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This thesis contains studies on a special class of topological insulators, so called anomalous Floquet topological insulators, which exclusively occur in periodically driven systems. At the boundary of an anomalous Floquet topological insulator, topologically protected transport occurs even though all of the Floquet bands are topologically trivial. This is in stark contrast to ordinary topological insulators of both static and Floquet type, where the topological invariants of the bulk bands completely determine the chiral boundary states via the bulk-boundary correspondence. In anomalous Floquet topological insulators, the boundary states are instead characterized by bulk invariants that account for the full dynamical evolution of the Floquet system.
Here, we explore the interplay between topology, symmetry, and non-Hermiticity in two-dimensional anomalous Floquet topological insulators. The central results of this exploration are (i) new expressions for the topological invariants of symmetry-protected anomalous Floquet topological phases which can be efficiently computed numerically, (ii) the construction of a universal driving protocol for symmetry-protected anomalous Floquet topological phases and its experimental implementation in photonic waveguide lattices, (iii) the discovery of non-Hermitian boundary state engineering which provides unprecedented possibilities to control and manipulate the topological transport of anomalous Floquet topological insulators.
This thesis describes recent developments in multi-reflection time-of-flight mass spectrometry (MR-ToF MS) with ions exhibiting large masses and mass differences at an MR-ToF setup at the University of Greifswald. A series of in-trap manipulation techniques to selectively retain or eject ion bunches of multiple species with disparate mass-to-charge ratios is investigated. These highlight the possibility to correct long-term flight-time drifts using a reference ion species far away in mass from the species of interest and also the ability to use such a pair to perform single-reference precision mass determinations. In both cases, the results obtained with disparate-mass ion pairs are comparable to those known from operation with isobaric species.
In addition, an in-trap photoexcitation technique is developed and applied to study the dissociation behavior of atomic bismuth clusters (systems of some number of bismuth atoms). Compared to previous works by other groups, the probed cluster-size range is expanded for both ion polarities, resulting in a more comprehensive picture of the underlying dissociation pathways. The known significance of neutral-tetramer breakoff is confirmed, however, evidence is also found for the loss of larger neutral fragments.
Lastly, the principle of tandem high-resolution MR-ToF MS is introduced. This new method allows the study of the change in dissociation behavior of the cationic bismuth octamer resulting from substituting one of its atoms for lead. It is found that the lead-doping opens new preferential fragmentation pathways that outstrip the dominant tetramer breakoff for this specific precursor cluster size. As a first proof-of-principle experiment, the case of the cationic octamer shows that tandem MR-ToF MS is well-suited for the investigation of compound clusters.
In this thesis, the transport properties of topological insulators are investigated. In contrast to trivial insulators, topological insulators possess conducting boundary states which cross the bulk energy gap that separates the highest occupied electronic band from the lowest unoccupied band. The materials used in this thesis are three-dimensional topological insulators with time-reversal symmetry. Their associated helical surface states are protected against elastic backscattering by Kramers degeneracy. The unique properties of the helical surface states can be utilized to generate spin-polarized currents at the surface of topological insulators and to control their propagation direction. This makes them a promising material class for the field of spintronics.
Here, we perform photocurrent scans of topological insulator Hall bar and nanowire devices. From these measurements, we obtained two-dimensional maps of the polarization-independent and helicity-dependent components of the photocurrents.
We find that the polarization-independent component is dominated by the Seebeck effect and thus driven by thermoelectric currents. On the other hand, the helicity-dependent component is driven by the spin-polarized currents that emerge from the topologically non-trivial helical surface states via the circular photogalvanic effect.
First and foremost, our experiments demonstrate that topological insulator nanowires provide a promising platform for the generation of spin-polarized currents, whose direction can be controlled via the helicity of the excitation light. They also highlight the importance of analysing the spatial distribution of the photocurrent, as we observe a strong enhancement of the spin-polarized current and the thermoelectric current at the interface between the nanowire and the metallic contacts. As our analysis shows, the thermoelectric current is enhanced by the Schottky effect and the spin-polarized current is amplified by the spin Nernst effect. In addition, the spin Nernst effect is also present in Hall bar devices and manifest as an enhancement of the spin-polarized current along the Hall bar sides.
Study of the effect of the podocyte-specific palladin knockout in mice with a 129 genetic background
(2023)
Worldwide, chronic kidney disease is one of the leading public health problems. Podocytes, highly specialized postmitotic cells in the filtration unit of the kidney glomerulus, are essential for the size selectivity of the filtration barrier. Loss of the complex 3D morphology of their interdigitating foot processes, effacement and detachment of the cells from the capillaries lead to proteinuria and often loss of kidney function.
Since the morphology of podocyte foot processes is highly dependent on an intact actin cytoskeleton and actin-binding proteins, we investigated the role of the actin-binding protein palladin in podocytes from mice with a 129 genetic background, that is more susceptible to kidney injury. PodoPalld129-/- mice were examined at 6 and 12 months of age using immunofluorescence staining, electron and 3D super-resolution microscopy as well as qRT-PCR.
Our analysis of PodoPalld129-/- mice at 6 and 12 months of age showed that podocyte- specific knockout of palladin results in dilation of the capillary tuft accompanied by loss of mesangial cells, indicating the influence of palladin on glomerular tuft formation. Besides, we observed morphological abnormalities such as an enlarged sub-podocyte space, cyst formations and an increased number of cell-cell contacts between podocytes and parietal epithelial cells in PodoPalld129-/- mice compared to controls. Moreover, palladin knockout resulted in downregulation of the slit diaphragm protein nephrin as well as an age-dependent significant increase in podocyte foot process effacement. Although there was a significant change in foot process morphology, we did not detect albuminuria in PodoPalld129-/- mice of both age groups. However, we found an increase of trefoil factor 1 (Tff1) in the urine of the mice, indicating an altered, more permeable filtration barrier.
Considering that palladin has several binding sites for important actin-binding and regulatory proteins, we studied the expression of Lasp-1, Pdlim2, VASP and Klotho in dependence on palladin. We found a remarkable reduction in, for example, phosphorylated Lasp-1 as well as Klotho, which could influence the morphology of podocyte foot processes.
Compared with PodoPalldBL/6-/- mice, PodoPalld129-/- mice showed stronger glomerular tuft dilation and developed podocytes with increased morphological abnormalities, underlining the importance of the genetic background.
In conclusion, these results demonstrate the essential role of palladin for podocyte morphology in mice with a 129 genetic background.