248 research outputs found
Low temperature transport and Coulomb drag studies of undoped electron-hole bilayers
No full textThis dissertation describes a series of three experiments focused on low electronic temperature transport and Coulomb drag in GaAs electron-hole bilayers. Electron-hole bilayers are of immense interest for exciton condensation studies since the exciton, predicted to form here, has a comparably light mass. This should lead to condensation at temperatures relatively easily obtained in a 3He-fridge, while the bilayer's device geometry allows for unambiguous detection of condensation effects via Coulomb drag measurements. General transport measurements of each layer are also of interest since an additional source of correlation, via the attractive Coulomb interaction from the nearby layer, is present. These interlayer effects are expected to become more visible as the layer separation is reduced and depend on the densities, relative and total, in each well, as well as the application of an external perpendicular or parallel magnetic field. Exploring these questions, measuring and discussing the results of experiments which probe them, was the main point of this dissertation.
The first experiment examined the layer interdependence of transport in an undoped electron-hole bilayer (uEHBL) device with a relatively large 30 nm AlGaAs barrier between the two quantum wells. The results here were consistent with mobility of each layer being only indirectly dependent on the adjacent layer density and dominated by background impurity scattering. A decreasing interlayer separation, estimated via Coulomb drag measurements, was also observed with increased interlayer electric-field.
The other two experiments were centered on the possibility of detecting electron-hole pairing and condensation in the bilayer using Coulomb drag measurements. Hints of condensation were observed in previous bilayer drag experiments and the follow-on experiments, described in this dissertation, sought to further elucidate the nature of these initial effects. The main result previously determined, which the experiments here were intended to elaborate upon, was an upturn in the Coulomb drag signal measured at low temperatures in the hole layer of bilayer devices with relatively small-barrier widths between the wells. The initial upturn results, which are fully reviewed and expanded upon here, ideally indicated a dramatic change in the coupling between the layers and, at least initially, appeared consistent with formation of a superfluid condensate, as predicted over a decade ago.\cite{Vignale1996, Hu2000} However, several issues with the upturn results have arisen since its initial discovery, the main one being the lack of drag symmetry under layer reversal. These issues are discussed in detail, along with some different mechanisms that might account for the phenomena.
The other two experiments, related to the upturn phenomena, were studies on the effects of a density imbalance and an external perpendicular or parallel magnetic field. The former showed the drag upturn to be inconsistent with the predictions of electron-hole pairing fluctuations, for which a set of numerical calculations were also done, and have a stronger density dependence than transport theory predicts. The perpendicular magnetic field study showed drag in the upturn regime was barely enhanced by a small perpendicular magnetic field up to the point where oscillations began occurring. A small phase offset between sets of oscillations in the layer resistivity and drag was also observed. Following this, a parallel magnetic field was found to diminish the upturn magnitude. At higher temperatures, above the upturn regime, the drag was enhanced, as expected, in concert with the rise in drive layer magnetoresistivity. All the magnetic field effects were weaker than expected.
Summarizing the conclusions from these experiments, the results indicated that in wide-barrier devices the transport was only indirectly affected by the appearance of a second layer, while in the narrow-barrier device there was enhanced drag and a strong departure from Fermi-liquid physics at low temperatures. Experiments on the latter here have shown that it may be due to a phase transition, but that it is not entirely consistent with formation of a superfluid condensate. In the future, newly designed dual-gate devices with separate gates for the central Hall bar and contact regions of the uEHBLs promise more robust fabrication and expanded experimental options, such as lower density and operation at lower temperatures.I acknowledge that this work has been supported by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.PhysicsDoctoralUniversity of New Mexico. Dept. of Physics & AstronomyDunlap, DavidDunlap, DavidLilly, MikeMalloy, KevinKrishna, SanjayCardimona, DavidHuang, Danhon
Effects of all-<i>trans</i>retinoic acid on renin-angiotensin system in rats with experimental nephritis
No full textWe previously demonstrated that all- trans retinoic acid (RA) preserves glomerular structure and function in anti-Thy1.1 nephritis (Wagner J, Dechow C, Morath C, Lehrke I, Amann K, Floege J, and Ritz E. J Am Soc Nephrol 11: 1479–1489, 2000). Because the renin-angiotensin system (RAS) contributes to renal damage, we 1) studied retinoid-specific effects on its components and 2) compared the effects of all- trans-RA with those of the AT1-receptor blocker candesartan. Rats were pretreated for 3 days before injection of the OX-7 antibody and continued with treatment with either vehicle or daily injections of 10 mg/kg all- trans-RA only ( study 1) or 10 mg/kg body wt all- trans-RA, 1 mg/kg candesartan, or both ( study 2) for an additional 7 days. The blood pressure increase observed in anti-Thy1.1 nephritic rats was equally normalized by all- trans-RA and candesartan ( P < 0.05). In nephritic rats, mRNAs of angiotensinogen and angiotensin-converting enzyme (ACE) in the kidney were unchanged, but renin mRNA was lower ( P < 0.01). Renal and glomerular AT1-receptor gene and protein expression levels were higher in anti-Thy1.1 nephritic rats ( P < 0.05). In the renal cortex of nephritic rats, pretreatment with all- trans-RA significantly reduced mRNAs of all the examined RAS components, but in the glomeruli it increased ACE gene and protein expression ( P < 0.01). In nephritic rats, candesartan reduced the number of glomerular cells and mitoses ( P < 0.05) less efficiently than all- trans-RA ( P < 0.01). Both substances reduced cellular proliferation (proliferating cell nuclear antigen) significantly ( P < 0.05). No additive effects were noted when both compounds were combined. In conclusion, all- trans-RA influences the renal RAS in anti-Thy1.1 nephritis by decreasing ANG II synthesis and receptor expression. The beneficial effect of retinoids may be explained, at least in part, by reduction of RAS activity.</jats:p
When should post-transplantation proteinuria be attributed to the renal allograft rather than to the native kidney?
No full textElectrical Characterization of Ion-Implanted 4H-Silicon Carbide
No full textElectrical characterization has been performed on ion-implanted p-type 4H-SiC to assess the activation efficiency and implantation-related damage recrystallization with the intention of developing an implantation/annealing scheme. Low doped (Na - Nd = 5x10(exp 15)/cu cm) epitaxial p-type layers grown by MOCVD were implanted with Al or B at doses ranging from 1x10(exp 13) to 1x10(exp 14)/sq cm at room temperature or 500 deg. C. The electrical technique of Temperature Dependent Hall Effect (TDHE) indicated that Al and B act as shallow acceptors 4H-SiC with ionization energies of ^252 and ^285 meV, respectively. The highest activation efficiency for Al and B implanted samples was found to occur at anneal temperatures of ^1650 deg C and ^1550 deg C, respectively. The implantation dose resulting in the highest concentration for Al and B implantation was found to be 3x10(exp 13)/sq cm. An average peak mobility of ^200 sq cm/ V s was found for an Al implanted sample; this is considerably higher than the average peak mobility for the B implanted samples, ^100 sq cm/ V s. No significant gains in activation efficiency or mobility were evident with high temperature implantation compared to the room temperature implantation. Overall, Al implantation of 4H-SiC appears superior with regard to these properties compared to B implantation
Transplant centers’ prophylaxis and monitoring strategies: a key determinant of current herpes and polyomavirus incidences – results from the DZIF kidney transplant cohort
No full textAbstract Background Herpes- and polyomaviruses are major opportunistic pathogens after renal transplantation. Despite established guidelines, there is limited data on transplant centers’ prophylaxis and monitoring strategies and centers’ adherence to these guidelines and their impact on infection rates and patient outcomes. Methods This multicenter cohort study, conducted by the German Center for Infection Research, included 1035 kidney transplant recipients from five centers (01/2014–02/2021), focusing on herpes- and polyomavirus viremia within the first year and adherence to prophylaxis strategies. Results Among 1035 recipients, 26.6% developed herpes- or polyomavirus viremia, predominantly Cytomegalovirus (CMV, 14.3%) and BK-virus (BKV, 13.2%). BKV monitoring frequency was below guideline recommendations. Deviations from guidelines were most common in CMV D-/R- (34.6% with prophylaxis) and D−/R + groups (37.3% without prophylaxis), doubling CMV-incidence in D−/R+ (28.9% vs. 12.5%, p < 0.01). In D+/R − group, six-month-prophylaxis reduced CMV-incidence compared to three months (22.5% vs. 38.4%, p < 0.01). Breakthrough-viremia was most commonly observed in D+/R − recipients who received a six-month-prophylaxis. Overall, viremia was associated with higher incidence of acute rejection (31.9% vs. 17.6%, p < 0.01), with most CMV-viremias occurring after rejection. CMV-viremia was associated with a higher risk of bacterial infection (HR = 1.77, [1.03;3.02]). Other herpesviruses were associated with a quadrupled risk for fungal infection (HR = 4.34, [1.03;18.30]) and the non-administration of CMV-prophylaxis (HR = 0.22, [0.11;0.47]). Graft survival and mortality were unaffected within the first year. Conclusion Clinical variability in guideline implementation drives high herpes- and polyomavirus infection rates with suboptimal outcomes. Future guidelines should focus on differentiated risk stratification to address breakthrough, post-prophylaxis, and post-rejection CMV, and include protocols for the early detection of secondary infections
Visualisierung sprachlicher Daten: Visual Linguistics – Praxis – Tools
No full textVisualisierungen spielen in den Wissenschaften eine wichtige Rolle im Forschungsprozess. Sie dienen der Illustration von gewonnener Erkenntnis, aber auch als eigenständiges Mittel der Erkenntnisgewinnung. Auch in der Linguistik sind solche Visualisierungen bedeutend. Beispielsweise in Form von Karten, Baumgraphen und Begriffsnetzen. Bei korpuslinguistischen Methoden sind explorative Visualisierungen oft ein wichtiges Mittel, um die Daten überblickbar und interpretierbar zu machen.Das Buch reflektiert die theoretischen Grundlagen wissenschaftlicher Visualisierungen in der Linguistik, zeigt Praxisbeispiele und stellt auch Visualisierungswerkzeuge vor.
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Die Forschungsdaten zum Beitrag von Wolfer/Hansen-Morath: "Visualisierung sprachlicher Daten mit R" sind abrufbar unter https://doi.org/10.11588/data/6SO6TG.
Visualizations play an important role in the scientific research process. They serve to illustrate the knowledge gained, but also as a tool to gain knowledge.Such visualizations are also important in linguistics. For example in the form of maps, tree graphs and word nets. In corpus linguistics, explorative visualizations are often a way to make the data accessible and interpretable.The book reflects on the theoretical basics of scientific visualizations in linguistics, shows practical examples and also introduces visualization tools.
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The research data to the contribution by Wolfer/Hansen-Morath: "Visualisierung sprachlicher Daten mit R" are available under https://doi.org/10.11588/data/6SO6TG
British journals and British-German relations, 1936-1939
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