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Momentum resolved tunneling in one-dimensional systems
No full textThis thesis presents measurements on one dimensional systems in GaAs cleaved
edge overgrowth (CEO) samples. Due to their unprecedented quality, the
quantum wires in these systems posses long mean free path of more than 10 μm
and large subband spacings exceeding 10 meV, which allow for the observation
of many effects characteristic to electronic transport in one dimension.
We investigate the finite size effects in tunneling between parallel quantum
wires with a density inhomogenuity induced by a top gate. This study dif-
fers from previous experiments in the inclusion of the gate voltage dependence
of conductance. By using both, experiments and numerical simulations, we
link the arising patterns in the tunnel conductance to the internal potential
landscape inside the quantum wire, enabling the characterization of various
sections of varying density. The finiteness of the wire implies that quantized
energy levels should exist. A combination of low temperature, good thermal-
ization and small DC-voltage bias and AC-excitations allows us to observe
Coulomb blockade in these long quantum wires, and extract the single particle
level spacing directly from the even/odd spacing of those Coulomb peaks.
We also investigate the dispersion of quantum Hall edge states and extract
their velocities using two independent methods, momentum resolved tunnel-
ing as a function of bias voltage (energy) and also as a function of gate voltage
(density), which agree well with each other. These measurements are per-
formed in the same CEO device and allow us to leverage our knowledge of the
quantum wire system, which is used as a spectrometer in these experiments.
We find excellent agreement between the extracted velocities and the predic-
tions for hard wall confined edge states.
The last two chapters cover phenomena of interacting electrons in one dimen-
sion. We observe further evidence for a spin selective Peierls transition by
measuring the increase of conductance on the non-universally quantized con-
ductance plateau as temperature is increased.
We also perform a systematic study of the zero bias anomaly in gate defined
quantum wire arrays, spanning a large temperature range of multiple decades,
and various densities, ranging from one to three subbands
Characterization of the genetic landscape of hereditary retinopathies in Pakistan
Full text linkHereditary retinopathies (HRPs) are a group of rare Mendelian disorders often leading to progressive vision loss and potential blindness. Despite their monogenic nature, HRPs exhibit extensive genetic heterogeneity, with pathogenic variants identified in nearly 300 genes. In this study, we investigated the genetic landscape of HRPs in Pakistan, a population with high consanguinity and unique demographic features. A cohort of 213 families (722 affected individuals) underwent genetic analysis, primarily through whole-exome sequencing, achieving a molecular diagnosis in 80.3% of cases. Pathogenic or likely pathogenic variants were identified in 60 HRP-associated genes, with ABCA4, CRB1, MYO7A, PDE6B, and RP1 being the most prevalent. The findings underscore a distinct mutational spectrum, shaped by endogamy and recurrent founder mutations, differing from other populations
Exploring the landscape of mTOR substrates: identification of two novel targets
Full text linkTOR (Target of Rapamycin), is a highly conserved serine/threonine protein kinase that plays a central role in regulating cell growth and metabolism. It is activated by nutrients, growth factors, and cellular energy. TOR forms two structurally and functionally distinct complexes, TORC1 and TORC2. TOR signaling activates cell growth, defined as an increase in biomass, by stimulating anabolic metabolism while inhibiting catabolic processes.
With emphasis on mammalian TOR (mTOR), we comprehensively reviewed the literature and identified all reported direct substrates. In the context of recent structural information, we discuss how mTORC1 and mTORC2, despite having a common catalytic subunit, phosphorylate distinct substrates. Based on this analysis we conclude that mTORC1 and mTORC2 phosphorylate a common, minimal motif but rely mostly on distinct binding sequences for substrate recognition. mTORC1, for instance, recognizes some of its substrates via a five amino acid target sequence called TOR signaling (TOS) motif. mTORC1 binding to a TOS peptide facilitates phosphorylation of the target site differently located than the binding site.
Furthermore, by exploiting the aforementioned information, we identified two previously uncharacterized mTOR substrates: LST2 and RTN4. Here we show that LST2, also known as ZFYVE28, contains a TOS motif and is phosphorylated by mTORC1 at serine 670 (S670). Phosphorylation on LST2 S670 by mTORC1 primes monoubiquitination on lysine 87 (K87). Monoubiquitinated and phosphorylated LST2 is stable and display a broad reticular distribution. When mTORC1 is inactive, unphosphorylated LST2 localizes to the endosome and is degraded by the proteasome. Cells deficient in LST2 have higher levels of the epidermal growth factor receptor (EGFR). We therefore propose a mechanism in which mTORC1, via LST2, negatively feeds back on its upstream receptor EGFR in order to maintain signaling homeostasis.
In the second part of this thesis, we searched previously reported phosphorproteomics datasets for the identification of novel mTOR substrates. We found that the ER residential protein reticulon 4 (RTN4) is phosphorylated by mTOR on S15, S107 and S181. Although still preliminary, our data suggest that mTOR-dependent phosphorylation of RTN4 regulates calcium and lipid homeostasis through its interaction with specific partners. RTN4 appears to also regulate the ER-mitochondria associated membranes (MAM) structure. In addition, both RTN4 mRNA and protein levels are increased in a mTOR-driven hepatocellular cancer (HCC) mouse model. Our data suggest that mTOR could regulate ER and mitochondria metabolism through phosphorylation of RTN4.
Collectively, this thesis offers a complete list of all reported mTOR substrates and provides insights into the discovery and characterization of two new mTOR substrates, LST2 and RTN4
Mycorrhiza within intercropping systems: Exploring AMF mediated complementary effects across staple crops from the drylands
No full textThe main aim of this thesis was to advance the ecological know-how for the effective use of AMF within intercropping systems, aiming to optimize complementary interactions between crops. In the first chapter of the thesis, I tested AMF-based biofertilizers in field trials in southern India. There, I examined how biofertilizers influenced intercropping performance across different spatial arrangements and crop mixtures across different sites. In the second chapter, I conducted greenhouse experiments where I tested the efficiency of AMF mediating complementary effects under increasing nutrient availabilities as well as different crop mixtures. In the third chapter, I tested the relevance of planting distance mediating interspecific water transfer by one AMF strain from deep-rooted legumes to shallow rooted cereals during drought
Effects of physical environmental variables on the behaviour of mountain woodland songbirds
Full text linkBackground: The behaviour of songbirds depends on several physical environmental factors that may change with increasing elevation. For example, temperature decreases and the vegetation period is shorter at higher elevations. Birds may need to adapt to changes in environmental conditions by changing their breeding or singing behaviour. This thesis aims to contribute to understand the effects of physical environmental variables on the behaviour of mountain woodland songbirds.
Location: Switzerland, Swiss National Park
Methods: To investigate the singing behaviour of mountain woodland songbirds (Alpine tit, coal tit, Eurasian chaffinch, song thrush, mistle thrush and ring ouzel) we used passive acoustic monitoring and observations in the field. We also used citizen science data to examine changes in breeding phenology along an elevational gradient.
Results and main conclusions: We observed species-specific differences in the singing behaviour according to the investigated variables, and found only small differences in the timing of breeding between higher and lower elevations independent of climatic conditions. The effect of anthropogenic noise seemed to be context- but not species-specific, since all investigated songbird species changed their singing behaviour in noisier areas and during nosier times. Physical environmental variables, like moon phase, temperature, cloud cover or aspect, had consistent but small effects on all investigated songbird species. The effects of elevation and date, in contrast, were more species-specific. Further, alpine songbirds seem to have evolved adaptations to cope with harsher environmental conditions at higher elevations, since the investigated species were only raised with a relatively small delay at higher compared to lower elevations
Disentangling Chemical Interaction and Electric Fields at Electrochemical Interfaces
No full textThe specific binding of solution components to a catalytic surface and the interfacial electric field are critical to electrocatalysis, but the two factors are entangled. Distinguishing whether adsorbate binding to the surface is governed by through-bond charge transfer that is modulated by a field or through-space electrostatic effects is essential for rational catalyst design. Here, we probed the binding of thiocyanate (SCN–) to Au and Pd as a function of applied potential (ϕapp) using surface-enhanced infrared absorption spectroscopy (SEIRAS). The IR data showed distinct shifts of the CN stretching wavenumbers (ν̃CN) with ϕapp for S- versus N-bound SCN– and on Au versus Pd. These ν̃CN shifts with ϕapp are the result of different extents of through-space electric field (Stark) effects and through-bond chemical binding effects. The presence of two binding modes of SCN– on the metal surfaces provided two separate interfacial probes, which enabled estimating the contributions of the two effects to the overall ν̃CN versus ϕapp slopes quantitatively. Competitive adsorption experiments using SCN– and different halides (Cl–, Br–, and I–) support our conclusions. While the through-bond effect for S-bound SCN– is more pronounced on Pd than on Au, that of N-bound SCN– is similar. Our data also suggest a similar electric field change with ϕapp on Pd and Au. This work offers a method to differentiate key contributors to interfacial chemistry in an external electric field and provides important groundwork for the development of future electrocatalysts
Volume rendering for surgical planning in virtual reality
No full textThe number of high-complexity surgeries has been steadily increasing over the last decades. One reason for this is the demographic trend of an aging population observed in many countries, which results in an increasing number of patients requiring complex interventions. Another important factor is the development and continued improvement of advanced surgical techniques and devices, such as minimally invasive surgeries, which enable surgeons to treat more challenging cases successfully. While these techniques can improve the surgical outcome, they are also considerably more difficult and require careful planning and training.
Surgical planning of complex cases is typically done using volumetric medical images, such as computed tomography (CT) scans or magnetic resonance imaging (MRI) data sets. These modalities capture the imaging subject in three dimensions, allowing a more detailed analysis of the patient's anatomy compared to the planar projection created by an X-ray device. Despite the three-dimensiona
Dialektik und Kritik: zur systematischen Bedeutung der Kantinterpretation von Theodor W. Adorno
Full text linkPhilosophie im Sinne Adornos bedeutet, die Traditionslinien der Dialektik und der Kritik miteinander engzuführen. Diese Engführung erfolgte bislang vor allem gemäß der Vorgabe Hegels – mit dem Resultat, dass das Profil einer dezidiert negativen Dialektik durch Identitätsdenken überformt und in der Forschung als mangelhafte Kopie der positiv-spekulativen Dialektik gehandelt wurde. Dagegen schlägt diese Arbeit vor, dieses Profil von Kant her zu sichten: Indem die Grundzüge von Adornos Kantinterpretation erhellt werden, soll deren systematische Bedeutung für das adornosche "Antisystem" zur Geltung gelangen. Negative Dialektik, so die These, ist nur als grenzbegriffliche Reflexion auf den Erkenntnisanspruch der traditionellen Dialektik möglich
Micro- and nanomagnet designs for the manipulation of spin qubits
Full text linkThis thesis discusses magnet designs that provide suitable stray fields for the manipulation of the spin direction of single electrons, enabling the implementation of a quantum computer. The building block, named qubit, is built by confining electrons in quantum dots with the quantum state encoded in the direction of each electron spin. The stray field allows for controlled rotations of the spin direction via an oscillatory displacement of the confinement potential minimum used to define the quantum dot. This technique, named electric dipole spin resonance (EDSR), has been implemented in several spin qubit processors with a Co micromagnet placed on top of the device. As a guideline, the requirements on the stray field of the magnet for successfully running quantum algorithms were set to a driving gradient > 1 mT/nm, a dephasing gradient < 0.1 mT/nm and single qubit addressability.
Extending this concept to a large number of qubits is challenging in terms of providing large enough driving gradients and individual addressability. Here we present three nanomagnet designs for this purpose, supported by micromagnetic simulations and the magnetic characterization of micro- and nanomagnets. We start by discussing the key parameters required in the modeling of magnets for EDSR manipulation. We demonstrate by scanning superconducting quantum interference device microscopy and simulations that the combination of polycrystallinity and large magnetocrystalline anisotropy energy inherent to Co micromagnets leads to random variations of the Larmor frequency of the qubits, negatively impacting qubit crosstalk and dephasing.
Building on these findings, we investigate the integration of dual-function Co magnetic structures within the qubit architecture. The magnets provide both the magnetic stray field for EDSR manipulation as well as the electric field modulation to confine the electron wavefunction. We fabricate a proof-of-concept architecture, demonstrating successful quantum confinement with Co nanogates. Moreover, we map their magnetization state in remanence by off-axis holography measurements. Micromagnetics simulations show that all the requirements of the stray field mentioned above are fullfilled with an applied external magnetic fields above 1 T.
Finally, we introduce two nanomagnet designs and discuss their advantages compared to micromagnets for the manipulation of spin qubits. The first design ("Horseshoe") is aimed at driving spin qubits arranged in linear chains and strongly confined in directions lateral to the chain. Spin-SEM measurements on arrays of such nanomagnets show reproducible magnetization patterns and good agreement with micromagnetic simulations. Based on these findings, we developed the design ("Block"), aimed at driving qubits arranged on a 2D lattice. Importantly, both designs are modular, ensuring efficient qubit manipulation independent of the number of qubits within the architecture.
These findings provide a novel concept for the development of a scalable electron spin qubit architecture manipulated with the stray field of magnets