Spin and density resolved microscopy of Hubbard chains

This thesis reports on the microscopic investigation of antiferromagnetic order in Hubbard chains, realized with ultracold repulsively interacting fermions in an optical lattice. Extending a quantum gas microscope on spin resolution opened the possibility to Access the full charge and spin statistics of a strongly correlated fermionic many body system. Using this technique, we measured antiferromagnetic correlations over distances up to three lattice sites. Starting with a repulsive degenerate two component mixture of 6Li atoms trapped in a single plane of a vertical lattice, we loaded the Atoms into one-dimensional tubes of a transverse optical superlattice. By adiabatically ramping up the lattice potential along the tubes, we isentropically entered the strongly correlated regime of the one-dimensional Fermi-Hubbard Hamiltonian. We spatially separated the two spin states into opposite sites of local double wells orthogonal to the direction of the lattice before single atom and site sensitive imaging. In this way we extracted the spin information of every single lattice site and were able to evaluate spin-spin correlations. The two component degenerate gases were produced with sub-Poissonian atom number fluctuations by magnetically driven evaporative cooling in vicinity of a Feshbach resonance, used to set the scattering length. The dependence of antiferromagnetic correlations on the driving superexchange coupling was experimentally investigated by varying the interparticle scattering length before loading the atoms into the optical lattice. Taking advantage of the underlying trapping potential of the optical lattice, which shaped the filling and the entropy of the gases, the influence of density excitations and different entropies on spin correlations was observed. By post-selecting the Hubbard chains based on their average filling and local atom number fluctuations, spin correlations corresponding to 58% of the zero temperature predictions in the Heisenberg regime were measured. By comparing the obtained antiferromagnetic correlations to Quantum-Monte-Carlo predictions for our System the lowest measured entropy per particle could be stated as s = 0.51(4)kB, clearly below the critical value of s = ln(2)kB, which is required to form longer ranged correlations. Below this critical entropy, in contrast to local density fluctuations, these spin correlations strongly depend on the entropy of the system. In the future, such a spin thermometer can be used to benchmark novel cooling techniques, which are required to enter the temperature regime of d-wave superconducting states. The access to the full particle and spin statistics will enable us to read out multi-point correlation functions, which characterize such exotic states of matter.Diese Dissertation berichtet von der mikroskopischen Untersuchung antiferromagnetischer Ordnung in Hubbard Ketten, die mit ultrakalten, repulsiv wechselwirkenden Fermionen in optischen Gittern realisiert wurden. Durch die Erweiterung eines Quantengasmikroskops um Spin Auflösung,eröffnete sich die Möglichkeit, die komplette Dichte- und Spinstatistik eines stark korrelierten fermionischen Vielteilchensystems auszulesen. Durch Anwendung dieser Technik haben wir antiferromagnetische Korrelationen, die sich über drei Gitterplätze erstrecken,nachgewiesen. Angefangen mit einer entarteten zwei-komponentigen Mischung von Li-6 Atomen, die ineiner einzelnen Ebene eines vertikalen Gitters gefangen waren, laden wir die Atome in eindimensionale Subsysteme eines transversalen optischen Supergitters. Durch adiabatisches Anrampen des Gitterpotentials entlang des letzten verbliebenen Bewegungsfreiheitsgrades geht das System in das Regime des stark korrelierten ein-dimensionalen Fermi-Hubbard Hamiltonians über. Anschließend trennten wir die Atome unterschiedlichen Spins räumlich in entgegengesetzte Gitterplätze lokaler Doppelmulden senkrecht zur Gitterrichtung, bevor die Atome mit einzel-Atom und Gitterplatz sensitiv abgebildet wurden. Dadurch konnten wir den Spin jedes einzelnen Gitterplatzes auslesen und Spin-Spin Korrelatoren berechnen. Die zweikomponentigen entarteten Gase wurden mit sub-Poisson Atomzahlfluktuationen hergestellt, und zwar mithilfe von magnetisch unterstützter evaporativer Kühlung in Gegenwart einer Feshbach Resonanz, die benutzt wurde um die Streulänge einzustellen. Die Abhängigkeit der antiferromagnetischen Korrelationen von der Austauschwechselwirkung wurde experimentell untersucht indem die Streulänge zwischen den Atomen vor dem Laden in das optische Gitter eingestellt wurde. Das dem optischen Gitter zugrundeliegende Fallenpotential modellierte die Dichte- und Entropieverteilung, sodass der Einfluss von Dichteanregungen und verschiedenen Entropien auf Spin-Korrelationen gemessen werden konnten. Durch Selektion der Hubbard Ketten, basierend auf deren durchschnittlichen Besetzung und lokalen Atomzahlfluktuationen, konnten Spin Korrelationen, die 58% des vorrausgesagten Wertes im Rahmen des Heisenberg Modells bei T = 0 entsprechen, gemessen werden. Durch Vergleich der erhaltenen Korrelationen mit Quanten-Monte-Carlo Vorhersagen für unser System konnte die niedrigste Entropie pro Teilchen von s = 0.51(4)kB nachgewiesen werden, die damit deutlich unter der für länger-reichweitige Korrelationen notwendigen Entropie von s = ln(2)kB, liegt. Für Entropien unterhalb dieser kritischen Entropie hängen diese Spin Korrelationen stark von der Entropie des Systems ab, im Gegensatz zu lokalen Dichtefluktuationen. Daher könnte ein solches Spin-Thermometer in der Zukunft dazu genutzt werden, neuartige Kühlmethoden zu charakterisieren, welche nötig sind, um das Temperaturregime der d-Wellensupraleitung zu erreichen. Der Zugang zur kompletten Atomzahl- und Spinstatistik ermöglicht es uns Multipunkt-Korrelatoren zu berechnen, welche solche exotischen Materiezustände charakterisieren.Deutsche Übersetzung des Titels: Spin und Dichte aufgelöste Mikroskopie von Hubbard Kette

Microscopic Observation of Pauli Blocking in Degenerate Fermionic Lattice Gases

The Pauli exclusion principle is one of the most fundamental manifestations of quantum statistics. Here, we report on its local observation in a spin-polarized degenerate gas of fermions in an optical lattice. We probe the gas with single-site resolution using a new generation quantum gas microscope avoiding the common problem of light induced losses. In the band insulating regime, we measure a strong local suppression of particle number fluctuations and a low local entropy per atom. Our work opens a new avenue for studying quantum correlations in fermionic quantum matter both in and out of equilibrium.Comment: 8 pages, 6 figure

Predesign Considerations for the DC Link Voltage Level of the CENTRELINE Fuselage Fan Drive Unit

Electric propulsion (EP) systems offer considerably more degrees of freedom (DOFs) within the design process of aircraft compared to conventional aircraft engines. This requires large, computationally expensive design space explorations (DSE) with coupled models of the single components to incorporate interdependencies during optimization. The purpose of this paper is to exemplarily study these interdependencies of system key performance parameters (KPIs), e.g., system mass and efficiency, for a varying DC link voltage level of the power transmission system considering the example of the propulsion system of the CENTRELINE project, including an electric motor, a DC/AC inverter, and the DC power transmission cables. Each component is described by a physically derived, analytical model linking specific subdomains, e.g., electromagnetics, structural mechanics and thermal analysis, which are used for a coupled system model. This approach strongly enhances model accuracy and simultaneously keeps the computational effort at a low level. The results of the DSE reveal that the system KPIs improve for higher DC link voltage despite slightly inferior performance of motor and inverter as the mass of the DC power transmission cable has a major share for a an aircraft of the size as in the CENTRELINE project. Modeling of further components and implementation of optimization strategies will be part of future work

Noise Reduction Using Wavelet Thresholding of Multitaper Estimators and Geometric Approach to Spectral Subtraction for Speech Coding Strategy

ObjectivesNoise reduction using wavelet thresholding of multitaper estimators (WTME) and geometric approach to spectral subtraction (GASS) can improve speech quality of noisy sound for speech coding strategy. This study used Perceptual Evaluation of Speech Quality (PESQ) to assess the performance of the WTME and GASS for speech coding strategy.MethodsThis study included 25 Mandarin sentences as test materials. Environmental noises including the air-conditioner, cafeteria and multi-talker were artificially added to test materials at signal to noise ratio (SNR) of -5, 0, 5, and 10 dB. HiRes 120 vocoder WTME and GASS noise reduction process were used in this study to generate sound outputs. The sound outputs were measured by the PESQ to evaluate sound quality.ResultsTwo figures and three tables were used to assess the speech quality of the sound output of the WTME and GASS.ConclusionThere is no significant difference between the overall performance of sound quality in both methods, but the geometric approach to spectral subtraction method is slightly better than the wavelet thresholding of multitaper estimators

A holistic system approach for short range passenger aircraft with cryogenic propulsion system

Currently, hybrid-electric aircraft are under investigation as one possible solution to reduce the emissions of the aviation industry according to Flightpath 2050 of the European Union. To meet the drive trainʼs requirements on low mass while aiming for highest efficiency, superconducting technologies are regarded as a key enabling technology for drive train powers of several tens of megawatts. Within the German nationally funded project TELOS an exemplary mission profile and the physical measures of a 220-passenger aircraft are used to derive the requirements for a cryogenic-cooled serial hybrid-electric propulsion system. To optimize the total system performance, we subsequently evaluated the superconducting and cryogenic-cooled components by using computationally fast, analytical models. This approach allows quantifying the system performance by using component technologies being available today. In particular, the system performance of geared drive to direct drive propulsion units are compared and the influence of the DC bus voltage and the electric frequencies of the AC circuits on the mass and the efficiency of the drive trains are analysed

High‐temperature ternary oxide phases in Ta/Nb‐Alumina composite materials

Coarse-grained composites of refractory ceramics and refractory metals are a novel approach for materials at application temperatures up to 1500 °C. Al$_{2}$O$_{3}$ and the refractory metals Nb and Ta are suitable candidates for enhanced thermal shock capability, as they show similar thermal expansion. During fabrication, a key aspect to consider is the possible formation of additional phases upon interaction of the constituent phases as well as through reaction with the environment. X-Ray diffraction (XRD) and investigations of the microstructure with scanning electron microscopy methods unveil Al$_{2}$O$_{3}$–Nb composite to form NbO, whereas for Al$_{2}$O$_{3}$–Ta the ternary compound aluminum tantalate (AlTaO$_{4}$) is found. Thermodynamic calculations show that the changing oxygen solubility in Nb accounts for the formation of NbO, and explain the absence of a corresponding niobate (AlNbO4) phase. AlTaO$_{4}$ is identified as the disordered tetragonal high-temperature modification

Phase transformation pathways in a Ti-5.9Cu alloy modified with Fe and Al

Titanium alloys have been gaining importance in various industries due to their advantageous combination of strength, low density, excellent corrosion/oxidation resistance, and superior mechanical properties at elevated temperatures. Recently, eutectoid Ti–Cu alloys have been explored as promising candidates for advanced processes. This work investigates the effects of Fe and Al on a Ti-5.9Cu alloy using multi-scale characterization techniques. While Fe acts as a β-stabilizing element (despite being a sluggish eutectoid former), Al acts as an α-stabilizer. This work focuses on the effects of combined addition of these elements, studied in different heat treatment conditions. The results show that a fine, equiaxed microstructure is obtained in the binary Ti-5.9Cu alloy, whereas the addition of 2 wt% Fe, or 2 wt% Fe combined with 2 wt% Al to the Ti-5.9Cu alloy deteriorates the effect of grain refinement and coarse, columnar grains result and a small amount of β-phase is retained. Further, the microstructure resulting from the eutectoid decomposition is altered dramatically from a lamellar pearlitic in the binary alloy to a lath-like α-phase with diverse decomposition products in the ternary and quaternary alloys accompanied by increasing hardness values. Evaluation of the α misorientation suggests that a substantial amount of non-Burgers α is present in the Ti-Cu alloy in contrast to the results of the ternary and quaternary alloys. The observed Cu-rich intermetallic compound was identified as Ti$_2$Cu phase with off-stoichiometric composition. Results obtained explain how adding either Fe or Fe and Al leads to substantial hardening