Direct Visualization of Laser-Driven Focusing Shock Waves

Cylindrically or spherically focusing shock waves have been of keen interest for the past several decades. In addition to fundamental study of materials under extreme conditions, cavitation, and sonoluminescence, focusing shock waves enable myriad applications including hypervelocity launchers, synthesis of new materials, production of high-temperature and high-density plasma fields, and a variety of medical therapies. Applications in controlled thermonuclear fusion and in the study of the conditions reached in laser fusion are also of current interest. Here we report on a method for direct real-time visualization and measurement of laser-driven shock generation, propagation, and 2D focusing in a sample. The 2D focusing of the shock front is the consequence of spatial shaping of the laser shock generation pulse into a ring pattern. A substantial increase of the pressure at the convergence of the acoustic shock front is observed experimentally and simulated numerically. Single-shot acquisitions using a streak camera reveal that at the convergence of the shock wave in liquid water the supersonic speed reaches Mach 6, corresponding to the multiple gigapascal pressure range 30 GPa

Vortex pinning by natural defects in thin films of YBa2Cu3O7−δ

Although vortex pinning in laser-ablated YBa2Cu3O7−δ films on (100) SrTiO3 is dominated by threading dislocations, many other natural pinning sites are present. To identify the contribution from twin planes, surface corrugations and point defects, we manipulate the relative densities of all defects by post-annealing films with various as-grown dislocation densities, ndisl. While a universal magnetic field B dependence of the transport current density js(B, T) is observed (independently of ndisl, temperature T and the annealing treatment), the defect structure changes considerably. Correlating the microstructure to js(B, T), it becomes clear that surface roughness, twins and point defects are not important at low magnetic fields compared to linear defect pinning. Transmission electron microscopy indicates that threading dislocations are not part of grain boundaries nor are they related to the twin domain structure. We conclude that js(B, T) is essentially determined by pinning along threading dislocations, naturally induced during the growth process. Even in high magnetic fields, where the vortex density outnumbers ndisl, it appears that linear defects stabilize the vortex lattice by means of the vortex–vortex interaction.

Field-effect transistors assembled from functionalized carbon nanotubes

We have fabricated field effect transistors from carbon nanotubes using a novel selective placement scheme. We use carbon nanotubes that are covalently bound to molecules containing hydroxamic acid functionality. The functionalized nanotubes bind strongly to basic metal oxide surfaces, but not to silicon dioxide. Upon annealing, the functionalization is removed, restoring the electronic properties of the nanotubes. The devices we have fabricated show excellent electrical characteristics.Comment: 5 pages, 6 figure

Global Production Increased by Spatial Heterogeneity in a Population Dynamics Model

Spatial and temporal heterogeneity are often described as important factors having a strong impact on biodiversity. The effect of heterogeneity is in most cases analyzed by the response of biotic interactions such as competition of predation. It may also modify intrinsic population properties such as growth rate. Most of the studies are theoretic since it is often difficult to manipulate spatial heterogeneity in practice. Despite the large number of studies dealing with this topics, it is still difficult to understand how the heterogeneity affects populations dynamics. On the basis of a very simple model, this paper aims to explicitly provide a simple mechanism which can explain why spatial heterogeneity may be a favorable factor for production.We consider a two patch model and a logistic growth is assumed on each patch. A general condition on the migration rates and the local subpopulation growth rates is provided under which the total carrying capacity is higher than the sum of the local carrying capacities, which is not intuitive. As we illustrate, this result is robust under stochastic perturbations

Maximal Spontaneous Photon Emission and Energy Loss from Free Electrons

Free electron radiation such as Cerenkov, Smith--Purcell, and transition radiation can be greatly affected by structured optical environments, as has been demonstrated in a variety of polaritonic, photonic-crystal, and metamaterial systems. However, the amount of radiation that can ultimately be extracted from free electrons near an arbitrary material structure has remained elusive. Here we derive a fundamental upper limit to the spontaneous photon emission and energy loss of free electrons, regardless of geometry, which illuminates the effects of material properties and electron velocities. We obtain experimental evidence for our theory with quantitative measurements of Smith--Purcell radiation. Our framework allows us to make two predictions. One is a new regime of radiation operation---at subwavelength separations, slower (nonrelativistic) electrons can achieve stronger radiation than fast (relativistic) electrons. The second is a divergence of the emission probability in the limit of lossless materials. We further reveal that such divergences can be approached by coupling free electrons to photonic bound states in the continuum (BICs). Our findings suggest that compact and efficient free-electron radiation sources from microwaves to the soft X-ray regime may be achievable without requiring ultrahigh accelerating voltages.Comment: 7 pages, 4 figure

Herschel observations of extra-ordinary sources: Detection of Hydrogen Fluoride in absorption towards Orion~KL

We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules ("weeds"), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 x 10^13 cm^-2 for the HF column density responsible for the broad absorption component. Using our best estimate of the H2 column density within the low-velocity molecular outflow, we obtain a lower limit of ~1.6 x 10^-10 for the HF abundance relative to hydrogen nuclei, corresponding to 0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J=2--1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 x 10^-9 derived by Neufeld et al. (2010) for cold, foreground clouds on the line of sight towards G10.6-0.4.Comment: 5 pages, 3 figures, paper to be published in the Herschel special issue of A&A letter

Herschel observations of EXtra-Ordinary Sources: The Terahertz spectrum of Orion KL seen at high spectral resolution

We present the first high spectral resolution observations of Orion KL in the frequency ranges 1573.4 - 1702.8 GHz (band 6b) and 1788.4 - 1906.8 GHz (band 7b) obtained using the HIFI instrument on board the Herschel Space Observatory. We characterize the main emission lines found in the spectrum, which primarily arise from a range of components associated with Orion KL including the hot core, but also see widespread emission from components associated with molecular outflows traced by H2O, SO2, and OH. We find that the density of observed emission lines is significantly diminished in these bands compared to lower frequency Herschel/HIFI bands.Comment: Accepted for publication in the Herschel HIFI special issue of Astronomy and Astrophysics Letters, 5 pages, 3 figure

The distribution of water in the high-mass star-forming region NGC 6334I

We present observations of twelve rotational transitions of H2O-16, H2O-18, and H2O-17 toward the massive star-forming region NGC 6334 I, carried out with Herschel/HIFI as part of the guaranteed time key program Chemical HErschel Surveys of Star forming regions (CHESS). We analyze these observations to obtain insights into physical processes in this region. We identify three main gas components (hot core, cold foreground, and outflow) in NGC 6334 I and derive the physical conditions in these components. The hot core, identified by the emission in highly excited lines, shows a high excitation temperature of 200 K, whereas water in the foreground component is predominantly in the ortho- and para- ground states. The abundance of water varies between 4 10^-5 (outflow) and 10^-8 (cold foreground gas). This variation is most likely due to the freeze-out of water molecules onto dust grains. The H2O-18/H2O-17 abundance ratio is 3.2, which is consistent with the O-18/O-17 ratio determined from CO isotopologues. The ortho/para ratio in water appears to be relatively low 1.6(1) in the cold, quiescent gas, but close to the equilibrium value of three in the warmer outflow material (2.5(0.8)).Comment: 7 pages, 3 figures, accepted by A&

Roadmap consensus on carotid artery plaque imaging and impact on therapy strategies and guidelines: An international, multispecialty, expert review and position statement

Current guidelines for primary and secondary prevention of stroke in patients with carotid atherosclerosis are based on the quantification of the degree of stenosis and symptom status. Recent publications have demonstrated that plaque morphology and composition, independent of the degree of stenosis, are important in the risk stratification of carotid atherosclerotic disease. This finding raises the question as to whether current guidelines are adequate or if they should be updated with new evidence, including imaging for plaque phenotyping, risk stratification, and clinical decision-making in addition to the degree of stenosis. To further this discussion, this roadmap consensus article defines the limits of luminal imaging and highlights the current evidence supporting the role of plaque imaging. Furthermore, we identify gaps in current knowledge and suggest steps to generate high-quality evidence, to add relevant information to guidelines currently based on the quantification of stenosis

Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines

We have observed CH absorption lines ($J=3/2, N=1 \leftarrow J=1/2, N=1$) against the continuum source Sgr~B2(M) using the \textit{Herschel}/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show `spiral arm' cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr~B2 envelope. The observations show that each `spiral arm' harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H$_2$ column densities found at lower $A_V$ by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds.Comment: Accepted for publication in A&A, HIFI Special Issu