494 research outputs found
Accurate near-field calculation in the rigorous coupled-wave analysis method
The rigorous coupled-wave analysis (RCWA) is one of the most successful and
widely used methods for modeling periodic optical structures. It yields fast
convergence of the electromagnetic far-field and has been adapted to model
various optical devices and wave configurations. In this article, we
investigate the accuracy with which the electromagnetic near-field can be
calculated by using RCWA and explain the observed slow convergence and
numerical artifacts from which it suffers, namely unphysical oscillations at
material boundaries due to the Gibb's phenomenon. In order to alleviate these
shortcomings, we also introduce a mathematical formulation for accurate
near-field calculation in RCWA, for one- and two-dimensional straight and
slanted diffraction gratings. This accurate near-field computational approach
is tested and evaluated for several representative test-structures and
configurations in order to illustrate the advantages provided by the proposed
modified formulation of the RCWA.Comment: 13 pages, 12 figure
Mapping Itinerant Electrons around Kondo Impurities
We investigate single Fe and Co atoms buried below a Cu(100) surface using
low temperature scanning tunneling spectroscopy. By mapping the local density
of states of the itinerant electrons at the surface, the Kondo resonance near
the Fermi energy is analyzed. Probing bulk impurities in this well-defined
scattering geometry allows separating the physics of the Kondo system and the
measuring process. The line shape of the Kondo signature shows an oscillatory
behavior as a function of depth of the impurity as well as a function of
lateral distance. The oscillation period along the different directions reveals
that the spectral function of the itinerant electrons is anisotropic.Comment: 5 pages, 4 figures, accepted by Physical Review Letter
Theory of real space imaging of Fermi surfaces
A scanning tunneling microscope can be used to visualize in real space Fermi
surfaces with buried impurities far below substrates acting as local probes. A
theory describing this feature is developed based on the stationary phase
approximation. It is demonstrated how a Fermi surface of a material acts as a
mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
Best Practices for Retaining Public Speaking Students
This article draws on existing communication research and praxes to share the best practices for retaining students enrolled in the introductory public speaking course. Among the many important pedagogical practices that communication scholars have documented, this article highlights the value of 10 best practices: instructor use of immediacy and confirmation; instructor inclusion of written prescriptive feedback, peer feedback workshops, low-stakes assignments, applied assignments, and individual speech preparation tools; and instructor participation in out-of-class communication, online office hours, and classroom-connectedness
Long-range Kondo signature of a single magnetic impurity
The Kondo effect, one of the oldest correlation phenomena known in condensed
matter physics, has regained attention due to scanning tunneling spectroscopy
(STS) experiments performed on single magnetic impurities. Despite the
sub-nanometer resolution capability of local probe techniques one of the
fundamental aspects of Kondo physics, its spatial extension, is still subject
to discussion. Up to now all STS studies on single adsorbed atoms have shown
that observable Kondo features rapidly vanish with increasing distance from the
impurity. Here we report on a hitherto unobserved long range Kondo signature
for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We
present a theoretical interpretation of the measured signatures using a
combined approach of band structure and many-body numerical renormalization
group (NRG) calculations. These are in excellent agreement with the rich
spatially and spectroscopically resolved experimental data.Comment: 7 pages, 3 figures + 8 pages supplementary material; Nature Physics
(Jan 2011 - advanced online publication
Fetal cardiac magnetic resonance imaging of the descending aorta in suspected left-sided cardiac obstructions
BackgroundSevere left-sided cardiac obstructions are associated with high morbidity and mortality if not detected in time. The correct prenatal diagnosis of coarctation of the aorta (CoA) is difficult. Fetal cardiac magnetic resonance imaging (CMR) may improve the prenatal diagnosis of complex congenital heart defects. Flow measurements in the ascending aorta could aid in predicting postnatal CoA, but its accurate visualization is challenging.ObjectivesTo compare the flow in the descending aorta (DAo) and umbilical vein (UV) in fetuses with suspected left-sided cardiac obstructions with and without the need for postnatal intervention and healthy controls by fetal phase-contrast CMR flow. A second objective was to determine if adding fetal CMR to echocardiography (echo) improves the fetal CoA diagnosis.MethodsProspective fetal CMR phase-contrast flow in the DAo and UV and echo studies were conducted between 2017 and 2022.ResultsA total of 46 fetuses with suspected left-sided cardiac obstructions [11 hypoplastic left heart syndrome (HLHS), five critical aortic stenosis (cAS), and 30 CoA] and five controls were included. Neonatal interventions for left-sided cardiac obstructions (n = 23) or comfort care (n = 1 with HLHS) were pursued in all 16 fetuses with suspected HLHS or cAS and in eight (27%) fetuses with true CoA. DAo or UV flow was not different in fetuses with and without need of intervention. However, DAo and UV flows were lower in fetuses with either retrograde isthmic systolic flow [DAo flow 253 (72) vs. 261 (97) ml/kg/min, p = 0.035; UV flow 113 (75) vs. 161 (81) ml/kg/min, p = 0.04] or with suspected CoA and restrictive atrial septum [DAo flow 200 (71) vs. 268 (94) ml/kg/min, p = 0.04; UV flow 89 vs. 159 (76) ml/kg/min, p = 0.04] as well as in those without these changes. Adding fetal CMR to fetal echo predictors for postnatal CoA did not improve the diagnosis of CoA.ConclusionFetal CMR-derived DAo and UV flow measurements do not improve the prenatal diagnosis of left-sided cardiac obstructions, but they could be important in identifying fetuses with a more severe decrease in blood flow across the left side of the heart. The physiological explanation may be a markedly decreased left ventricular cardiac output with subsequent retrograde systolic isthmic flow and decreased total DAo flow
Giant Nonlinear Optical Activity from Planar Metasurfaces
Second harmonic generation circular dichroism (CD) is more sensitive to the handedness of
chiral materials than its linear optical counterpart. In this work, we show that 3D chiral structures are not
necessary for introducing strong CD for harmonic generations. Specifically, we demonstrate giant CD for
both second harmonic generation and third harmonic generation on suitably designed ultrathin plasmonic
metasurfaces. It is experimentally and theoretically verified that the overwhelming contribution to this
nonlinear CD is of achiral origin. The results shed new light on the origin of the nonlinear CD effect in
achiral planar surfaces
Ceramides : a potential cardiovascular biomarker in young adult childhood cancer survivors?
Aims The aim of this study was to investigate circulating ceramides involved in cardiovascular disease (CVD) in young adult childhood cancer survivors (CCS) and their correlations to previously reported adverse cardiovascular changes in this cohort. Methods Fifty-seven CCS and 53 healthy controls (age 20-30 years) were studied. Plasma long-chain ceramides, known to be cardi- and results otoxic (C16:0, C18:0, C24:0, and C24:1), were analysed by mass spectrometry. The coronary event risk test 2 (CERT2) score was calculated from the ceramide data. Cardiac and carotid artery ultrasound data and lipid data available from previous studies of this cohort were used to study partial correlations with ceramide and CERT2 score data. All four analysed ceramides were elevated in CCS compared with controls (P ≤ 0.012). The greatest difference was noted for C18:0, which was 33% higher in CCS compared with controls adjusted for sex, age, and body mass index (BMI) (P < 0.001). The CERT2 score was higher in CCS compared with controls (P < 0.001). In the CCS group, 35% had a high to very high CERT2 score (7-12) when compared with 9% in the control group (P < 0.001). The CCS subgroup with a CERT2 score ≥ 7 had higher heart rate, systolic blood pressure, and higher levels of apolipoprotein B compared with CCS with a CERT2 score < 6 (P ≤ 0.011). When adjusted for age, sex, and BMI, CERT2 score was significantly correlated with arterial stiffness, growth hormone, and cranial radiotherapy (P < 0.044). Conclusion Ceramides could be important biomarkers in understanding the pathophysiology of CVD and in predicting CVD disease risk in young adult CCS.Peer reviewe
Learning From Early Attempts to Generalize Darwinian Principles to Social Evolution
Copyright University of Hertfordshire & author.Evolutionary psychology places the human psyche in the context of evolution, and addresses the Darwinian processes involved, particularly at the level of genetic evolution. A logically separate and potentially complementary argument is to consider the application of Darwinian principles not only to genes but also to social entities and processes. This idea of extending Darwinian principles was suggested by Darwin himself. Attempts to do this appeared as early as the 1870s and proliferated until the early twentieth century. But such ideas remained dormant in the social sciences from the 1920s until after the Second World War. Some lessons can be learned from this earlier period, particularly concerning the problem of specifying the social units of selection or replication
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