Anthropogenic input of heavy metals to near-coastal sediment depocenters in the eastern North Sea and the Hauraki Gulf in historical times

Since the beginning of the industrial revolution, and even earlier, near-coastal marine ecosystems are affected by various anthropogenic influences such as the enhanced input of heavy metals. In this thesis, the anthropogenically induced historical heavy metal input to near-coastal sediment depocenters in the eastern North Sea, the Helgoland mud area and the Skagerrak, and the Hauraki Gulf, NZ, is analysed and discussed. Enhanced heavy metal inputs to the Helgoland mud area and the Skagerrak commenced in medieval times and are attributed to early mining and smelting activities in medieval mining centres in Germany and Sweden. Due to the low present-day sediment input and the strong reworking of the sediments, the sediments of the Hauraki Gulf do not reveal any signal of heavy metal inputs, but provide new insights in the deposition and reworking of surficial sediments as a consequence of the interactions of wind-generated waves and the modern hydrodynamic regime

Cooling of radiative quantum-dot excitons by terahertz radiation: A spin-resolved Monte Carlo carrier dynamics model

We have developed a theoretical model to analyze the anomalous cooling of radiative quantum dot (QD) excitons by THz radiation reported by Yusa et al [Proc. 24th ICPS, 1083 (1998)]. We have made three-dimensional (3D) modeling of the strain and the piezoelectric field and calculated the 3D density of states of strain induced quantum dots. On the basis of this analysis we have developed a spin dependent Monte Carlo model, which describes the carrier dynamics in QD's when the intraband relaxation is modulated by THz radiation. We show that THz radiation causes resonance transfer of holes from dark to radiative states in strain-induced QD's. The transition includes a spatial transfer of holes from the piezoelectric potential mimima to the deformation potential minimum. This phenomenon strongly enhances the QD ground state luminescence at the expense of the luminescence from higher states. Our model also reproduces the delayed flash of QD ground state luminescence, activated by THz radiation even $\sim1$ s after the carrier generation. Our simulations suggest a more general possibility to cool the radiative exciton subsystem in optoelectronic devices.Comment: 18 pages, 1 table, 8 figures, submitted to Physical Review B v2: major conceptual changes. The article was extended considerably to suit Physical Review B (instead of Physical Review Letters

Modeling the effect of elastic strain on ballistic transport and photonic properties of semiconductor quantum structures

The recent progress in microelectronic processing techniques has made it possible to fabricate artificial materials, dedicated and tailored directly for nanoelectronics and nanophotonics. The materials are designed to achieve a confinement of electrons to nanometer size foils or grains, often called quantum structures because of the quantization of the electron energies. In this work I have developed computationalmodels for the electronic structure, photonic recombination and carrier dynamics of quantum confined charge carriers of artificial materials. In this thesis I have studied in particular the effect of elastic strain on the ballistic transport of electrons, in silicon electron wave guides; and on the electronic structure and photonic properties of III-V compound semiconductor heterostructures. I have simulated two types of elastic strain. The strain in the silicon wave guides is induced by the thermal oxidation of the silicon processing and the strain of the III-V compound semiconductor structures is a result of a pseudomorphic integration of lattice mismatched materials. As one of the main results of this work, we have shown that the oxidation-induced strain can lead to current channeling effects in electron wave guides and a doubling of the conductance steps of the wave guide. In the case of the III-V compound semiconductor heterostructures, it was shown that piezoelectric potential (which is due to the elastic strain) complicates considerably the electron-hole confinement potential of strain-induced quantum dots. This has several consequences on the optical properties of these systems. Our results are well in agreement with experimental observations and do explain a set of experiments, which have so far lacked any explanation. This work does, thereby, imply a much better understanding of both silicon electron wave guides and strain-induced quantum dots. This could have implications for both further detailed experiments and future technological applications of the studied devices.reviewe

Topographically specific effects of ELF-1 on retinal axon guidance in vitro and retinal axon mapping in vivo

Peer reviewedPublisher PD

B-Cos Aligned Transformers Learn Human-Interpretable Features

Vision Transformers (ViTs) and Swin Transformers (Swin) are currently state-of-the-art in computational pathology. However, domain experts are still reluctant to use these models due to their lack of interpretability. This is not surprising, as critical decisions need to be transparent and understandable. The most common approach to understanding transformers is to visualize their attention. However, attention maps of ViTs are often fragmented, leading to unsatisfactory explanations. Here, we introduce a novel architecture called the B-cos Vision Transformer (BvT) that is designed to be more interpretable. It replaces all linear transformations with the B-cos transform to promote weight-input alignment. In a blinded study, medical experts clearly ranked BvTs above ViTs, suggesting that our network is better at capturing biomedically relevant structures. This is also true for the B-cos Swin Transformer (Bwin). Compared to the Swin Transformer, it even improves the F1-score by up to 4.7% on two public datasets.Comment: Accepted at MICCAI 2023 (oral). Camera-ready available at https://doi.org/10.1007/978-3-031-43993-3_5

Molecular mechanisms separating two axonal pathways during embryonic development of the avian optic tectum

During embryonic development of the avian optic tectum, retinal and tectobulbar axons form an orthogonal array of nerve processes. Growing axons of both tracts are transiently very closely apposed to each other. Despite this spatial proximity, axons from the two pathways do not intermix, but instead restrict their growth to defined areas, thus forming two separate plexiform layers, the stratum opticum and the stratum album centrale. In this study we present experimental evidence indicating that the following three mechanisms might play a role in segregating both axonal populations: Retinal and tectobulbar axons differ in their ability to use the extracellular matrix protein laminin as a substrate for axonal elongation; the environment in the optic tectum is generally permissive for retinal axons, but is specifically nonpermissive for tectobulbar axons, resulting in a strong fasciculation of the latter; and growth cones of temporal retinal axons are reversibly inhibited in their motility by direct contact with the tectobulbar axon's membrane

Sediment deposition in the central Hauraki Gulf, New Zealand

Based on the analysis of 14 short sediment cores, we present new insights into the distribution of surficial sediments in the central Hauraki Gulf, a semi-enclosed coastal embayment on the northeast coast of New Zealand’s North Island. We identify and discuss the effects of interaction of modern wind-generated waves and currents with regard to deposition and reworking of sediments in the Gulf. The modern hydrodynamic regime is controlled by tidal currents, oceanic inflows, and wave-induced currents and it is responsible for a N-S gradient in sediment texture and elemental concentrations in the central Hauraki Gulf sediments. The present-day sediment input into the system is generally low and consists of fine-grained fluvial sediments mostly deposited in the southern study area and comparatively high inputs of relict carbonate material to the northern study sites. The central Hauraki Gulf sediments, which show numerous age reversals in the sedimentary record, can be characterised as palimpsest sediments, as a consequence of continuous reworking and storm-induced sediment transport. In view of the new data, a previously assumed significant post-transgression accumulation of sediments of > 10 m in the central Hauraki Gulf appears to be very unlikely