Current amplification and relaxation in Dirac systems

We study how electron-electron (e-e) interactions affect the current carried by excited electrons and holes in Dirac systems such as graphene or topological insulators. We find that the current shows distinctly different behavior for e-e collisions involving the electron or the hole. Most surprisingly, for positive Fermi energy, collisions of the electrons can substantially increase the current. This remarkable amplification of the current can be of the order of $\sim 10\%$ per scattering event. It also causes a strong suppression of the overall current relaxation rate of a photoexcited electron-hole pair, with the amplification of the electron current making up for a fast decay of the hole current.Comment: 4 pages, 4 figures + references + supplemental materia

Informed Consent: A Cultural Dilemma

Obtaining informed consent is a process unique to Western medicine. Cultural differences are high-lighted and conflict can arise when trying to explain consent to immigrant or ethnic patients. This essay describes an example case and provides suggestions of important points to include in the phy-sician-patient discussion in order to improve understanding

Cultivating suspicion: an ethnography of corporeal strategies deployed against vulnerability to crime in Observatory, Cape Town

This ethnographic study explores how people deal with suspicion and navigate the fear of crime in the Observatory suburb of Cape Town, South Africa. The study grapples with the question of how the neighbourhood watch, as a recently revived institution, operates. It analyses the institution and relationships within and around it as an alternative source of trust to the state in combatting crime and its wider impact on lived sociality in the suburb and, perhaps, beyond. The focus of the study lies in understanding the strategies people employ habitually in order to create a sense of security in a context where the anticipation of violence permeates various everyday routines. In analysing strategies of living through insecurities, I focus on examining material and highly visible security measures, such as patrol cars and barbed wires, and engage with the body as a site of social and political memory and struggle, while considering the roles it takes on in the face of perceived precariousness. This dissertation offers an insight in to how the body is deployed as an instrument or buffer to deal with insecurity and crime vulnerability. The quality of public life becomes compromised through embodied strategies of (in)security and vulnerability as employed by the neighbourhood watch. The capacity of a constantly perceived presence of criminal violence in shaping individual and institutional bodies and strategies constitutes the main focus of this study. While the study does not identify the roots of crime as is currently practice with related studies of crime in South Africa, it illuminates the engagement with its perceived presence and thus moves away from a fixed victim-perpetrator dichotomy that has dominated the public discourse

Multimodal view on resting-state brain activity in Parkinson’s disease: examining the relation between functional resting-state networks and metabolic network activity

Research focusing on the pathophysiology of neurodegenerative disorders has undergone a fundamental shift towards a network perspective in the last decades. Besides regional aggregation of misfolded proteins and changes in cellular metabolism, accompanying changes of synaptic activity evolve and evoke dysregulation within neural circuits including remote brain regions. Modern theories of neurodegeneration propose a stereotypic pattern of these cerebral pathologies, which partly are in vivo accessible by multimodal neuroimaging techniques. The most often used indirect measurement of functional network integrity is resting-state functional magnetic resonance imaging, which depends on a complex interplay of hemodynamics, blood volume, and blood flow. Less is known about a potential metabolic component underlying resting-state networks in healthy brains and changes thereof in neurodegeneration and the influence of different transmitter systems. The current work therefore sought to investigate the association between functional resting-state networks and metabolic network activity and focused on metabolic consequences of nigrostriatal and striatocortical dysfunction in Parkinson’s disease. In the current work, a multimodal data set of the TP10 KFO219 cohort was analyzed regarding 1) the impact of nigrostriatal dopamine depletion on resting-state networks and 2) the relation between changes in functional connectivity and metabolic network activity. The first study addressed the subset of the KFO219 TP10 cohort who completed the trimodal imaging protocol (42 patients vs. 14 controls). Dopamine deficiency in Parkinson’s patients was examined by voxel-wise comparison of 6-[18F]fluoro-L-Dopa positron emission tomography scans. Resulting clusters served as seeds for restingstate functional connectivity maps that were compared between both groups by voxelwise t-tests. Metabolic activity was extracted from 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography scans for respective cortical clusters with striatocortical dysconnectivity and the relation to functional connectivity values was analyzed. In a separate study, functional and metabolic resting-state networks were obtained by performing spatial independent component analyses in a subset of the same cohort who underwent 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography and functional magnetic resonance imaging (56 vs 16) and completed neuropsychological testing. Multimodally obtained regions of interest in the default mode network were defined and metabolic activity as well as metabolic connectivity compared to functional connectivity differences between patients without or with mild cognitive impairment and healthy controls. Moreover, a third study was initiated in the context of the present work with the aim of establishing a dynamic 2-[18F]fluoro-2-deoxy-D-glucose positronemission tomography acquisition with a constant infusion protocol for examining interregional metabolic connectivity on single subject level and enable comparable analysis of hemodynamic and metabolic fluctuations in Parkinson’s disease. In the first study, a significant association between striatocortical functional connectivity changes of the data-driven defined dopamine depleted posterior putamen and metabolic activity of the cortical target area in the inferior parietal cortex was found in Parkinson’s disease. Interestingly, striatocortical connectivity of the inferior parietal cortex was associated with motor and cognitive impairment. In a second study, the multivariate approach revealed a moderate spatial convergence for the posterior default mode network in functional and metabolic data. For all multimodally obtained default mode network regions, a significant trend towards an increment of metabolic deficits from healthy controls via unimpaired patients to patients with mild cognitive impairment was identified. In addition, posterior default mode network regions with the strongest metabolic deficits and gradual decline in comparison to controls, also showed the strongest increases in both metabolic and functional connectivity compared to controls. The verification of the applicability of a constant infusion dynamic 2-[18F]fluoro-2-deoxy- D-glucose positron emission tomography protocol in Parkinson’s disease patients was started in a self-initiated study, which finished the acquisition phase with 10 participants per group by the time the current work was submitted. Together the first two studies highlight the added value of multimodal imaging in investigating human brain function and the pathophysiology of neurodegenerative disorders, in particular their great potential for identifying links between individual pathologies. The second study partly continued, and answered questions raised in response to the first study, which hinted at an involvement of default mode network regions in cognitive symptoms of Parkinson’s disease and a relation between functional network degeneration and metabolic activity. The current work shows exemplary the complementarity of both measures of brain network activity and their individual significance for cognitive symptoms in Parkinson’s disease. The presented work highlights how multimodal resting-state studies can provide new insights into the (patho-)physiological network organization of brain activity by confirming insights obtained by one modality and deepen our understanding of disease processes. The selfinitiated study further laid the ground for multimodal characterization of metabolic and hemodynamic network changes on single-subject level and the evaluation of dynamic positron emission tomography-based connectivity as metabolic network marker for Parkinson’s disease

Coping in pandemic times: bricolage employed by first-generation engineering students

First-generation students have been a focus in higher education research over the past ten years. However, limited attention has been paid to engineering students who are the first in their generation to enter university. The paper reports on data collected as part of a longitudinal study of first-generation engineering students at a South African university during the early stages of the pandemic. First-generation students, who already face multiple difficulties in their educational journey, were confronted with a juxtaposition during the lockdown. As engineering students, they are inducted into technical approaches to problem-solving via systematic and analytical exploration. Levi-Srauss contrasts this notion of the ingenieur, grounded in the Enlightenment belief in the superiority of rational scientific reasoning, with the bricoleur, who finds solutions by “doing things with whatever is at hand”. With the lockdown period being less amenable to structured problem-solving, students often had to resort to more improvised approaches to accommodate their studies and their shifted precarious everyday routines. The study not only adds to literature on firstgeneration engineering students, but also provides insight into the ways in which these students cope with obstacles over which they have little control. In the process a picture of resilient agency emerges that challenges a narrow deficit view of students with limited resources

Transport through a disordered topological-metal strip

Features of a topological phase, and edge states in particular, may be obscured by overlapping in energy with a trivial conduction band. The topological nature of such a conductor, however, is revealed in its transport properties, especially in the presence of disorder. In this work, we explore the conductance behavior of such a system with disorder present, and contrast it with the quantized conductance in an ideal two-dimensional topological insulator. Our analysis relies on numerics on a lattice system and analytics on a simple toy model. Interestingly, we find that as disorder is increased from zero, the edge conductivity initially falls from its quantized value; yet, as disorder continues to increase, the conductivity recovers, and saturates at a value slightly below the quantized value of the clean system. We discuss how this effect can be understood from the tendency of the bulk states to localize, while the edge states remain delocalized

Photocurrent response of topological insulator surface states

We study the photocurrent response of topological insulator surface states to circularly polarized light for arbitrary oblique incidence. We describe the surface states within a Dirac model, including several perturbations such as hexagonal warping, nonlinear corrections to the mode velocity, and applied magnetic fields. We find that the photogalvanic current is strongly suppressed for the usual orbital coupling, prompting us to include the weaker Zeeman coupling. We find that the helicity-independent photocurrent dominates over the helicity-dependent contributions