TY - THES A1 - Kagerer, Philipp Thomas T1 - Two-Dimensional Ferromagnetism and Topology at the Surface of MnBi\(_2\)Te\(_4\) - Bi\(_2\)Te\(_3\) Heterostructures - MBE Growth, Magnetism and Electronic Properties T1 - Zweidimensionaler Ferromagnetismus und Topologie an der Oberfläche von MnBi\(_2\)Te\(_4\) - Bi\(_2\)Te\(_3\) Heterostrukturen - MBE Wachstum, Magnetismus und elektronische Eigenschaften N2 - In this thesis, a model system of a magnetic topological heterostructure is studied, namely a heterosystem consisting of a single ferromagnetic septuple-layer (SL) of \(MnBi_2Te_4\) on the surface of the three-dimensional topological insulator \(Bi_2Te_3\). Using MBE and developing a specialized experimental setup, the first part of this thesis deals with the growth of \(Bi_2Te_3\) and thin films of \(MnBi_2Te_4\) on \(BaF_2\)-substrates by the co-evaporation of its binary constituents. The structural analysis is conducted along several suitable probes such as X-ray diffraction (XRD, XRR), AFM and scanning tunnelling electron microscopy (STEM). It is furthermore found that the growth of a single septuple-layer of \(MnBi_2Te_4\) on the surface of \(Bi_2Te_3\) can be facilitated. By using X-ray absorption and circular magnetic dichroism (XAS, XMCD), the magnetic properties of \(MnBi_2Te_4\) are explored down to the monolayer limit. The layered nature of the vdW crystal and a strong uniaxial magnetocrystalline anisotropy establish stable out-of plane magnetic order at the surface of \(MnBi_2Te_4\), which is stable even down to the 2D limit. Pushing the material system to there, i.e. a single SL \(MnBi_2Te_4\) further allows to study the phase transition of this 2D ferromagnet and extract its critical behaviour with \(T_c \, = \, 14.89~k\) and \(\beta \, = \, 0.484\). Utilizing bulk crystals of the ferromagnetic \(Fe_3GeTe_2\) as substrate allows to influence, enhance and bias the magnetism in the single SL of \(MnBi_2Te_4\). By growing heterostructures of the type \(MnBi_2Te_4\) -- n layer \(Bi_2Te_3\) -- \(Fe_3GeTe_2\)for n between 0 and 2, it is shown, that a considerable magnetic coupling can be introduced between the \(MnBi_2Te_4\) top-layer and the substrate. Finally the interplay between topology and magnetism in the ferromagnetic extension is studied directly by angle-resolved photoemission spectroscopy. The heterostructure is found to host a linearly dispersing TSS at the centre of the Brillouin zone. Using low temperature and high-resolution ARPES a large magnetic gap opening of \(\sim\) 35 meV is found at the Dirac point of the TSS. By following its temperature evolution, it is apparent that the scaling behaviour coincides with the magnetic order parameter of the modified surface. N2 - In dieser Arbeit wird ein Modellsystem einer magnetischen topologischen Heterostruktur untersucht, genauer ein Heterosystem, das aus einer einzelnen ferromagnetischen Septupellage (SL) aus \(MnBi_2Te_4\) auf der Oberfläche des dreidimensionalen topologischen Isolators \(Bi_2Te_3\) besteht. Mittels MBE und eines eigens entwickelten Setups befasst sich der erste Teil mit dem Wachstum von \(Bi_2Te_3\) und \(MnBi_2Te_4\) auf \(BaF_2\)-Substraten durch die Ko-Verdampfung ihrer binären Bestandteile. Die Strukturanalyse wird mit Hilfe von Röntgenbeugung (XRD, XRR), AFM und Rastertunnel-Elektronenmikroskopie (STEM) durchgeführt. Darüber hinaus wird festgestellt, dass das Wachstum einer einzelnen SL von \(MnBi_2Te_4\) auf der Oberfläche von \(Bi_2Te_3\) möglich ist. Mit Hilfe von Röntgenabsorption und des zirkularen magnetischen Dichroismus (XAS, XMCD) werden die magnetischen Eigenschaften von \(MnBi_2Te_4\) bis zur Monolage untersucht. Die geschichtete Natur des vdW-Kristalls und eine starke uniaxiale magnetokristalline Anisotropie stabilisieren eine magnetische Ordnung an der Oberfläche von \(MnBi_2Te_4\), die sogar bis zum 2D-Limit stabil ist. Bei Betrachtung einer einzigen SL \(MnBi_2Te_4\), kann man den Phasenübergang dieses 2D-Ferromagneten weiter untersuchen und sein kritisches Verhalten mit \(T_c \, = \, 14,89~k\) und \(\beta \, = \, 0,484\) extrahieren. Die Verwendung von \(Fe_3GeTe_2\) als Substrat ermöglicht es, den Magnetismus in einzelnen SL von \(MnBi_2Te_4\) zu beeinflussen und zu verstärken. Durch Aufwachsen von Heterostrukturen des Typs \(MnBi_2Te_4\) -- n Schicht \(Bi_2Te_3\) -- \(Fe_3GeTe_2\) für n zwischen 0 und 2 wird gezeigt, dass eine beträchtliche magnetische Kopplung zwischen der \(MnBi_2Te_4\) Deckschicht und dem Substrat erreicht werden kann. Schließlich wird das Zusammenspiel zwischen Topologie und Magnetismus in dem System direkt durch winkelaufgelöstes ARPES untersucht. Die Heterostruktur weist einen linear dispergierendes TSS im Zentrum der Brillouin-Zone auf. In der Temperaturabhängigkeit wird eine große magnetische Lücke von \(\sim\) 35 meV am Dirac-Punkt des TSS gefunden, deren Skalierungsverhalten mit dem magnetischen Ordnungsparameter der modifizierten Oberfläche übereinstimmt. KW - Molekularstrahlepitaxie KW - ARPES KW - Magnetischer Röntgenzirkulardichroismus KW - Topologie KW - Magnetismus KW - Topological Insulator KW - Heterostructure Growth KW - Electronic Structure KW - Ferromagnetism Y1 - 2024 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-360121 ER - TY - JOUR A1 - China, Swarup A1 - Burrows, Susannah M. A1 - Wang, Bingbing A1 - Harder, Tristan H. A1 - Weis, Johannes A1 - Tanarhte, Meryem A1 - Rizzo, Luciana V. A1 - Brito, Joel A1 - Cirino, Glauber G. A1 - Ma, Po-Lun A1 - Cliff, John A1 - Artaxo, Paulo A1 - Gilles, Mary K. A1 - Laskin, Alexander T1 - Fungal spores as a source of sodium salt particles in the Amazon basin JF - Nature Communications N2 - In the Amazon basin, particles containing mixed sodium salts are routinely observed and are attributed to marine aerosols transported from the Atlantic Ocean. Using chemical imaging analysis, we show that, during the wet season, fungal spores emitted by the forest biosphere contribute at least 30% (by number) to sodium salt particles in the central Amazon basin. Hydration experiments indicate that sodium content in fungal spores governs their growth factors. Modeling results suggest that fungal spores account for ~69% (31–95%) of the total sodium mass during the wet season and that their fractional contribution increases during nighttime. Contrary to common assumptions that sodium-containing aerosols originate primarily from marine sources, our results suggest that locally-emitted fungal spores contribute substantially to the number and mass of coarse particles containing sodium. Hence, their role in cloud formation and contribution to salt cycles and the terrestrial ecosystem in the Amazon basin warrant further consideration. KW - atmospheric chemistry KW - biogeochemistry KW - environmental sciences Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-222492 VL - 9 ER - TY - JOUR A1 - Kern, Christian S. A1 - Haags, Anja A1 - Egger, Larissa A1 - Yang, Xiaosheng A1 - Kirschner, Hans A1 - Wolff, Susanne A1 - Seyller, Thomas A1 - Gottwald, Alexander A1 - Richter, Mathias A1 - de Giovannini, Umberto A1 - Rubio, Angel A1 - Ramsey, Michael G. A1 - Bocquet, François C. A1 - Soubatch, Serguei A1 - Tautz, F. Stefan A1 - Puschnig, Peter A1 - Moser, Simon T1 - Simple extension of the plane-wave final state in photoemission: bringing understanding to the photon-energy dependence of two-dimensional materials JF - Physical Review Research N2 - Angle-resolved photoemission spectroscopy (ARPES) is a method that measures orbital and band structure contrast through the momentum distribution of photoelectrons. Its simplest interpretation is obtained in the plane-wave approximation, according to which photoelectrons propagate freely to the detector. The photoelectron momentum distribution is then essentially given by the Fourier transform of the real-space orbital. While the plane-wave approximation is remarkably successful in describing the momentum distributions of aromatic compounds, it generally fails to capture kinetic-energy-dependent final-state interference and dichroism effects. Focusing our present study on quasi-freestanding monolayer graphene as the archetypical two-dimensional (2D) material, we observe an exemplary E\(_{kin}\)-dependent modulation of, and a redistribution of spectral weight within, its characteristic horseshoe signature around the \(\bar {K}\) and \(\bar {K´}\) points: both effects indeed cannot be rationalized by the plane-wave final state. Our data are, however, in remarkable agreement with ab initio time-dependent density functional simulations of a freestanding graphene layer and can be explained by a simple extension of the plane-wave final state, permitting the two dipole-allowed partial waves emitted from the C 2p\(_z\) orbitals to scatter in the potential of their immediate surroundings. Exploiting the absolute photon flux calibration of the Metrology Light Source, this scattered-wave approximation allows us to extract E\(_{kin}\)-dependent amplitudes and phases of both partial waves directly from photoemission data. The scattered-wave approximation thus represents a powerful yet intuitive refinement of the plane-wave final state in photoemission of 2D materials and beyond. Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-350330 VL - 5 IS - 3 ER - TY - JOUR T1 - Search for Multimessenger Sources of Gravitational Waves and High-energy Neutrinos with Advanced LIGO during Its First Observing Run, ANTARES, and IceCube JF - The Astrophysical Journal N2 - Astrophysical sources of gravitational waves, such as binary neutron star and black hole mergers or core-collapse supernovae, can drive relativistic outflows, giving rise to non-thermal high-energy emission. High-energy neutrinos are signatures of such outflows. The detection of gravitational waves and high-energy neutrinos from common sources could help establish the connection between the dynamics of the progenitor and the properties of the outflow. We searched for associated emission of gravitational waves and high-energy neutrinos from astrophysical transients with minimal assumptions using data from Advanced LIGO from its first observing run O1, and data from the Antares and IceCube neutrino observatories from the same time period. We focused on candidate events whose astrophysical origins could not be determined from a single messenger. We found no significant coincident candidate, which we used to constrain the rate density of astrophysical sources dependent on their gravitational-wave and neutrino emission processes. KW - gravitational waves KW - neutrinos KW - Electromagnetic signals KW - Events GW150914 KW - ray KW - emission Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-360189 VL - 870 IS - 2 PB - The American Astronomical Society ER - TY - JOUR A1 - Schade, A. A1 - Bader, A. A1 - Huber, T. A1 - Kuhn, S. A1 - Czyszanowski, T. A1 - Pfenning, A. A1 - Rygała, M. A1 - Smołka, T. A1 - Motyka, M. A1 - Sęk, G. A1 - Hartmann, F. A1 - Höfling, S. T1 - Monolithic high contrast grating on GaSb/AlAsSb based epitaxial structures for mid-infrared wavelength applications JF - Optics Express N2 - We demonstrate monolithic high contrast gratings (MHCG) based on GaSb/AlAs0.08Sb0.92 epitaxial structures with sub-wavelength gratings enabling high reflection of unpolarized mid-infrared radiation at the wavelength range from 2.5 to 5 µm. We study the reflectivity wavelength dependence of MHCGs with ridge widths ranging from 220 to 984 nm and fixed 2.6 µm grating period and demonstrate that peak reflectivity of above 0.7 can be shifted from 3.0 to 4.3 µm for ridge widths from 220 to 984 nm, respectively. Maximum reflectivity of up to 0.9 at 4 µm can be achieved. The experiments are in good agreement with numerical simulations, confirming high process flexibility in terms of peak reflectivity and wavelength selection. MHCGs have hitherto been regarded as mirrors enabling high reflection of selected light polarization. With this work, we show that thoughtfully designed MHCG yields high reflectivity for both orthogonal polarizations simultaneously. Our experiment demonstrates that MHCGs are promising candidates to replace conventional mirrors like distributed Bragg reflectors to realize resonator based optical and optoelectronic devices such as resonant cavity enhanced light emitting diodes and resonant cavity enhanced photodetectors in the mid-infrared spectral region, for which epitaxial growth of distributed Bragg reflectors is challenging. KW - monolithic grating KW - high contrast KW - mid-infrared wavelength applications KW - epitaxial structures KW - GaSb/AlAsSb Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-350346 VL - 31 IS - 10 ER - TY - JOUR A1 - Stebani, Jannik A1 - Blaimer, Martin A1 - Zabler, Simon A1 - Neun, Tilmann A1 - Pelt, Daniël M. A1 - Rak, Kristen T1 - Towards fully automated inner ear analysis with deep-learning-based joint segmentation and landmark detection framework JF - Scientific Reports N2 - 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. KW - anatomy KW - bone imaging KW - diagnosis KW - medical imaging KW - software KW - three-dimensional imaging KW - tomography Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-357411 VL - 13 ER - TY - JOUR A1 - Vogel, P. A1 - Rückert, M. A. A1 - Greiner, C. A1 - Günther, J. A1 - Reichl, T. A1 - Kampf, T. A1 - Bley, T. A. A1 - Behr, V. C. A1 - Herz, S. T1 - iMPI: portable human-sized magnetic particle imaging scanner for real-time endovascular interventions JF - Scientific Reports N2 - Minimally invasive endovascular interventions have become an important tool for the treatment of cardiovascular diseases such as ischemic heart disease, peripheral artery disease, and stroke. X-ray fluoroscopy and digital subtraction angiography are used to precisely guide these procedures, but they are associated with radiation exposure for patients and clinical staff. Magnetic Particle Imaging (MPI) is an emerging imaging technology using time-varying magnetic fields combined with magnetic nanoparticle tracers for fast and highly sensitive imaging. In recent years, basic experiments have shown that MPI has great potential for cardiovascular applications. However, commercially available MPI scanners were too large and expensive and had a small field of view (FOV) designed for rodents, which limited further translational research. The first human-sized MPI scanner designed specifically for brain imaging showed promising results but had limitations in gradient strength, acquisition time and portability. Here, we present a portable interventional MPI (iMPI) system dedicated for real-time endovascular interventions free of ionizing radiation. It uses a novel field generator approach with a very large FOV and an application-oriented open design enabling hybrid approaches with conventional X-ray-based angiography. The feasibility of a real-time iMPI-guided percutaneous transluminal angioplasty (PTA) is shown in a realistic dynamic human-sized leg model. KW - biomedical engineering KW - electrical and electronic engineering KW - imaging KW - three-dimensional imaging Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-357794 VL - 13 ER - TY - JOUR A1 - Rudno-Rudziński, W. A1 - Syperek, M. A1 - Andrezejewski, J. A1 - Maryński, A. A1 - Misiewicz, J. A1 - Somers, A. A1 - Höfling, S. A1 - Reithmaier, J. P. A1 - Sęk, G. T1 - Carrier delocalization in InAs/InGaAlAs/InP quantum-dash-based tunnel injection system for 1.55 μm emission JF - AIP Advances N2 - We have investigated optical properties of hybrid two-dimensional-zero-dimensional (2D-0D) tunnel structures containing strongly elongated InAs/InP(001) quantum dots (called quantum dashes), emitting at 1.55 μm. These quantum dashes (QDashes) are separated by a 2.3 nm-width barrier from an InGaAs quantum well (QW), lattice matched to InP. We have tailored quantum-mechanical coupling between the states confined in QDashes and a QW by changing the QW thickness. By combining modulation spectroscopy and photoluminescence excitation, we have determined the energies of all relevant optical transitions in the system and proven the carrier transfer from the QW to the QDashes, which is the fundamental requirement for the tunnel injection scheme. A transformation between 0D and mixed-type 2D-0D character of an electron and a hole confinement in the ground state of the hybrid system have been probed by time-resolved photoluminescence that revealed considerable changes in PL decay time with the QW width changes. The experimental discoveries have been explained by band structure calculations in the framework of the eight-band k·p model showing that they are driven by delocalization of the lowest energy hole state. The hole delocalization process from the 0D QDash confinement is unfavorable for optical devices based on such tunnel injection structures. KW - physics KW - surface collisions KW - electronic coupling KW - transition radiation KW - time-resolved photoluminescence KW - photoluminescence excitation KW - modulation spectroscopy KW - quantum dots KW - quantum wells KW - delocalization Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-181787 VL - 7 IS - 1 ER - TY - JOUR A1 - Ryczko, K. A1 - Misiewicz, J. A1 - Hofling, S. A1 - Kamp, M. A1 - Sęk, G. T1 - Optimizing the active region of interband cascade lasers for passive mode-locking JF - AIP Advances N2 - The work proposes possible designs of active regions for a mode-locked interband cascade laser emitting in the mid infrared. For that purpose we investigated the electronic structure properties of respectively modified GaSb-based type II W-shaped quantum wells, including the effect of external bias in order to simultaneously fulfil the requirements for both the absorber as well as the gain sections of a device. The results show that introducing multiple InAs layers in type II InAs/GaInSb quantum wells or introducing a tensely-strained GaAsSb layer into “W-shaped” type II QWs offers significant difference in optical transitions’ oscillator strengths (characteristic lifetimes) of the two oppositely polarized parts of such a laser, being promising for utilization in mode-locked devices. KW - physics KW - electrostatics KW - transition radiation KW - oscillator strengths KW - laser spectroscopy KW - optical spectroscopy KW - atomic and molecular spectroscopy, KW - frequency combs KW - quantum wells KW - laser physics Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-181790 VL - 7 IS - 1 ER - TY - THES A1 - Miller, Kirill T1 - Untersuchung von Nanostrukturen basierend auf LaAlO\(_3\)/SrTiO\(_3\) für Anwendungen in nicht von-Neumann-Rechnerarchitekturen T1 - Investigation of nanostructures based on LaAlO\(_3\)/SrTiO\(_3\) for applications in non von Neumann architectures N2 - Die Dissertation beschäftigt sich mit der Analyse von oxidischen Nanostrukturen. Die Grundlage der Bauelemente stellt dabei die LaAlO3/SrTiO3-Heterostruktur dar. Hierbei entsteht an der Grenzfläche beider Übergangsmetalloxide ein quasi zweidimensionales Elektronengas, welches wiederum eine Fülle von beachtlichen Eigenschaften und Charakteristika zeigt. Mithilfe lithographischer Verfahren wurden zwei unterschiedliche Bauelemente verwirklicht. Dabei handelt es sich einerseits um einen planaren Nanodraht mit lateralen Gates, welcher auf der Probenoberfläche prozessiert wurde und eine bemerkenswerte Trialität aufweist. Dieses Bauelement kann unter anderem als ein herkömmlicher Feldeffekttransistor agieren, wobei der Ladungstransport durch die lateral angelegte Spannung manipuliert wird. Zusätzlich konnten auch Speichereigenschaften beobachtet werden, sodass das gesamte Bauelement als ein sogenannter Memristor fungieren kann. In diesem Fall hängt der Ladungstransport von der Elektronenakkumulation auf den lateralen potentialfreien Gates ab. Die Memristanz des Nanodrahts lässt sich unter anderem durch Lichtleistungen im Nanowattbereich und mithilfe von kurzen Spannungspulsen verändern. Darüber hinaus kann die Elektronenakkumulation auch in Form einer memkapazitiven Charakteristik beobachtet werden. Neben dem Nanodraht wurde auch eine Kreuzstruktur, die eine ergänzende ferromagnetischen Elektrode beinhaltet, realisiert. Mit diesem neuartigen Bauteil wird die Umwandlung zwischen Spin- und Ladungsströmen innerhalb der nanoskaligen Struktur untersucht. Hierbei wird die starke Spin-Bahn-Kopplung im quasi zweidimensionalen Elektronengas ausgenutzt. N2 - The dissertation focuses on the analysis of oxide nanostructures. The basis of the devices consists of the LaAlO3/SrTiO3 heterostructure. A quasi two-dimensional electron gas is formed at the interface of the two transition metal oxides, which in turn exhibits a plethora of remarkable properties and characteristics. Two different components were realized using lithographic processes. The first is a planar nanowire with lateral gates, which was processed on the sample surface and exhibits remarkable triality. Among other things, this device can act as a conventional field-effect transistor, whereby the charge transport is manipulated by the laterally applied voltage. In addition, storage properties could also be observed, so that the entire component can function as a so-called memristor. In this case, the charge transport depends on the accumulation of electrons on the floating gates. The memristance of the nanowire can be altered using light power in the nanowatt range and with the aid of short voltage pulses. In addition, electron accumulation can also be observed in the form of a memcapacitive characteristic. In addition to the nanowire, a cross structure containing a complementary ferromagnetic electrode was also realized. This novel device is used to investigate the conversion between spin and charge currents within the nanoscale structure. Here, the strong spin-orbit coupling in the quasi two-dimensional electron gas is utilized. KW - Memristor KW - Heterostruktur-Bauelement KW - Spin-Bahn-Wechselwirkung KW - Grenzfläche KW - Übergangsmetalloxide KW - LaAlO\(_3\)/SrTiO\(_3\) KW - Transportspektroskopie KW - Spin-Ladungs-Umwandlung KW - Memkondensator KW - Nanoelektronik Y1 - 2024 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-354724 ER -