation of the lattice Green’s function, which also incorporates the effect of nearest neighbor correlations due to the non-condensed particles. Then we focus on a quasiparticle expansion of the Bose-Hubbard model, which has its foundation in linearized fluctuations of a static mean-field ground-state, allowing for the prediction of a vast range of experimentally relevant observables. Lastly, we introduce an efficient truncation scheme for the local bosonic Fock-basis, which allows for the simulation of phases with high condensate density at a vastly reduced computational effort.
In the final part (chapters 7 and 8), we discuss the application of both methods to itinerant bosonic gases in two-dimensional optical lattices, in order to predict the equilibrium ground-state phases, as well as the signatures of supersolidity and its formation in spectral functions and the dynamic and static structure factor. Specifically, we focus on two limiting cases. Firstly, we consider a two-component gas, as realized by two hyperfine ground states, for example, of rubidium-87, where one component is off-resonantly excited to a Rydberg state, which generates a soft-core shaped interaction potential. Secondly, we discuss the opposing limit, using near-resonant excitations
of Rydberg states, where the interacting component now directly corresponds to the Rydberg state, which interacts via a van der Waals potential. In both cases we discuss the rich variety of supersolid phases, which are found for a wide range of parameters. We also discuss how some of these phases can be realized in experiment.
In the subsequent appendices (A to D) we discuss some methodological details. Most notably, we consider the possible Fock-extension of the Hartree approximation (appendix A), introduced in the RB-DMFT treatment of the extended Bose-Hubbard model.
This thesis presents the first measurement of the proton capture reaction on the isotope 124Xe performed in inverse kinematics. The experiment was carried out in June 2016 at the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany.
124Xe is one of about 35 p-nuclei that cannot be produced via neutron-induced nucleo- synthesis as the vast majority of heavy elements. Its production and destruction provide important information about the nucleosynthesis of the p-nuclei. Measuring the 124Xe(p,g)125Cs reaction also gives strong constraints for its reverse 125Cs(g,p)124Xe reaction.
Fully stripped 124Xe ions repeatedly passed a H2 gas jet target at five different energies between 5.5 MeV/u and 8 MeV/u. An electron cooler compensated for the energy loss in the target and reduced the beam momentum spread. The reaction product 125Cs55+ has a smaller magnetic rigidity than 124Xe54+. Therefore 125Cs55+ was deflected towards smaller radii in the first dipole after the target area and thereby separated from 124Xe54+. It was detected with a position-sensitive Double-Sided Silicon Strip Detector (DSSSD). The novelty of this experiment was the installation of the DSSSD inside the ultra-high vacuum of the storage ring using a newly designed manipulator.
Three High-Purity Germanium X-ray detectors were used to measure the X-rays following the Radiative Electron Capture (REC) events into 124Xe53+. The REC cross sections are well-known and were used to determine the luminosity.
The 124Xe(p,g)125Cs cross sections at ion beam energies between 5.5 MeV/u and 8 MeV/u were determined relatively to the K-REC cross sections and finally compared to the theoretically predicted cross sections. While theoretical predictions of the TENDL database are lower than the measured ones by a factor of up to seven, the NON-SMOKER data are higher by a factor of up to two, except of the cross section at 7 MeV/u, where NON-SMOKER data are slightly lower than the experimental value.
For the first time, a proton capture cross section could be measured in inverse kinematics close to the astrophysically relevant Gamow window. This allows the direct determination of the (p,g) cross section of isotopes with half-lives down to several minutes, which is not possible with any other technique.
In this thesis we work on the theoretical description of relativistic heavy-ion collisions, focussing on electromagnetic probes. We present mainly four topics: electric conductivity and diffusion properties of the hot plasma and hadronic matter, response of the quark-gluon plasma to external magnetic fields, direct photon production in the quark-gluon plasma and a study about initial and final state effects in small systems. The latter topic aims, i.a., at a better understanding of the initial state, which is crucial for electromagnetic probes. In all research areas we make use of the Boltzmann transport equation, whereby the presented methods provide analytical and numerical solutions. We pay particular attention to the construction of complete leading order photon production processes in numerical transport simulations of the quark-gluon plasma.
To begin with, our findings are the complete conserved charge diffusion matrix and electric conductivity. Those properties are important ingredients, e.g., for future simulations of baryon rich collisions. Next, we find that the influence of external magnetic fields to the QGP dynamics is not quantifiable in observables.
We present results for a variety of direct photon observables and we can partly explain experimental data. We emphasize the importance of the chemical composition and non-equilibrium nature of the medium to the direct photon puzzle. Lastly, we observe the interesting dynamic behavior of azimuthal correlations in small systems and identify signatures of the initial state in final observables. This will also be of interest for more precise simulations of electromagnetic probes and allows for various future studies.
Most of superconductors in a magnetic field are penetrated by a lattice of quantized flux vortices. In the presence of a transport current causing the vortices to cross sample edges, emission of electromagnetic waves is expected due to the continuity of tangential components of the fields at the surface. Yet, such a radiation has not been observed so far due to low radiated power levels and lacking coherence in the vortex motion. Here, we clearly evidence the emission of electromagnetic waves from vortices crossing the layers of a superconductor/insulator Mo/Si superlattice. The emission spectra consist of narrow harmonically related peaks which can be finely tuned in the GHz range by the dc bias current and, coarsely, by the in-plane magnetic field value. Our findings show that superconductor/insulator superlattices can act as dc-tunable microwave generators bridging the frequency gap between conventional radiofrequency oscillators and (sub-)terahertz generators relying upon the Josephson effect.
Die vorliegende Arbeit präsentiert die wissenschaftlichen Erkenntnisse, welche im Rahmen dreier verschiedener Messreihen gewonnen wurden. Kernthema ist in allen Fällen die Ionisation von molekularem Wasserstoff mit Photonen.
Im Rahmen der Messung sollte eine 2014 veröffentlichte Vorhersage der theoretischen Physiker Vladislav V. Serov und Anatoli S. Kheifets im Experiment überprüft werden. Ihren Berechnungen zufolge kann ein sich langsam vom Wasserstoff Molekülion entfernendes Photoelektron durch sein elektrisches Feld das Mutterion polarisieren und dafür sorgen, dass beim anschließenden Aufbruch in ein Proton und ein Wasserstoffatom eine asymmetrische Emissionswinkelverteilung zu beobachten ist [SK14]. Diese Vorhersage konnte mit den Ergebnissen der hier vorgestellten Messung zweifelsfrei untermauert werden. Für drei verschiedene Photonenenergien, welche im relevanten Reaktionskanal Photoelektronenenergien von 1, 2 und 3 eV entsprechen, wurden die prognostizierten Symmetrien in den Messdaten herauspräpariert. Es zeigte sich, dass diese sowohl in qualitativer wie auch in quantitativer Hinsicht gut bis sehr gut mit den Vorhersagen übereinstimmen.
Im zweiten Teil dieser Arbeit wurde erneut die Dissoziationsreaktion, allerdings bei deutlich höheren Photonenenergien, untersucht. Ziel war es, den in Zusammenarbeit mit den Physikern um Fernando Martin gelungenen theoretischen Nachweis der Möglichkeit einer direkten Abbildung von elektronischen Wellenfunktionen auch im Experiment zu vollziehen. Der überwiegende Teil aller Veröffentlichungen im Vorfeld dieser Messung fokussierte sich bei den Untersuchungen der Wellenfunktion entweder auf die rein elektronischen Korrelationen - so zum Beispiel in Experimenten zur Ein-Photon-Doppelionisation, wo Korrelationen zwischen beiden beteiligten Elektronen den Prozess überhaupt erst möglich machen - oder aber auf den Einfluss, welchen das Molekülpotential auf das emittierte Elektron ausübt. Die wenigen Arbeiten, die sich bis heute an einer unmittelbaren Abbildung elektronischer Wellenfunktionen versuchten, gingen meist den im Vergleich zu dieser Arbeit umgekehrten Weg: Man untersuchte hier das Licht höherer Harmonischer, wie sie bei der lasergetriebenen Ionisation und anschließenden Rekombination eines Photoelektrons mit seinem Mutterion entstehen.
In dieser Arbeit wurde ein Ansatz präsentiert, der zwei überaus gängige und verbreitete Messtechniken geschickt kombiniert - Während das Photoelektron direkt nachgewiesen und seine wesentlichen Eigenschaften abgefragt werden, kann der quantenmechanische Zustand des zweiten, gebunden verbleibenden Elektrons über einen koinzident dazu geführten Nachweis des ionischen Reaktionsfragments bestimmt werden. Dieser Vorgang stützt sich wesentlich auf Berechnungen der Gruppe um Fernando Martín, welche eine Quantifizierung der Beiträge einzelner Zustande zum gesamten Wechselwirkungsquerschnitt dieser Reaktion erlauben. Diese unterscheiden sich je nach Energie der Fragmente signifikant, so dass über eine Selektion des untersuchten KER-Intervalls Kenntnis vom elektronischen Zustand des H2 +-Ions nach der Photoemission erlangt werden kann. Die experimentellen Daten unterstützen die Theorie von Martin et al. nicht nur mit verblüffend guter Übereinstimmung, die gemessenen Emissionswinkelverteilungen stehen darüber hinaus auch in sehr gutem Einklang mit ihren theoretisch berechneten Gegenstücken. Die Ergebnisse wurden zwischenzeitlich in der renommierten Fachzeitschrift Nature Communications veröffentlicht [WBM+17].
Die dritte Messreihe innerhalb dieser Arbeit beschäftigt sich mit der Photodoppelionisation von Wasserstoff. Im Rahmen des selben Experiments wie die weiter vorn beschriebene Dissoziationsmessung bei 400 eV Photonenenergie aufgenommen, belegen die Ergebnisse auf wunderbar anschauliche Art und Weise, dass die Natur in unserer Umgebung voller Prozesse ist, die ursprünglich als rein quantenmechanische Laborkonstrukte angesehen wurden. Es konnte zweifelsfrei gezeigt werden, dass die beiden Elektronen, die bei der Photodoppelionisation freigesetzt werden, als ein Quasiteilchen aufgefasst werden können. Sie befinden sich in einem verschränkten Zweiteilchenzustand, und nur eine koinzidente Messung beider Elektronen vermag es, Interferenzeffekte in ihren Impulsverteilungen sichtbar zu machen - betrachtet man beide hingegen individuell, so treten keinerlei derartige Phänomene auf. Es gelang dabei zudem, eine beispielhafte Übereinstimmung zwischen den gemessenen Daten und einer theoretischen Berechnung der Kollegen um Fernando Martín zu erreichen.
Fabrication of three-dimensional (3D) nanoarchitectures by focused electron beam induced deposition (FEBID) has matured to a level that highly complex and functional deposits are becoming available for nanomagnetics and plasmonics. However, the generation of suitable pattern files that control the electron beam’s movement, and thereby reliably map the desired target 3D structure from a purely geometrical description to a shape-conforming 3D deposit, is nontrivial. To address this issue we developed several writing strategies and associated algorithms implemented in C++. Our pattern file generator handles different proximity effects and corrects for height-dependent precursor coverage. Several examples of successful 3D nanoarchitectures using different precursors are presented that validate the effectiveness of the implementation.
The ATP-binding cassette transporter TAPL translocates polypeptides from the cytosol into the lysosomal lumen. TAPL can be divided into two functional units: coreTAPL, active in ATP-dependent peptide translocation, and the N-terminal membrane spanning domain, TMD0, responsible for cellular localization and interaction with the lysosomal associated membrane proteins LAMP-1 and LAMP-2. Although the structure and function of ABC transporters were intensively analyzed in the past, the knowledge about accessory membrane embedded domains is limited. Therefore, we expressed the TMD0 of TAPL via a cell-free expression system and confirmed its correct folding by NMR and interaction studies. In cell as well as cell-free expressed TMD0 forms oligomers, which were assigned as dimers by PELDOR spectroscopy and static light scattering. By NMR spectroscopy of uniformly and selectively isotope labeled TMD0 we performed a complete backbone and partial side chain assignment. Accordingly, TMD0 has a four transmembrane helix topology with a short helical segment in a lysosomal loop. The topology of TMD0 was confirmed by paramagnetic relaxation enhancement with paramagnetic stearic acid as well as by nuclear Overhauser effects with c6-DHPC and cross-peaks with water.
Self-organized robots may develop attracting states within the sensorimotor loop, that is within the phase space of neural activity, body and environmental variables. Fixpoints, limit cycles and chaotic attractors correspond in this setting to a non-moving robot, to directed, and to irregular locomotion respectively. Short higher-order control commands may hence be used to kick the system from one self-organized attractor robustly into the basin of attraction of a different attractor, a concept termed here as kick control. The individual sensorimotor states serve in this context as highly compliant motor primitives. We study different implementations of kick control for the case of simulated and real-world wheeled robots, for which the dynamics of the distinct wheels is generated independently by local feedback loops. The feedback loops are mediated by rate-encoding neurons disposing exclusively of propriosensoric inputs in terms of projections of the actual rotational angle of the wheel. The changes of the neural activity are then transmitted into a rotational motion by a simulated transmission rod akin to the transmission rods used for steam locomotives. We find that the self-organized attractor landscape may be morphed both by higher-level control signals, in the spirit of kick control, and by interacting with the environment. Bumping against a wall destroys the limit cycle corresponding to forward motion, with the consequence that the dynamical variables are then attracted in phase space by the limit cycle corresponding to backward moving. The robot, which does not dispose of any distance or contact sensors, hence reverses direction autonomously.
Das Standardmodell der Elementarteilchenphysik beschreibt nach aktuellem Kenntnisstand die Entstehung, den Aufbau und das Verhalten der Materie in unserem Universum am erfolgreichsten. Dennoch gibt es einige Phänomene, die sich nicht in dessen Rahmen beschreiben lassen, wie z. B. die Existenz von dunkler Materie und Energie, nicht-verschwindende Neutrinomassen oder die Baryonenasymmetrie. Speziell im Hinblick auf die starke Wechselwirkung, welche im Standardmodell durch die Quantenchromodynamik (QCD) beschrieben wird, gibt es noch immer viele offene Fragen.
Eine Umgebung, in der man die QCD experimentell ergründen kann, bieten vor allem Schwerionenkollisionen, die insbesondere am Large Hadron Collider (LHC) oder am Relativistic Heavy Ion Collider (RHIC) durchgeführt werden.
In dieser Arbeit soll ein Beitrag von theoretischer Seite aus hinsichtlich eines besseren Verständnisses dieser Schwerionenkollisionen und der zugrundeliegenden QCD erbracht werden. Der Fokus liegt dabei auf dem Isotropisierungsprozess unmittelbar nach der Kollision der beiden Kerne.
Neben etlichen effektiven Theorien, die sehr gute Ergebnisse in den entsprechenden Grenzbereichen liefern, ist die Beschreibung der QCD im Rahmen der Gittereichtheorie (Gitter-QCD) die am meisten etablierte. Diese beinhaltet in den meisten Fällen einen Übergang zur euklidischen Raumzeit, da somit ein Auswerten der hochdimensionalen Pfadintegrale mithilfe von Monte-Carlo-Simulation basierend auf dem sogenannten Importance Sampling ermöglicht wird. Aufgrund der Komplexwertigkeit der euklidischen Zeitkomponente ist man jedoch an das Studieren von statischen Observablen gebunden. Da wir aber gerade an einer Zeitentwicklung des Systems interessiert sind, sehen wir von dem Übergang zur euklidischen Raumzeit ab, was den Namen “real-time” im Titel der Arbeit erklärt.
Wir folgen dem sogenannten Hamilton-Ansatz und leiten damit Feldgleichungen in Form von partiellen Differentialgleichungen her, die wir dann mit den Methoden der Gitter-QCD numerisch lösen. Dabei bedienen wir uns der effektive Theorie des Farb-Glas-Kondensats (CGC, aus dem Englischen: “Color Glass Condensate”), um geeignete Anfangsbedingungen zu erhalten. Genauer gesagt basieren unsere Gitter-Anfangsbedingungen auf dem McLerran-Venugopalan-Modell (MV-Modell), das eine klassische Approximation in niedrigster Ordnung darstellt und nur Beiträge rein gluonischer Felder berücksichtigt.
Die klassische Näherung sowie das Vernachlässigen der fermionischen Felder wird insbesondere mit den hohen Besetzungszahlen der Feldmoden begründet. Einerseits dominieren Infrarot-Effekte, welche klassischer Natur sind, und andererseits ist dadurch der Einfluss der Fermionen, die dem Pauli-Prinzip gehorchen, unterdrückt. Gerade bei letzterer Aussage fehlt es jedoch an numerischen Belegen. Wir erweitern daher die klassische MV-Beschreibung durch stochastische Gitter-Fermionen, um diesem Punkt nachzugehen. Da sich Fermionen nicht klassisch beschreiben lassen, spricht man hierbei oft von einem semi-klassischen Ansatz.
Eines der Hauptziele dieser Arbeit liegt darin, den Isotropisierungsprozess, der bislang noch viele Fragen aufwirft, aber unter anderem Voraussetzung für das Anwenden von hydrodynamischen Modellen ist, zu studieren. Wir legen dabei einen besonderen Fokus auf die systematische Untersuchung der verschiedenen Parameter, die durch die CGC-Anfangsbedingungen in unsere Beschreibung einfließen, und deren Auswirkungen auf etwa die Gesamtenergiedichte des Systems oder die zugehörigen Isotropisierungszeiten. Währenddessen überprüfen wir zudem den Einfluss von unphysikalischen Gitter-Artefakten und präsentieren eine eichinvariante Methode zur Analyse der Güte unserer klassischen Näherung. Die Zeitentwicklung des Systems betrachten wir dabei sowohl in einer statischen Box als auch in einem expandierenden Medium, wobei Letzteres durch sogenannte comoving Koordinaten beschrieben wird. Zudem liefern wir einen Vergleich von der realistischen SU(3)-Eichgruppe und der rechentechnisch ökonomischeren SU(2)-Eichgruppe.
Mit unseren numerischen Ergebnissen zeigen wir, dass das System hochempfindlich auf die verschiedenen Modellparameter reagiert, was das Treffen quantitativer Aussagen in dieser Formulierung deutlich erschwert, insbesondere da einige dieser Parameter rein technischer Natur sind und somit keine zugehörigen physikalisch motivierten Größen, die den Definitionsbereich einschränken könnten, vorhanden sind. Es ist jedoch möglich, die Anzahl der freien Parameter zu reduzieren, indem man ihren Einfluss auf die Gesamtenergie des Systems analysiert und sich diesen zunutze macht. Dadurch gelingt es uns mithilfe von Konturdiagrammen einige Abhängigkeiten zu definieren und somit die Unbestimmtheit des Systems einzuschränken. Des Weiteren finden wir dynamisch generierte Filamentierungen in der Ortsdarstellung der Energiedichte, die ein starkes Indiz für die Präsenz von sogenannten chromo-Weibel-Instabilitäten sind. Unsere Studie des fermionischen Einflusses auf den Isotropisierungsprozess des CGC-Systems weist auf, dass dieser bei kleiner Kopplung vernachlässigbar ist. Bei hinreichend großen Werten für die Kopplungskonstante sehen wir allerdings einen starken Effekt hinsichtlich der Isotropisierungszeiten, was ein bemerkenswertes Resultat ist.
Particle physics is living it’s golden age: petabytes of high precision data are being recorded at experimental facilities such as the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Despite the significant theoretical progress achieved in the last years the complete understanding of the internal structure of protons, not computable with perturbative QCD, remains as one of the most challenging unsolved problems in the physics of elementary interactions. Besides its fundamental interest, pinning down the relevant degrees of freedom and their properties, such as their spatial distribution, has profound implications in several phenomenological aspects of high-energy collisions. Currently one of the subjects undergoing intense study is the possibility that droplets of quark-gluon plasma (QGP) are being created not only in heavy ion collisions but in more dilute systems such as high-multiplicity proton-proton interactions. This is a data driven debate as it is rooted in the similar patterns observed across the different collision systems at the LHC (p+p,p+Pb and Pb+Pb) in the flow harmonic analyses: one of the golden probes of QGP formation specially sensitive to the initial collision geometry. Another domain in which the proton structure plays a central role is the description of multi-parton interactions, the mechanism that dominates the underlying event at LHC energies, in Monte Carlo event generators. All in all a precise characterization of the hadronic structure is a crucial ingredient of the physics program of the LHC.
A full characterization of a hadron would require momentum, spatial and spin information, the so-called Wigner distribution. So far obtaining this information experimentally has not been achieved. From a theoretical point of view several complications arise such as non-universality and breaking of factorization theorems. Then, in general, the description of the hadron structure relies on phenomenological tools that require theoretical modeling constrained by experimental data. The main goal of this thesis to characterize the transverse structure of the proton. For that purpose, a wide variety of phenomenological problems that are sensitive to the proton structure have been addressed.
First, elastic scattering data on proton-proton interactions constitutes a powerful probe of the geometry of the collision. A dedicated analysis of this observable focusing on the extraction of the inelasticity density from it at √s=62.5 GeV and √s=7 TeV is presented. In the TeV regime, a unexpected phenomenon, dubbed the hollowness effect, arises: the inelasticity density, a measurement of how effective is the collision producing secondary particles, reaches its maximum at non-zero impact parameter. We provide the first dynamical explanation of the hollowness effect by constructing the elastic scattering amplitude in impact parameter representation according to the Glauber model. For that purpose we relied on a composite description of the proton. More concretely, the relevant degrees of freedom that participate in the scattering process were considered to be gluonic hot spots. The probability distribution for the transverse positions of hot spots inside the proton includes repulsive short-range correlations between all pairs of hot spots controlled by an effective repulsive core rc that effectively enlarges the mean transverse separation distance between them. The main results extracted from this work are as follows. To begin with, we found that our model was not able to describe a growing behavior of the inelasticity density at zero impact parameter in the absence of non-trivial spatial correlations. However, even in the presence of correlations, the emergence of the hollowness effect couldn’t be described when the number of hot spots was smaller than 3. Both features set solid constraints in the proton structure within our model. Finally, we pinpoint the transverse diffusion of the hot spots with increasing collision energy to be the dynamical mechanism underlying the onset of the hollowness effect.
A convenient playground to test further implications of this novel geometric description of the proton are the initial state properties of high energy proton-proton interactions in the context of QGP physics. The parametrization of the geometry of the collision is mandatory in any theoretical model attempting to describe the striking experimental results that suggest collective behavior in proton-proton interactions at the LHC, such as the non-zero value of the flow harmonic coefficients (vn). A quantitative way to characterize the initial geometry anisotropy of the overlap region is to compute the spatial eccentricity moments (εn) that fluctuate on an event by event basis. For that purpose we develop a Monte Carlo Glauber event generator. A systematic investigation of the effect of non-trivial spatial correlations in the spatial eccentricity moments from ISR to LHC energies within our Monte Carlo Glauber approach is presented. We found that both the eccentricity (ε2) and the triangularity (ε3) are affected by the inclusion of short-range repulsive correlations. In particular, the correlated scenario yielded larger values of ε2(3) in ultra-central collisions while reducing them in minimum bias.
Moreover, we explore not only the eccentricities mean but their fluctuations in terms of symmetric cumulants. The experimental measurement by the CMS Collaboration at √s=13 TeV indicates a anti-correlation of v2 and v3 around the same number of tracks in the three collision systems available at the LHC. We lay out, for the first time in the literature, a particular mechanism that permits an anti-correlation of ε2 and ε3 in the highest centrality bins as dictated by data. When modeling the proton as composed by 3 gluonic hot spots, the most common assumption in the literature, we find that the inclusion of spatial correlations is indispensable to reproduce the negative sign. Further, we perform a systematic investigation of the parameter space of the model i.e. radius of the hot spot, radius of the proton, repulsive core and number of hot spots in each proton. Our results suggest that the interplay of the different scales is decisive and confirm the discriminating power of this observable on initial state models. Together with their drastic impact on the description of the hollowness effect and the absolute values of the eccentricities, the symmetric cumulant study adds evidence to the fact that the inclusion of spatial correlations between the sub nucleonic degrees of freedom of the proton modifies the initial state properties of p+p interactions at LHC energies.
Complex I couples the free energy released from quinone (Q) reduction to pump protons across the biological membrane in the respiratory chains of mitochondria and many bacteria. The Q reduction site is separated by a large distance from the proton-pumping membrane domain. To address the molecular mechanism of this long-range proton-electron coupling, we perform here full atomistic molecular dynamics simulations, free energy calculations, and continuum electrostatics calculations on complex I from Thermus thermophilus. We show that the dynamics of Q is redox-state-dependent, and that quinol, QH2, moves out of its reduction site and into a site in the Q tunnel that is occupied by a Q analog in a crystal structure of Yarrowia lipolytica. We also identify a second Q-binding site near the opening of the Q tunnel in the membrane domain, where the Q headgroup forms strong interactions with a cluster of aromatic and charged residues, while the Q tail resides in the lipid membrane. We estimate the effective diffusion coefficient of Q in the tunnel, and in turn the characteristic time for Q to reach the active site and for QH2 to escape to the membrane. Our simulations show that Q moves along the Q tunnel in a redox-state-dependent manner, with distinct binding sites formed by conserved residue clusters. The motion of Q to these binding sites is proposed to be coupled to the proton-pumping machinery in complex I.
Strong electron correlations can give rise to extraordinary properties of metals with renormalized Landau quasiparticles. Near a quantum critical point, these quasiparticles can be destroyed and non-Fermi liquid behavior ensues. YbRh2Si2 is a prototypical correlated metal exhibiting the formation of quasiparticle and Kondo lattice coherence, as well as quasiparticle destruction at a field-induced quantum critical point. Here we show how, upon lowering the temperature, Kondo lattice coherence develops at zero field and finally gives way to non-Fermi liquid electronic excitations. By measuring the single-particle excitations through scanning tunneling spectroscopy, we find the Kondo lattice peak displays a non-trivial temperature dependence with a strong increase around 3.3 K. At 0.3 K and with applied magnetic field, the width of this peak is minimized in the quantum critical regime. Our results demonstrate that the lattice Kondo correlations have to be sufficiently developed before quantum criticality can set in.
Most of the elements heavier than iron are produced through neutron capture reactions in the s- and r -process. The overall path of the s-process is well understood and can be accurately reproduced in network simulations. However, there are still some neutron capture reactions of unstable nuclei involved in the s-process, which were not yet measured due to the difficulty in producing suitable targets. In those cases, theoretical models have to be used to estimate the missing cross section.
One example is the branching point nucleus 86Rb, whose neutron capture cross section cannot be directly measured due to its short half life of 18.86 days. It is, however, also possible to measure its inverse, the 87Rb(g,n) reaction in order to obtain the 86Rb(n,g) cross section through the principle of detailed balance.
Natural rubidium was irradiated with a quasi-monoenergetic photon beam in the energy range between 10.7 MeV and 16 MeV in order to investigate the photo-dissociation cross section of 87Rb. The results are presented in this thesis. Not only the total cross section of 87Rb(g,n), but also the partial production cross section of the ground and isomeric state of 84Rb through the 85Rb(g,n) reaction was measured.
Not all isotopes can be reached via neutron capture reaction, and are therefore bypassed by the s- and r -process. These 35 proton-rich isotopes are called p-nuclei and are produced in the γ-process by a chain of photo-disintegration reactions in Type II supernovae. Network calculations of Type II supernova show that the γ-process can explain the production of most p-nuclei, but some – especially 92/94Mo and 96/98Ru – are heavily underproduced. While this could be the result of deficiencies in the corresponding stellar models or insufficient knowledge of the involved reaction rates, it is also possible that the missing p-nuclei are synthesized in other production scenarios.
An alternative scenario for 92Mo is the production via a chain of proton capture reactions in Type Ia supernovae. One important reaction in this chain is the 90Zr(p,g) reaction. The reaction cross section was already measured several times, but the results were inconclusive. In the present work, the 90 Zr(p,g) reaction was measured using the in-beam gamma-ray spectroscopy technique and the discrepancies between the data sets could be largely explained.
This paper presents an imaging radar system for structural health monitoring (SHM) of wind turbine blades. The imaging radar system developed here is based on two frequency modulated continuous wave (FMCW) radar sensors with a high output power of 30 dBm. They have been developed for the frequency bands of 24,05 GHz-24,25 GHz and 33.4 GHz-36.0 GHz, respectively. Following the successful proof of damage detection and localization in laboratory conditions, we present here the installation of the sensor system at the tower of a 2 MW wind energy plant at 95 m above ground. The realization of the SHM-system will be introduced including the sensor system, the data acquisition framework and the signal processing procedures. We have achieved an imaging of the rotor blades using inverse synthetic aperture radar techniques under changing environmental and operational condition. On top of that, it was demonstrated that the front wall and back wall radar echo can be extracted from the measured signals demonstrating the full penetration of wind turbine blades during operation.
Far outside the surface of slabs, the exact exchange (EXX) potential vx falls off as −1/z , if z denotes the direction perpendicular to the surface and the slab is localized around z=0 . Similarly, the EXX energy density ex behaves as −n/(2z) , where n is the electron density. Here, an alternative proof of these relations is given, in which the Coulomb singularity in the EXX energy is treated in a particularly careful fashion. This new approach allows the derivation of the next-to-leading order contributions to the asymptotic vx and ex . It turns out that in both cases, the corrections are proportional to 1/z2 in general.
An empirical study of the per capita yield of science Nobel prizes : is the US era coming to an end?
(2018)
We point out that the Nobel prize production of the USA, the UK, Germany and France has been in numbers that are large enough to allow for a reliable analysis of the long-term historical developments. Nobel prizes are often split, such that up to three awardees receive a corresponding fractional prize. The historical trends for the fractional number of Nobelists per population are surprisingly robust, indicating in particular that the maximum Nobel productivity peaked in the 1970s for the USA and around 1900 for both France and Germany. The yearly success rates of these three countries are to date of the order of 0.2–0.3 physics, chemistry and medicine laureates per 100 million inhabitants, with the US value being a factor of 2.4 down from the maximum attained in the 1970s. The UK in contrast managed to retain during most of the last century a rate of 0.9–1.0 science Nobel prizes per year and per 100 million inhabitants. For the USA, one finds that the entire history of science Noble prizes is described on a per capita basis to an astonishing accuracy by a single large productivity boost decaying at a continuously accelerating rate since its peak in 1972.
By the fabrication of periodically arranged nanomagnetic systems it is possible to engineer novel physical properties by realizing artificial lattice geometries that are not accessible via natural crystallization or chemical synthesis. This has been accomplished with great success in two dimensions in the fields of artificial spin ice and magnetic logic devices, to name just two. Although first proposals have been made to advance into three dimensions (3D), established nanofabrication pathways based on electron beam lithography have not been adapted to obtain free-form 3D nanostructures. Here we demonstrate the direct-write fabrication of freestanding ferromagnetic 3D nano-architectures. By employing micro-Hall sensing, we have determined the magnetic stray field generated by our free-form structures in an externally applied magnetic field and we have performed micromagnetic and macro-spin simulations to deduce the spatial magnetization profiles in the structures and analyze their switching behavior. Furthermore we show that the magnetic 3D elements can be combined with other 3D elements of different chemical composition and intrinsic material properties.
Fluctuation spectroscopy measurements of quasi-two-dimensional organic charge-transfer salts (BEDT-TTF) 2 X are reviewed. In the past decade, the method has served as a new approach for studying the low-frequency dynamics of strongly correlated charge carriers in these materials. We review some basic aspects of electronic fluctuations in solids, and give an overview of selected problems where the analysis of 1/f -type fluctuations and the corresponding slow dynamics provide a better understanding of the underlying physics. These examples are related to (1) an inhomogeneous current distribution due to phase separation and/or a percolative transition; (2) slow dynamics due to a glassy freezing either of structural degrees of freedom coupling to the electronic properties or (3) of the electrons themselves, e.g., when residing on a highly-frustrated crystal lattice, where slow and heterogeneous dynamics are key experimental properties for the vitrification process of a supercooled charge-liquid. Another example is (4), the near divergence and critical slowing down of charge carrier fluctuations at the finite-temperature critical endpoint of the Mott metal-insulator transition. Here also indications for a glassy freezing and temporal and spatial correlated dynamics are found. Mapping out the region of ergodicity breaking and understanding the influence of disorder on the temporal and spatial correlated fluctuations will be an important realm of future studies, as well as the fluctuation properties deep in the Mott or charge-ordered insulating states providing a connection to relaxor or ordered ferroelectric states studied by dielectric spectroscopy.
Temperature- and field-dependent 1H-, 19F-, and 79,81Br-NMR measurements together with zero - field 79,81Br-NQR measurements on polycrystalline samples of barlowite, Cu4(OH)6FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the 79,81Br-NMR spin-lattice relaxation rates 1/T1 indicate a phase transition at TN ≃ 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below TN. Moreover, 1/T1T scales linear with the bulk susceptibility which indicates persisting spin fluctuations down to 2 K. Quadupolare resonance (NQR) studies reveal a pair of zero-field NQR- lines associated with the two isotopes of Br with the nuclear spins of I = 3/2. Quadrupole coupling constants of vQ ≃ 28.5 MHz and 24.7 MHz for 79Br- and 81Br-nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to η ≃ 0.2. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above TN. This broadening and the relative large η value suggests a symmetry reduction at the Br- site reflecting the presence of a local distortion in the lattice. Our density-functional calculations show that the displacements of Cu2 atoms located between the kagome planes do not account for this relatively large η. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.
This thesis is concerned with systematic investigations of electronic noise in novel condensed matter systems. Although fluctuations are frequently considered a nuisance, that is, a disturbance limiting the accuracy of scientific measurements, in many cases they can reveal fundamental information about the inherent system dynamics. During the past decades, the study of electronic fluctuations has evolved into an indispensable tool in condensed matter physics.
The focus of the present work lies both in a further development of the fluctuation spectroscopy technique and in the study of materials of current interest. In particular, a comprehensive study of the charge carrier dynamics in the archetypal diluted magnetic semiconductors (Ga,Mn)As and (Ga,Mn)P was performed. In spite of extensive research work carried out during the last years, there still exists no theoretical consensus on the precise mechanism of ferromagnetic order and the electronic structure in these materials. Moreover, disorder and correlation effects complicate the understanding of these compounds.
Fluctuation spectroscopy experiments presented in this work provide strong evidence that a percolation transition is observed in samples with localized charge carriers, since the normalized resistance noise magnitude displays a significant enhancement around the Curie temperature. In addition, this quantity exhibits a power law scaling behavior as a function of the resistance, which is in good agreement with theoretical models of percolating systems.
By contrast, it was found that the resistance noise in metallic samples is mainly dominated by the physics of defects such as manganese interstitials and arsenic antisites. Furthermore, first noise studies were carried out on hafnia- and yttria-based resistive random access memories. In these memristor devices, the rupture and re-formation of oxygen deficient conducting filaments caused by the electric field and Joule heating driven motion of mobile anions lead to an unusual resistance switching behavior. For the first time, comparative noise measurements on oxygen deficient and stoichiometric hafnium oxide devices, as well as on novel yttrium oxide based devices were performed in this work. Finally, new strategies for noise measurements of highly insulating and extremely low-resistive samples were developed and realized. In detail, an experimental setup for the measurements of dielectric polarization fluctuations in insulating systems was designed and successfully tested. Here, the polarization noise of a sample is measured as current or voltage fluctuations produced within a capacitance cell. The study of dielectric polarization noise allows for conclusions to be drawn regarding equilibrium structural dynamics in insulators such as relaxor ferroelectrics. On the other hand, as successfully demonstrated for a heavy-fermion compound, focused ion beam etching enables to introduce a meander-shaped geometry in single crystal platelets, in order to strongly enhance the sample resistance and thus make resistance noise measurements possible. First results indicate a connection of the noise properties with the Kondo effect in the investigated material.
Crystal growth and characterization of cerium- and ytterbium-based quantum critical materials
(2018)
In der Festkörperphysik werden heutzutage Themen wie Supraleitung, Magnetismus und Quantenkritikalität sowohl von experimenteller als auch von theoretischer Seite stark untersucht. Quantenkritikalität und Quantenphasenübergänge können in Systemen erforscht werden, für welche ein Kontroll Parameter existiert, durch den z.B. eine magnetische Ordnung soweit unterdrückt wird, bis der Phasenübergang bei Null Kelvin, bei einem quantenkritischen Punkt (QCP), stattfindet. Vorzugsweise wird quantenkritisches Verhalten an Einkristallen untersucht, da diese in sehr reiner Qualität gezüchtet werden können und da deren gemessenen physikalischen Eigenschaften ausschließlich intrinsisch sind und nicht durch Verunreinigungseffekte überlagert werden. Der Schwerpunkt dieser Arbeit lag auf der Züchtung von Einkristallen und der Charakterisierung von Materialien, die quantenkritische Phänomene aufweisen. Als Ausgangsstoffe dienten dabei Elemente höchstmöglicher Reinheit. Es wurden die Serie YbNi4(P1-xAsx)2 mit einem ferromagnetischen QCP bei x=0,1, die Verbindung YbRh2Si2 mit einem feldinduzierten QCP bei Bcrit = 60mT und die Serie Ce(Ru1-xFex)PO mit einem QCP bei x = 0,86 untersucht. Für alle Verbindungen wurde das Züchtungsverfahren entwickelt, dann wurden Einkristalle gezüchtet und charakterisiert. Die Züchtung wurde zum einen mittels der Bridgman-Methode, zum anderen mit der Czochralski Methode durchgeführt. Neben struktureller und chemischer Charakterisierung der Einkristalle mittels Röntgen-Pulverdiffraktometrie, Laue-Methode und Energie-dispersiver Röntgen-Spektroskopie, wurden auch deren spezifische Wärme, elektrischer Widerstand und Magnetisierung im Temperaturbereich 1,8 – 300 K untersucht. Im weiteren Verlauf wurden die Kristalle in verschiedenen Kooperationen untersucht und bis in den Tieftemperatur- Bereich (20 mK), bei YbRh2Si2 bis in den Submillikelvin-Bereich, charakterisiert. Ausserdem wurden im Rahmen dieser Dissertation Einkristalle weiterer antiferromagnetischer Verbindungen SmRh2Si2, GdRh2Si2, GdIr2Si2, HoRh2Si2 und HoIr2Si2 gezüchtet. Bei diesen Verbindungen stand die Untersuchung elektronischer Oberflächenzustände mittels winkelaufgelöster Photoemissionsspektroskopie im Vordergrund.
Spontaneous brain activity is characterized in part by a balanced asynchronous chaotic state. Cortical recordings show that excitatory (E) and inhibitory (I) drivings in the E-I balanced state are substantially larger than the overall input. We show that such a state arises naturally in fully adapting networks which are deterministic, autonomously active and not subject to stochastic external or internal drivings. Temporary imbalances between excitatory and inhibitory inputs lead to large but short-lived activity bursts that stabilize irregular dynamics. We simulate autonomous networks of rate-encoding neurons for which all synaptic weights are plastic and subject to a Hebbian plasticity rule, the flux rule, that can be derived from the stationarity principle of statistical learning. Moreover, the average firing rate is regulated individually via a standard homeostatic adaption of the bias of each neuron’s input-output non-linear function. Additionally, networks with and without short-term plasticity are considered. E-I balance may arise only when the mean excitatory and inhibitory weights are themselves balanced, modulo the overall activity level. We show that synaptic weight balance, which has been considered hitherto as given, naturally arises in autonomous neural networks when the here considered self-limiting Hebbian synaptic plasticity rule is continuously active.
Recent experiments have demonstrated that visual cortex engages in spatio-temporal sequence learning and prediction. The cellular basis of this learning remains unclear, however. Here we present a spiking neural network model that explains a recent study on sequence learning in the primary visual cortex of rats. The model posits that the sequence learning and prediction abilities of cortical circuits result from the interaction of spike-timing dependent plasticity (STDP) and homeostatic plasticity mechanisms. It also reproduces changes in stimulus-evoked multi-unit activity during learning. Furthermore, it makes precise predictions regarding how training shapes network connectivity to establish its prediction ability. Finally, it predicts that the adapted connectivity gives rise to systematic changes in spontaneous network activity. Taken together, our model establishes a new conceptual bridge between the structure and function of cortical circuits in the context of sequence learning and prediction.
Quantum chromodynamics (QCD) is the theory of the strong interaction between quarks and gluons. Due to Confinement, at lower energies quarks and gluons are bound into colorless states called hadrons. QCD is also asymptotically free, i.e. at large energies or densities it enters a deconfined state, termed quark-gluon plasma (QGP), where quarks and gluons are quasi-free. This transition occurs at an energy scale around 200 MeV where QCD cannot be treated perturbatively. Instead it can be formulated on a space-time grid. The resulting theory, lattice quantum chromodynamics (LQCD), can be simulated efficiently on high performance parallel-computing clusters. In recent years graphic processing units (GPUs), which outperform CPUs in terms of parallel-computing and memory bandwidth capabilities, became very popular for LQCD computations. In this work the QCD deconfinement transition is studied using CL2QCD, a LQCD application that runs efficiently on GPUs. Furthermore, CL2QCD is extended by a Rational Hybrid Monte Carlo algorithm for Wilson fermions to allow for simulations of an odd number of quark flavors.
Due to the sign-problem LQCD simulations are restricted to zero or very small baryon densities, where, in the limit of infinite quark mass QCD has a first order deconfinement phase transition associated to the breaking of the global centre symmetry. Including dynamical quarks breaks this symmetry explicitly. Lowering their mass weakens the first order transition until it terminates in a second order Z2 point. Beyond this point the transition is merely an analytic crossover. As the lattice spacing is decreased, the reduction of discretization errors causes the region of first order transitions to expand towards lower masses. In this work the deconfinement critical point with 2 and 3 flavors of standard Wilson fermions is studied. To this end several kappa values are simulated on temporal lattice extents 6,8,10 (4) for two flavors (three flavors) and various aspect ratios (spatial lattice extent / temporal lattice extent) so as to extrapolate to the thermodynamic limit, applying finite size scaling. For two flavors an estimate is done if and when a continuum extrapolation is possible.
The chiral and deconfinement phase transitions at zero density for light and heavy quarks, respectively, have analytic continuations to purely imaginary chemical potential, where no sign-problem exists and LQCD simulations can be applied. At some critical value of the imaginary chemical potential, the transitions meet the endpoint of the Roberge-Weiss transition between adjacent Z3 sectors. For light and heavy quarks the transition lines meet in a triple point, while for intermediate masses they meet in a second order point. At the boundary between these regimes the junction is a tricritical point, as shown in studies with two and three flavors of staggered and Wilson quarks on lattices with a temporal lattice extent of 4. Employing finite size scaling the nature of this point as a function of the quark mass is studied in this work for two flavors of Wilson fermions with a temporal lattice extent of 6. Of particular interest is the change of the location of tricritical points compared to an earlier study on lattices with temporal extent of 4.
The quasi-two-dimensional organic charge-transfer salt κ -(BEDT-TTF) 2 Cu 2 (CN) 3 is one of the prime candidates for a quantum spin-liquid due the strong spin frustration of its anisotropic triangular lattice in combination with its proximity to the Mott transition. Despite intensive investigations of the material’s low-temperature properties, several important questions remain to be answered. Particularly puzzling are the 6 K anomaly and the enigmatic effects observed in magnetic fields. Here we report on low-temperature measurements of lattice effects which were shown to be particularly strongly pronounced in this material (R. S. Manna et al., Phys. Rev. Lett. 2010, 104, 016403)). A special focus of our study lies on sample-to-sample variations of these effects and their implications on the interpretation of experimental data. By investigating overall nine single crystals from two different batches, we can state that there are considerable differences in the size of the second-order phase transition anomaly around 6 K, varying within a factor of 3. In addition, we find field-induced anomalies giving rise to pronounced features in the sample length for two out of these nine crystals for temperatures T< 9 K. We tentatively assign the latter effects to B-induced magnetic clusters suspected to nucleate around crystal imperfections. These B-induced effects are absent for the crystals where the 6 K anomaly is most strongly pronounced. The large lattice effects observed at 6 K are consistent with proposed pairing instabilities of fermionic excitations breaking the lattice symmetry. The strong sample-to-sample variation in the size of the phase transition anomaly suggests that the conversion of the fermions to bosons at the instability is only partial and to some extent influenced by not yet identified sample-specific parameters.
Für das bessere Verständnis der Nukleosynthese der schweren Elemente im s-Prozess wurde im Rahmen dieser Arbeit die Messung zur Bestimmung der Neutroneneinfangsreaktion von 83Kr durchgeführt. Als Messinstrument wurde DANCE am LANL verwendet, ein 4pi-Kalorimeter zur Detektion der entstehenden g-Kaskaden bei (n,g)-Reaktionen. Darüber hinaus wurden außerdem noch Proben mit 85Kr und 86Kr vermessen.
Die Herausforderung an diesem Experiment bestand vor allem in der Probenherstellung. Das Edelgas Kr erforderte eine Neukonstruktion der normalerweise bei DANCE verwendeten Probenhalterung. Das Hauptaugenmerk lag auf der Maximierung der Kr-Exposition durch den Neutronenstrahl. Im Gegenzug wurde versucht das umgebende Material nach Möglichkeit keinen Neutronen auszusetzen. Für die Isotope 83,86Kr wurden Hochdruckgaskugeln verwendet, die an der Goethe-Universität Frankfurt gefüllt und in eine der neuen Probenhalterungen eingesetzt wurden. Zur Beachtung des bei der Messung entstehenden Untergrundes wurde eine Messung mit baugleicher Probenhalterung und leerer Gaskugel durchgeführt. Da bereits kleine Mengen 85Kr eine hohe Radioaktivität aufweisen, wurde eine in einen Stahlzylinder eingeschweißte, existierende Quelle verwendet.
Bei der Analyse zu 86Kr wurde schnell eine zu starke Verunreinigung der Kr-Probe mit Xe offensichtlich, einen signifikanten Anteil des Spektrums ausmachte. Aus diesem Grund kam es vor allem zu Problemen den korrekten Untergrund von den 86Kr Messdaten zu subtrahieren. Die weitere Bestimmung inklusive Streukorrekturen, Normierung anhand des Flussmonitors und DICEBOX/GEANT3 Effizienzbestimmung lieferte zwar einen energieabhängigen Wirkungsquerschnitt, dieser zeigte allerdings große Abweichungen von den evaluierten ENDF/B-VII.1 Daten, was besonders ersichtlich in der deutlichsten 86Kr Resonanz bei 5515 eV zu erkennen war. Aus diesem Grund konnte aus den Messdaten kein MACS extrahiert werden.
Bei einer Untersuchung der Aktivität der 85Kr-Probe mit Hilfe der einzelnen BaF2-Detektoren in der DANCE Kugel zeigte sich zunächst eine um fast einen Faktor vier geringere Aktivität als vom Hersteller angegeben. Auch bei der weiteren Analyse traten massive Untergrundprobleme auf. Die Form des Stahlzylinders, in dem das Kr-Gasgemisch eingeschweißt war, konnte aufgrund seiner Form nur schwer im Strahlrohr untergebracht werden. Beim Experiment selbst zeigte sich dann, dass Teile der Halterung vom Neutronenstrahl getroffen wurden, was einen Untergrund mit sehr hohem Q-Wert erzeugte, der nicht durch ein Esum Fenster entfernt werden konnte. Durch eine Beschädigung der Halterung mit der Probe kam es darüber hinaus zu Abweichungen mit der verwendeten Leerhalterung. All das führte trotz einer langen Messzeit von fast 18 d dazu, dass nur ein sehr schwaches Signal von der eigentlichen Kr-Probe zu erkennen war. Es wurde eine mögliche 85Kr Resonanz bei 675 eV gefunden, allerdings ist die endgültige Zuordnung aufgrund der nicht eindeutigen Untergrundsituation äußerst schwierig. Im Vorfeld des Kr-Experimentes wurde eine Messung von RbCl an DANCE durchgeführt, da ursprünglich zu erwarten war, dass bereits ein Teil des 85Kr zu 85Rb zerfallen war. Durch diese Messung sollte dieser Anteil leicht von der späteren Messung zu subtrahieren sein. Allerdings trat ein unerwartetes Problem während der Datenaufnahme auf. Die Verbindung der DAQ Boards wurde getrennt, wodurch ca. 3/4 der Detektoren nicht mehr zeitsynchron liefen. Im Zuge dieser Arbeit wurde eine Rekonstruktion dieser Daten angestrebt. Durch Modifikationen am FARE Code, der zur Auswertung verwendet wurde, konnte Flugzeitspektren für jeden Beschleunigerpuls erzeugt werden. Es zeigte sich zunächst ein offensichtlicher Trend einer Verschiebung der getrennten Boards zu späteren Zeiten. Durch mehrere Fits an die Abweichungsverteilung und anschließende Korrektur konnte zunächst ein Spektrum wiederhergestellt werden, das vergleichbar mit den unbeschädigten Daten war. Bei einer detaillierten Analyse dieser neu gewonnen Daten zeigte sich jedoch eine Nichtlinearität in der Zeitverschiebung. Dies resultierte letztlich in einer Korrektur des Spektrums, allerdings nicht in einem Koinzidenzfenster von 10 ns, das für eine Wirkungsquerschnittsanalyse notwendig ist. Es wurde geschlussfolgert, dass durch die geringe Statistik in den einzelnen Flugzeitspektren solch eine Genauigkeit nicht zu erreichen ist.
Die Messung des Neutroneneinfangsquerschnitts von 83Kr konnte im Zuge dieser Arbeit erfolgreich durchgeführt werden. Es wurden zwei Messungen mit verschiedenen Strömen kombiniert. Eine Messung mit 40 µA wurde durchgeführt, um Pile-Up in der größten Resonanz bei 28 eV zu reduzieren. Die zweite Messung diente dann dem Sammeln von ausreichend Statistik in den nicht resonanten Bereichen. Die eingesetzte Leerkugel erlaubte eine saubere Subtraktion des Untergrundes von Probenhalterung, Gaskugel und Umgebung. Für die Skalierung der Messergebnisse wurde eine weitere Messung mit einer 5000 Å dicken Goldfolie durchgeführt. Zur Bestimmung der Detektoreffizienz konnten zunächst die durch den Neutroneneinfang entstandenen Abregungskaskaden der 84Kr Kerne mit DICEBOX modelliert werden. Diese Kaskaden wurden dann anschließend in GEANT3 Simulationen verwendet, um die Effizienz bestimmen zu können. Mit diesen Methoden erhielt man die Maxwell-gemittelten Wirkungsquerschnitte von kT = 5 keV - 100 keV. Bei der für den s-Prozess wichtigen Temperatur von kT = 30 keV wurde der Querschnitt bestimmt zu: MACS (30 keV) = (256,6 +- 14,2 (stat) +- 18,1(sys)) mb.
Dieser Wert ist in guter Übereinstimmung mit dem in der KADoNIS v0.3 Datenbank angegebenen Wert von MACS;KADoNIS (30 keV) = (243 +- 15) mb. Mit den so gewonnenen Wirkungsquerschnitten wurden außerdem die Reaktionsraten berechnet. Bei den anschließenden Netzwerkrechnungen mit dem Programm NETZ wurden die Auswirkungen der in dieser Arbeit gewonnenen Wirkungsquerschnitte im Vergleich zu den KADoNIS v0.3 Werten betrachtet. Dabei zeigte sich eine leicht erhöhte Produktion der stabilen Isotope 84Kr, 86Kr, 85Rb und 87Rb, sowie eine leichte Unterproduktion der stabilen Isotope 86-88Sr in der Hauptkomponente des s-Prozess. Ein ähnliches Bild zeigte sich in der He-Brennphase der schwachen Komponente. Der in dieser Arbeit gemessene Wirkungsquerschnitt bei hohen Temperaturen ist geringer als der in KADoNIS v0.3 angegebene, weswegen es bei der Simulation mit NETZ zu einer stark erhöhten Produktion von 83Kr in der C-Brennphase kommt.
Compact objects - black holes and neutron stars - are fascinating objects, not only for the astrophysicists, but for a wide range of researchers, including astronomers, theoretical physicists, particle and nuclear physicists, condensed matter physicists and arguably for the layman as well.
First theorized in the first part of the twentieth century, for a long time these objects have been considered just exotic ideas or mathematical curiosities. Pulsar were however detected in the late 1960s and readily identified as rotating, radiating neutron stars, while the first candidate black hole, Cygnus X-1, was observed in 1972. Since then the interest in these objects has steadily grown.
The reasons behind this interest are easily understood considering that compact object dwell at the intersection of many different areas of physics, and are ideal laboratories to explore the interplay between these areas.
Black holes, which are purely gravitational objects, are perfectly suited to study the nature of gravity, its manifestations such as gravitational waves, and the differences between various theories of gravity in the regime where they are expected to be most relevant, i.e. the strong field regime. However, just like any massive astrophysical object, black holes are interested by accretion phenomena, which are thought to be the power source of some very bright astrophysical emitters of electromagnetic signals, such as active galactic nuclei or X-ray binaries.
At the same time, black holes exist in a variety of different mass scales, from stellar mass to supermassive black holes billions of times heavier. The latter play a very important and yet not fully understood role in the formation and evolution of galaxies, as well as in shaping the large scale structure of the universe, making them relevant to cosmology as well.
Neutron stars share with black holes the characteristic of being gravitationally dominated systems; but because they are composed of baryon matter, they display a much richer behaviour. It has been realized early on that the matter in neutron star cores reaches extreme densities, exceeding the one in atomic nuclei. This means that neutron stars could provide invaluable information on the behaviour of matter in such extreme conditions (which are impossible to achieve in laboratory experiments), such as details of the nucleonic interaction, the properties of hyperons or of quark-gluon plasmas.
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