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Titanium Dioxide Nanoparticles: Synthesis, X-Ray Line Analysis and Chemical Composition Study
(2016)
TiO2 nanoparticleshave been synthesized by the sol-gel method using titanium alkoxide and isopropanolas a precursor. The structural properties and chemical composition of the TiO2 nanoparticles were studied usingX-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy.The X-ray powder diffraction pattern confirms that the particles are mainly composed of the anatase phase with the preferential orientation along [101] direction. The physical parameters such as strain, stress and energy density were investigated from the Williamson- Hall (W-H) plot assuming a uniform deformation model (UDM), and uniform deformation energy density model (UDEDM). The W-H analysis shows an anisotropic nature of the strain in nanopowders. The scanning electron microscopy image shows clear TiO2 nanoparticles with particle sizes varying from 60 to 80nm. The results of mean particle size of TiO2 nanoparticles show an inter correlation with the W-H analysis and SEM results. Our X-ray photoelectron spectroscopy spectra show that nearly a complete amount of titanium has reacted to TiO2
The top-quark mass is measured in the all-hadronic top-antitop quark decay channel using proton-proton collisions at a centre-of-mass energy of \(\sqrt{s}=8\) TeV with the ATLAS detector at the CERN Large Hadron Collider. The data set used in the analysis corresponds to an integrated luminosity of 20.2 fb\(^{−1}\). The large multi-jet background is modelled using a data-driven method. The top-quark mass is obtained from template fits to the ratio of the three-jet to the dijet mass. The three-jet mass is obtained from the three jets assigned to the top quark decay. From these three jets the dijet mass is obtained using the two jets assigned to the W boson decay. The top-quark mass is measured to be 173.72 ± 0.55 (stat.) ± 1.01 (syst.) GeV.
Topolectrical Circuits
(2018)
Invented by Alessandro Volta and Félix Savary in the early 19th century, circuits consisting of resistor, inductor and capacitor (RLC) components are omnipresent in modern technology. The behavior of an RLC circuit is governed by its circuit Laplacian, which is analogous to the Hamiltonian describing the energetics of a physical system. Here we show that topological insulating and semimetallic states can be realized in a periodic RLC circuit. Topological boundary resonances (TBRs) appear in the impedance read-out of a topolectrical circuit, providing a robust signal for the presence of topological admittance bands. For experimental illustration, we build the Su-Schrieffer–Heeger circuit, where our impedance measurement detects the TBR midgap state. Topolectrical circuits establish a bridge between electrical engineering and topological states of matter, where the accessibility, scalability, and operability of electronics synergizes with the intricate boundary properties of topological phases.
The fascination of microcavity exciton-polaritons (polaritons) rests upon the combination of advanced technological control over both the III-V semiconductor material platform as well as the precise spectroscopic access to polaritonic states, which provide access to the investigation of open questions and complex phenomena due to the inherent nonlinearity and direct spectroscopic observables such as energy-resolved real and Fourier space information, pseudospin and coherence. The focus of this work was to advance the research area of polariton lattice simulators with a particular emphasis on their lasing properties. Following the brief introduction into the fundamental physics of polariton lattices in chapter 2, important aspects of the sample fabrication as well as the Fourier spectroscopy techniques used to investigate various features of these lattices were summarized in chapter 3. Here, the implementation of a spatial light modulator for advanced excitation schemes was presented.
At the foundation of this work is the capability to confine polaritons into micropillars or microtraps resulting in discrete energy levels. By arranging these pillars or traps into various lattice geometries and ensuring coupling between neighbouring sites, polaritonic band structures were engineered. In chapter 4, the formation of a band structure was visualised in detail by investigating ribbons of honeycomb lattices. Here, the transition of the discrete energy levels of a single chain of microtraps to the fully developed band structure of a honeycomb lattice was observed. This study allows to design the size of individual domains in more complicated lattice geometries such that a description using band structures becomes feasible, as it revealed that a width of just six unit cells is sufficient to reproduce all characteristic features of the S band of a honeycomb lattice. In particular in the context of potential technological applications in the realms of lasing, the laser-like, coherent emission from polariton microcavities that can be achieved through the excitation of polariton condensates is intriguing. The condensation process is significantly altered in a lattice potential environment when compared to a planar microcavity. Therefore, an investigation of the polariton condensation process in a lattice with respect to the characteristics of the excitation laser, the exciton-photon detuning as well as the reduced trap distance that represents a key design parameter for polaritonic lattices was performed. Based on the demonstration of polariton condensation into multiple bands, the preferred condensation into a desired band was achieved by selecting the appropriate detuning. Additionally, a decreased condensation threshold in confined systems compared to a planar microcavity was revealed.
In chapter 5, the influence of the peculiar feature of flatbands arising in certain lattice geometries, such as the Lieb and Kagome lattices, on polaritons and polariton condensates was investigated. Deviations from a lattice simulator described by a tight binding model that is solely based on nearest neighbour coupling cause a remaining dispersiveness of the flatbands along certain directions of the Brillouin zone. Therefore, the influence of the reduced trap distance on the dispersiveness of the flatbands was investigated and precise technological control over the flatbands was demonstrated. As next-nearest neighbour coupling is reduced drastically by increasing the distance between the corresponding traps, increasing the reduced trap distance enables to tune the S flatbands of both Lieb and Kagome lattices from dispersive bands to flatbands with a bandwidth on the order of the polariton linewidth. Additionally to technological control over the band structures, the controlled excitation of large condensates, single compact localized state (CLS) condensates as well as the resonant excitation of polaritons in a Lieb flatband were demonstrated. Furthermore, selective condensation into flatbands was realised. This combination of technological and spectroscopic control illustrates the capabilities of polariton lattice simulators and was used to study the coherence of flatband polariton condensates. Here, the ability to tune the dispersiveness from a dispersive band to an almost perfect flatband in combination with the selectivity of the excitation is particularly valuable. By exciting large flatband condensates, the increasing degree of localisation to a CLS with decreasing dispersiveness was demonstrated by measurements of first order spatial coherence. Furthermore, the first order temporal coherence of CLS condensates was increased from τ = 68 ps for a dispersive flatband, a value typically achieved in high-quality microcavity samples, to a remarkable τ = 459 ps in a flatband with a dispersiveness below the polarion linewidth. Corresponding to this drastic increase of the first order coherence time, a decrease of the second order temporal coherence function from g(2)(τ =0) = 1.062 to g(2)(0) = 1.035 was observed. Next to laser-like, coherent emission, polariton condensates can form vortex lattices. In this work, two distinct vortex lattices that can form in polariton condensates in Kagome flatbands were revealed. Furthermore, chiral, superfluid edge transport was realised by breaking the spatial symmetry through a localised excitation spot. This chirality was related to a change in the vortex orientation at the edge of the lattice and thus opens the path towards further investigations of symmetry breaking and chiral superfluid transport in Kagome lattices.
Arguably the most influential concept in solid-state physics of the recent decades is the idea of topological order that has also provided a new degree of freedom to control the propagation of light. Therefore, in chapter 6, the interplay of topologically non-trivial band structures with polaritons, polariton condensates and lasing was emphasised. Firstly, a two-dimensional exciton-polariton topological insulator based on a honeycomb lattice was realised. Here, a topologically non-trivial band gap was opened at the Dirac points through a combination of TE-TM splitting of the photonic mode and Zeeman splitting of the excitonic mode. While the band gap is too small compared to the linewidth to be observed in the linear regime, the excitation of polariton condensates allowed to observe the characteristic, topologically protected, chiral edge modes that are robust against scattering at defects as well as lattice corners. This result represents a valuable step towards the investigation of non-linear and non-Hermitian topological physics, based on the inherent gain and loss of microcavities as well as the ability of polaritons to interact with each other. Apart from fundamental interest, the field of topological photonics is driven by the search of potential technological applications, where one direction is to advance the development of lasers. In this work, the starting point towards studying topological lasing was the Su-Schrieffer-Heeger (SSH) model, since it combines a simple and well-understood geometry with a large topological gap. The coherence properties of the topological edge defect of an SSH chain was studied in detail, revealing a promising degree of second order temporal coherence of g(2)(0) = 1.07 for a microlaser with a diameter of only d = 3.5 µm. In the context of topological lasing, the idea of using a propagating, topologically protected mode to ensure coherent coupling of laser arrays is particularly promising. Here, a topologically non-trivial interface mode between the two distinct domains of the crystalline topological insulator (CTI) was realised. After establishing selective lasing from this mode, the coherence properties were studied and coherence of a full, hexagonal interface comprised of 30 vertical-cavity surface-emitting lasers (VCSELs) was demonstrated. This result thus represents the first demonstration of a topological insulator VCSEL array, combining the compact size and convenient light collection of vertically emitting lasers with an in-plane topological protection.
Finally, in chapter 7, an approach towards engineering the band structures of Lieb and honeycomb lattices by unbalancing the eigenenergies of the sites within each unit cell was presented. For Lieb lattices, this technique opens up a path towards controlling the coupling of a flatband to dispersive bands and could enable a detailed study of the influence of this coupling on the polariton flatband states. In an unbalanced honeycomb lattice, a quantum valley Hall boundary mode between two distinct, unbalanced honeycomb domains with permuted sites in the unit cells was demonstrated. This boundary mode could serve as the foundation for the realisation of a polariton quantum valley Hall effect with a truly topologically protected spin based on vortex charges. Modifying polariton lattices by unbalancing the eigenenergies of the sites that comprise a unit cell was thus identified as an additional, promising path for the future development of polariton lattice simulators.
Thin films of transition metal oxides open up a gateway to nanoscale electronic devices beyond silicon characterized by novel electronic functionalities. While such films are commonly prepared in an oxygen atmosphere, they are typically considered to be ideally terminated with the stoichiometric composition. Using the prototypical correlated metal SrVO\(_{3}\) as an example, it is demonstrated that this idealized description overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not explicitly exposed to air, are shown to severely affect the intrinsic electronic structure of a transition metal oxide film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in the thin films. These findings highlight that it is important to take into account surface apical oxygen or—mutatis mutandis—the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of transition metal oxides near the single unit-cell limit.
This thesis reports a successful fabrication and characterisation of ferromagnetic/superconductor junction (F/S) on graphene. The thesis preposes a fabrication method to produce F/S junctions on graphene which make use of ALD grown Al2O3 as the tunnel barrier for the ferromagnetic contacts. Measurements done on F/G/S/G/F suggests that by injecting spin polarised current into the superconductor, a spin imbalance is created in the quasiparticle density of states of the superconductor which then diffuses through the graphene channel. The observed characteristic curves are similar to the ones which are already reported on metallic ferromagnet/superconductor junctions where the spin imbalance is created using Zeeman splitting. Further measurements also show that the curves loose their characteristic shapes when the temperature is increased above the critical temperature (Tc) or when the external magnetic field is higher then the critical field (Hc) of the superconducting contact. But to prove conclusively and doubtlessly the existence of spin imbalance in ferromagnet/superconductor junctions on graphene, more devices have to be made and characterised preferably in a dilution refrigerator.
Automated analysis of the inner ear anatomy in radiological data instead of time-consuming manual assessment is a worthwhile goal that could facilitate preoperative planning and clinical research. We propose a framework encompassing joint semantic segmentation of the inner ear and anatomical landmark detection of helicotrema, oval and round window. A fully automated pipeline with a single, dual-headed volumetric 3D U-Net was implemented, trained and evaluated using manually labeled in-house datasets from cadaveric specimen (N = 43) and clinical practice (N = 9). The model robustness was further evaluated on three independent open-source datasets (N = 23 + 7 + 17 scans) consisting of cadaveric specimen scans. For the in-house datasets, Dice scores of 0.97 and 0.94, intersection-over-union scores of 0.94 and 0.89 and average Hausdorf distances of 0.065 and 0.14 voxel units were achieved. The landmark localization task was performed automatically with an average localization error of 3.3 and 5.2 voxel units. A robust, albeit reduced performance could be
attained for the catalogue of three open-source datasets. Results of the ablation studies with 43 mono-parametric variations of the basal architecture and training protocol provided task-optimal parameters for both categories. Ablation studies against single-task variants of the basal architecture showed a clear performance beneft of coupling landmark localization with segmentation and a dataset-dependent performance impact on segmentation ability.
Oxide heterostructures attract a lot of attention as they display a vast range of physical phenomena like conductivity, magnetism, or even superconductivity. In most cases, these effects are caused by electron correlations and are therefore interesting for studying fundamental physics, but also in view of future applications. This thesis deals with the growth and characterization of several prototypical oxide heterostructures. Fe3O4 is highly ranked as a possible spin electrode in the field of spintronics. A suitable semiconductor for spin injection in combination with Fe3O4 is ZnO due to its oxide character and a sufficiently long spin coherence length. Fe3O4 has been grown successfully on ZnO using pulsed laser deposition and molecular beam epitaxy by choosing the oxygen partial pressure adequately. Here, a pressure variation during growth reduces an FeO-like interface layer. Fe3O4 films grow in an island-like growth mode and are structurally nearly fully relaxed, exhibiting the same lattice constants as the bulk materials. Despite the presence of a slight oxygen off-stoichiometry, indications of the Verwey transition hint at high-quality film properties. The overall magnetization of the films is reduced compared to bulk Fe3O4 and a slow magnetization behavior is observed, most probably due to defects like anti-phase boundaries originating from the initial island growth. LaAlO3/SrTiO3 heterostructures exhibit a conducting interface above a critical film thickness, which is most likely explained by an electronic reconstruction. In the corresponding model, the potential built-up owing to the polar LaAlO3 overlayer is compensated by a charge transfer from the film surface to the interface. The properties of these heterostructures strongly depend on the growth parameters. It is shown for the first time, that it is mainly the total pressure which determines the macroscopic sample properties, while it is the oxygen partial pressure which controls the amount of charge carriers near the interface. Oxygen-vacancy-mediated conductivity is found for too low oxygen pressures. A too high total pressure, however, destroys interface conductivity, most probably due to a change of the growth kinetics. Post-oxidation leads to a metastable state removing the arbitrariness in controlling the electronic interface properties by the oxygen pressure during growth. LaVO3/SrTiO3 heterostructures exhibit similar behavior compared to LaAlO3/SrTiO3 when it comes to a thickness-dependent metal-insulator transition. But in contrast to LaAlO3, LaVO3 is a Mott insulator exhibiting strong electron correlations. Films have been grown by pulsed laser deposition. Layer-by-layer growth and a phase-pure pervoskite lattice structure is observed, indicating good structural quality of the film and the interface. An electron-rich layer is found near the interface on the LaVO3 side for conducting LaVO3/SrTiO3. This could be explained by an electronic reconstruction within the film. The electrostatic doping results in a band-filling-controlled metal-insulator transition without suffering from chemical impurities, which is unavoidable in conventional doping experiments.
Spin-lock based functional magnetic resonance imaging (fMRI) has the potential for direct spatially-resolved detection of neuronal activity and thus may represent an important step for basic research in neuroscience. In this work, the corresponding fundamental effect of Rotary EXcitation (REX) is investigated both in simulations as well as in phantom and in vivo experiments. An empirical law for predicting optimal spin-lock pulse durations for maximum magnetic field sensitivity was found. Experimental conditions were established that allow robust detection of ultra-weak magnetic field oscillations with simultaneous compensation of static field inhomogeneities. Furthermore, this work presents a novel concept for the emulation of brain activity utilizing the built-in MRI gradient system, which allows REX sequences to be validated in vivo under controlled and reproducible conditions. Via transmission of Rotary EXcitation (tREX), we successfully detected magnetic field oscillations in the lower nano-Tesla range in brain tissue. Moreover, tREX paves the way for the quantification of biomagnetic fields.
An attempt has been made to define the extent to which metabolic flux in central plant metabolism is reflected by changes in the transcriptome and metabolome, based on an analysis of in vitro cultured immature embryos of two oilseed rape (Brassica napus) accessions which contrast for seed lipid accumulation. Metabolic flux analysis (MFA) was used to constrain a flux balance metabolic model which included 671 biochemical and transport reactions within the central metabolism. This highly confident flux information was eventually used for comparative analysis of flux vs. transcript (metabolite). Metabolite profiling succeeded in identifying 79 intermediates within the central metabolism, some of which differed quantitatively between the two accessions and displayed a significant shift corresponding to flux. An RNA-Seq based transcriptome analysis revealed a large number of genes which were differentially transcribed in the two accessions, including some enzymes/proteins active in major metabolic pathways. With a few exceptions, differential activity in the major pathways (glycolysis, TCA cycle, amino acid, and fatty acid synthesis) was not reflected in contrasting abundances of the relevant transcripts. The conclusion was that transcript abundance on its own cannot be used to infer metabolic activity/fluxes in central plant metabolism. This limitation needs to be borne in mind in evaluating transcriptome data and designing metabolic engineering experiments.