1,874 research outputs found
Self-consistent calculation of electric potentials in Hall devices
Using a first-principles classical many-body simulation of a Hall bar, we
study the necessary conditions for the formation of the Hall potential: (i)
Ohmic contacts with metallic reservoirs, (ii) electron-electron interactions,
and (iii) confinement to a finite system. By propagating thousands of
interacting electrons over million time-steps we capture the build-up of the
self-consistent potential, which resembles results obtained by
conformal-mapping methods. As shown by a microscopic model of the current
injection, the Hall effect is linked to specific boundary conditions at the
particle reservoirs.Comment: 6 pages, 7 figure
Dust and gas emission from cometary nuclei: the case of comet 67P/Churyumov-Gerasimenko
Comets display with decreasing solar distance an increased emission of gas
and dust particles, leading to the formation of the coma and tail. Spacecraft
missions provide insight in the temporal and spatial variations of the dust and
gas sources located on the cometary nucleus. For the case of comet
67P/Churyumov-Gerasimenko (67P/C-G), the long-term observations from the
Rosetta mission point to a homogeneous dust emission across the entire
illuminated surface. Despite the homogeneous initial distribution, a
collimation in jet-like structures becomes visible. We propose that this
observation is linked directly to the complex shape of the nucleus and projects
concave topographical features into the dust coma. To test this hypothesis, we
put forward a gas-dust description of 67P/C-G, where gravitational and gas
forces are accurately determined from the surface mesh and the rotation of the
nucleus is fully incorporated. The emerging jet-like structures persist for a
wide range of gas-dust interactions and show a dust velocity dependent bending.Comment: 17 pages, with 7 figures. To appear in Advances in Physics X (2018
A unified approach to inverse robust optimization problems
A variety of approaches has been developed to deal with uncertain
optimization problems. Often, they start with a given set of uncertainties and
then try to minimize the influence of these uncertainties. Depending on the
approach used, the corresponding price of robustness is different. The reverse
view is to first set a budget for the price one is willing to pay and then find
the most robust solution.
In this article, we aim to unify these inverse approaches to robustness. We
provide a general problem definition and a proof of the existence of its
solution. We study properties of this solution such as closedness, convexity,
and boundedness. We also provide a comparison with existing robustness concepts
such as the stability radius, the resilience radius, and the robust feasibility
radius. We show that the general definition unifies these approaches. We
conclude with examples that demonstrate the flexibility of the introduced
concept.Comment: 25 pages, 3 figure
Theory of the quantum Hall effect in graphene
We study the quantum Hall effect (QHE) in graphene based on the current
injection model. In our model, the presence of disorder, the edge-state
picture, extended states and localized states, which are believed to be
indispensable ingredients in describing the QHE, do not play an important role.
Instead the boundary conditions during the injection into the graphene sheet,
which are enforced by the presence of the Ohmic contacts, determine the
current-voltage characteristics.Comment: 4 pages, 3 figures, rewritten, role of contacts for boundary
conditions in small device
Outcomes of stable and unstable patterns of subjective cognitive decline: results from the Leipzig Longitudinal Study of the Aged (LEILA75+)
Background: Subjective cognitive decline (SCD), i.e., the self-perceived feeling of worsening cognitive function, may be the first notable syndrome of preclinical Alzheimer’s disease and other dementias. However, not all individuals with SCD progress. Stability of SCD, i.e., repeated reports of SCD, could contribute to identify individuals at risk, as stable SCD may more likely reflect the continuous neurodegenerative process of Alzheimer’s and other dementias. Methods: Cox regression analyses were used to assess the association between stability of SCD and progression to MCI and dementia in data derived from the population-based Leipzig Longitudinal Study of the Aged (LEILA75+). Results: Of 453 cognitively unimpaired individuals with a mean age of 80.5 years (SD = 4.2), 139 (30.7 %) reported SCD at baseline. Over the study period (M = 4.8 years, SD = 2.2), 84 (18.5 %) individuals had stable SCD, 195 (43.1 %) unstable SCD and 174 (38.4 %) never reported SCD. Stable SCD was associated with increased risk of progression to MCI and dementia (unadjusted HR = 1.8, 95 % CI = 1.2–2.6; p < .01), whereas unstable SCD yielded a decreased progression risk (unadjusted HR = 0.5, 95 % CI = 0.4–0.7; p < .001) compared to no SCD. When adjusted for baseline cognitive functioning, progression risk in individuals with stable SCD was significantly increased in comparison to individuals with unstable SCD, but not compared to individuals without SCD. Conclusions: Our results, though preliminary, suggest that stable SCD, i.e., repeated reports of SCD, may yield an increased risk of progression to MCI and dementia compared to unstable SCD. Baseline cognitive scores, though within a normal range, seem to be a driver of progression in stable SCD. Future research is warranted to investigate whether stability could hold as a SCD research feature
Revivals of quantum wave-packets in graphene
We investigate the propagation of wave-packets on graphene in a perpendicular
magnetic field and the appearance of collapses and revivals in the
time-evolution of an initially localised wave-packet. The wave-packet evolution
in graphene differs drastically from the one in an electron gas and shows a
rich revival structure similar to the dynamics of highly excited Rydberg
states.
We present a novel numerical wave-packet propagation scheme in order to solve
the effective single-particle Dirac-Hamiltonian of graphene and show how the
collapse and revival dynamics is affected by the presence of disorder. Our
effective numerical method is of general interest for the solution of the Dirac
equation in the presence of potentials and magnetic fields.Comment: 22 pages, 10 figures, 3 movies, to appear in New Journal of Physic
The Impact of the COVID-19 Pandemic on Avoidance of Health Care, Symptom Severity, and Mental Well-Being in Patients With Coronary Artery Disease
The COVID-19 pandemic affected regular health care for patients with chronic diseases.
However, the impact of the pandemic on primary care for patients with coronary artery
disease (CAD) who are enrolled in a structured disease management program (DMP) in
Germany is not clear. We investigated whether the pandemic affected primary care and
health outcomes of DMP-CAD patients (n = 750) by using a questionnaire assessing
patients’ utilization of medical care, CAD symptoms, as well as health behavior and
mental health since March 2020. We found that out of concern about getting infected
with COVID-19, 9.1% of the patients did not consult a medical practitioner despite
having CAD symptoms. Perceived own influence on infection risk was lower and anxiety
was higher in these patients compared to symptomatic CAD patients who consulted
a physician. Among the patients who reported chest pain lasting longer than 30 min,
one third did not consult a medical practitioner subsequently. These patients were
generally more worried about COVID-19. Patients with at least one worsening CAD
symptom (chest pain, dyspnea, perspiration, or nausea without apparent reason) since
the pandemic showed more depressive symptoms, higher anxiety scores, and were less
likely to consult a doctor despite having CAD symptoms out of fear of infection. Our
results provide evidence that the majority of patients received sufficient medical care
during the COVID-19 pandemic in Germany. However, one in ten patients could be
considered particularly at risk for medical undersupply and adverse health outcomes.
The perceived infection risk with COVID-19 might have facilitated the decision not to
consult a medical doctor
Pursuing More Aggressive Timelines in the Surgical Treatment of Traumatic Spinal Cord Injury (TSCI): A Retrospective Cohort Study with Subgroup Analysis
Background: The optimal timing of surgical therapy for traumatic spinal cord injury (TSCI) remains unclear. The purpose of this study is to evaluate the impact of “ultra-early” (<4 h) versus “early” (4–24 h) time from injury to surgery in terms of the likelihood of neurologic recovery. Methods: The effect of surgery on neurological recovery was investigated by comparing the assessed initial and final values of the American Spinal Injury Association (ASIA) Impairment Scale (AIS). A post hoc analysis was performed to gain insight into different subgroup regeneration behaviors concerning neurological injury levels. Results: Datasets from 69 cases with traumatic spinal cord injury were analyzed. Overall, 19/46 (41.3%) patients of the “ultra-early” cohort saw neurological recovery compared to 5/23 (21.7%) patients from the “early” cohort (p = 0.112). The subgroup analysis revealed differences based on the neurological level of injury (NLI) of a patient. An optimal cutpoint for patients with a cervical lesion was estimated at 234 min. Regarding the prediction of neurological improvement, sensitivity was 90.9% with a specificity of 68.4%, resulting in an AUC (area under the curve) of 84.2%. In thoracically and lumbar injured cases, the estimate was lower, ranging from 284 (thoracic) to 245 min (lumbar) with an AUC of 51.6% and 54.3%. Conclusions: Treatment within 24 h after TSCI is associated with neurological recovery. Our hypothesis that intervention within 4 h is related to an improvement in the neurological outcome was not confirmed in our collective. In a clinical context, this suggests that after TSCI there is a time frame to get the right patient to the right hospital according to advanced trauma life support (ATLS) guidelines
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