182 research outputs found
Power and Energy Student Summit 2019: 9 – 11 July 2019 Otto von Guericke University Magdeburg ; Conference Program
The book includes a short description of the conference program of the "Power and Energy Student Summit 2019". The conference, which is orgaized for students in the area of electric power systems, covers topics such as renewable energy, high voltage technology, grid control and network planning, power quality, HVDC and FACTS as well as protection technology. Besides the overview of the conference venue, activites and the time schedule, the book includes all papers presented at the conference
Angle-dependence of quantum oscillations in YBa2Cu3O6.59 shows free spin behaviour of quasiparticles
Measurements of quantum oscillations in the cuprate superconductors afford a
new opportunity to assess the extent to which the electronic properties of
these materials yield to a description rooted in Fermi liquid theory. However,
such an analysis is hampered by the small number of oscillatory periods
observed. Here we employ a genetic algorithm to globally model the field,
angular, and temperature dependence of the quantum oscillations observed in the
resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set
to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key
feature of the data is the first identification of the effect of Zeeman
splitting, which separates spin-up and spin-down contributions, indicating that
the quasiparticles in the cuprates behave as nearly free spins, constraining
the source of the Fermi surface reconstruction to something other than a
conventional spin density wave with moments parallel to the CuO2 planes.Comment: 8 pages, 4 figure
Construction, management and maintenance of embankments used for road and rail infrastructure: implications of weather induced pore water pressures
Understanding the age and construction quality of embankments used for road and rail infrastructure is critical in the effective management and maintenance of our transport networks, worth £billions to the UK economy. This paper presents for the first time results from full-scale, carefully controlled experiments on a unique model embankment conducted over the 4-year period between 2008 and 2011. It combines point location and spatially distributed measurements of pore water pressures and water content with outputs from hydrological modelling to draw conclusions of significance to both ongoing research in this field and to the asset management practices of infrastructure owners. For researchers, the paper highlights the crucial importance of transient permeability and soil water retention behaviour of fill materials in controlling the magnitude and distribution of pore water pressure in response to climate and weather events. For practitioners, the work demonstrates that there are significant differences in pore water pressure behaviour across the embankment, which is influenced by construction-related issues such as compaction level, aspect and presence of a granular capping material. Permeability was also observed to vary across the embankment both spatially and with depth, being dependent on degree of saturation and macroscale effects, particularly within a ‘near surface zone’. It is proposed that this ‘near surface zone’ has a critical effect on embankment stability and should be the focus of both ongoing scientific research and inspection and monitoring as encompassed by asset management regimes
Advances in single crystal growth and annealing treatment of electron-doped HTSC
High quality electron-doped HTSC single crystals of and have been
successfully grown by the container-free traveling solvent floating zone
technique. The optimally doped and crystals have transition temperatures
of \,K and \,K, respectively, with a transition width of less than
\,K. We found a strong dependence of the optimal growth parameters on the Ce
content . We discuss the optimization of the post-growth annealing treatment
of the samples, the doping extension of the superconducting dome for both
compounds as well as the role of excess oxygen. The absolute oxygen content of
the as-grown crystals is determined from thermogravimetric experiments and is
found to be . This oxygen surplus is nearly completely removed by a
post-growth annealing treatment. The reduction process is reversible as
demonstrated by magnetization measurements. In as-grown samples the excess
oxygen resides on the apical site O(3). This apical oxygen has nearly no doping
effect, but rather influences the evolution of superconductivity by inducing
additional disorder in the CuO layers. The very high crystal quality of
is particularly manifest in magnetic quantum
oscillations observed on several samples at different doping levels. They
provide a unique opportunity of studying the Fermi surface and its dependence
on the carrier concentration in the bulk of the crystals.Comment: 19 pages, 7 figures, submitted to Eur. Phys. J.
Identification and Characterization of a Liver Stage-Specific Promoter Region of the Malaria Parasite Plasmodium
During the blood meal of a Plasmodium-infected mosquito, 10 to 100 parasites are inoculated into the skin and a proportion of these migrate via the bloodstream to the liver where they infect hepatocytes. The Plasmodium liver stage, despite its clinical silence, represents a highly promising target for antimalarial drug and vaccine approaches. Successfully invaded parasites undergo a massive proliferation in hepatocytes, producing thousands of merozoites that are transported into a blood vessel to infect red blood cells. To successfully develop from the liver stage into infective merozoites, a tight regulation of gene expression is needed. Although this is a very interesting aspect in the biology of Plasmodium, little is known about gene regulation in Plasmodium parasites in general and in the liver stage in particular. We have functionally analyzed a novel promoter region of the rodent parasite Plasmodium berghei that is exclusively active during the liver stage of the parasite. To prove stage-specific activity of the promoter, GFP and luciferase reporter assays have been successfully established, allowing both qualitative and accurate quantitative analysis. To further characterize the promoter region, the transcription start site was mapped by rapid amplification of cDNA ends (5′-RACE). Using promoter truncation experiments and site-directed mutagenesis within potential transcription factor binding sites, we suggest that the minimal promoter contains more than one binding site for the recently identified parasite-specific ApiAP2 transcription factors. The identification of a liver stage-specific promoter in P. berghei confirms that the parasite is able to tightly regulate gene expression during its life cycle. The identified promoter region might now be used to study the biology of the Plasmodium liver stage, which has thus far proven problematic on a molecular level. Stage-specific expression of dominant-negative mutant proteins and overexpression of proteins normally active in other life cycle stages will help to understand the function of the proteins investigated
Missing evidence of widespread subglacial lakes at Recovery Glacier, Antarctica
Recovery Glacier reaches far into the East Antarctic Ice Sheet. Recent projections point out that its dynamic behavior has a considerable impact on future Antarctic ice loss (Golledge et al., 2017, https://doi.org/10.1002/2016GL072422). Subglacial lakes are thought to play a major role in the initiation
of the rapid ice flow (Bell et al., 2007, https://doi.org/10.1038/nature05554). Satellite altimetry observations have even suggested several actively filling and draining subglacial lakes beneath the main trunk (B. E. Smith et al., 2009, https://doi.org/10.3189/002214309789470879). We present new data of the geometry
of this glacier and investigate its basal properties employing radio-echo sounding. Using ice sheet modeling, we were able to constrain estimates of radar absorption in the ice, but uncertainties remain large. The magnitude of the basal reflection coefficient is thus still poorly known. However, its spatial variability, in conjunction with additional indicators, can be used to infer the presence of subglacial water. We find no clear evidence of water at most of the previously proposed lake sites. Especially, locations, where altimetry detected active lakes, do not exhibit lake characteristics in radio-echo sounding. We argue that lakes
far upstream the main trunk are not triggering enhanced ice flow, which is also supported by modeled subglacial hydrology
Ice flow and the conditions of the ice-bed interface at the onset of the Northeast Greenland Ice Stream
The Northeast Greenland Ice Stream (NEGIS) is an important dynamic component contributing to the total mass balance of the Greenland ice sheet, as it reaches up to the central divide and drains 12% of the ice sheet. The ice stream geometry and surface velocities in the onset region of the NEGIS are not yet sufficiently well reproduced by ice sheet models. We present an assessment of the basal conditions of the onset region in a systematic analysis of airborne ultra-wideband radar data. Our data yield a new detailed model of ice-thickness distribution and basal topography in the upstream part of the ice stream. We observe a change from a smooth to a rougher bed where the ice stream widens from 10 to 60 km, and a distinct roughness anisotropy, indicating a preferred orientation of subglacial structures. The observation of off-nadir reflections that are symmetrical to the bed reflection in the radargrams suggests that these structures are elongated subglacial landforms, which in turn indicate potential streamlining of the bed. Together with basal water routing pathways, our observations hint to two different zones in this part of the NEGIS: an accelerating and smooth upstream region, which is collecting water, with reduced basal traction, and in the further downstream part, where the ice stream is slowing down and is widening, with a distribution of basal water towards the shear margins. Our findings support the hypothesis that the NEGIS is strongly interconnected to the subglacial water system in its onset region, but also to the subglacial substrate and morphology
Where is the quantum critical point in the cuprate superconductors?
Transport measurements in the hole-doped cuprates show a "strange metal"
normal state with an electrical resistance which varies linearly with
temperature. This strange metal phase is often identified with the quantum
critical region of a zero temperature quantum critical point (QCP) at hole
density x=x_m, near optimal doping. A long-standing problem with this picture
is that low temperature experiments within the superconducting phase have not
shown convincing signatures of such a optimal doping QCP (except in some
cuprates with small superconducting critical temperatures). I review
theoretical work which proposes a simple resolution of this enigma.
The crossovers in the normal state are argued to be controlled by a QCP at
x_m linked to the onset of spin density wave (SDW) order in a "large" Fermi
surface metal, leading to small Fermi pockets for x<x_m. A key effect is that
the onset of superconductivity at low temperatures disrupts the simplest
canonical quantum critical crossover phase diagram. In particular, the
competition between superconductivity and SDW order_shifts_ the actual QCP to a
lower doping x_s < x_m in the underdoped regime, so that SDW order is only
present for x<x_s. I review the phase transitions and crossovers associated
with the QCPs at x_m and x_s: the resulting phase diagram as a function of x,
temperature, and applied magnetic field consistently explains a number of
recent experiments.Comment: 6 pages, 5 figures, Talk at the Conference on Quantum Criticality and
Novel Phases, Dresden; (v2)+(v3) added clarifications and refs; (v4) added
discussion on electron-doped superconductors and three-dimensional figure;
(v5) Included addendum based on discussions at conference, and 2 new figures;
(v7) discussion of early experimental work of Panagopoulos et a
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