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Persistence of Tc across superconductor interfaces challenges leading theories
Brookhaven National Lab
Two non-conducting materials can be layered to produce high Tc superconductivity with tantalizing and mystifying ramifications. Scientists at BNL routinely grow layered thin film samples containing lanthanum, strontium, copper, and oxygen. The conductivity of the layers can be controlled based on doping level, transforming between an under-doped insulator, a well doped superconductor, or an over-doped and non-superconducting metal. Doping generally involves adding impurities or charge carriers — which can be electrons or "holes" — to inert materials. In this case it involves changing the local stoichiometry of the layers, which the BNL team can do with precise control.
As reported in Nature Materials researchers tested a sample set of unprecedented size — more than 800 distinct, custom-made materials. Theory predicted that Tc in these interface samples would depend strongly on the electron content. But the team saw no such dependence, finding instead that Tc in these systems remained constant across a wide range of atomic compositions.
Radio waves measure atmospheric temperature changes
Earth's atmosphere has 5 distinct layers. The upper atmosphere consists of the exosphere, the thermosphere and then the mesophere. The mesopause, which is the lower boundary of the upper atmosphere and separates the thermosphere from the mesophere, is marked by the temperature minimum of the atmosphere. (The rest of the lower atmosphere consist of the stratosphere and troposphere.)
In recent years the mesopause has been the focus for studies on global climate change associated with increases in CO2. Direct studies of temperatures in the mesopause are difficult and expensive to undertake — with the region being both too low for in situ measurement by orbital satellites, and yet too high for airplanes or weather balloons.
But new work reported in the Journal of Geophysical Research provides a cheap ground-based method to measure mesopause temperatures. When very low frequency radio waves are transmitted into space, by navigation beams for instance, the ionosphere bounces some of them back to Earth. This new method measures the amplitude of the reflected waves, which can be correlated to the temperature, hence CO2 density in the mesopause.
X-Ray pulsars boil 'nuclear pasta' to keep spinning
Most pulsars emit electromagnetic radiation in the microwave region, and they spin extremely fast, with periods ranging from a few seconds down to 100 ms. Some neutron stars, however, have magnetic fields that are an order of magnitude or two larger than ordinary. These pulsars emit x-rays and gamma rays. Astronomers have long expected to find that the spin rate of such stars to slow down as they age, with periods reaching 30 seconds. But no such spin rates have been found. Instead spin periods have been found to cluster in range from 2-12 seconds. A recent paper in Nature Physics offers an explanation for why long-period x-ray pulsars are missing. The nuclear pasta in the stellar interior changes the way energy is dissipated and transported within the star. The magnetic field generates currents in the pasta region, which provides an intermediate step in converting magnetic energy to rotational energy. Without the nuclear pasta transfering magnetic energy into rotational energy, the star's spin rate would decrease indefinitely.
Designed as a unique and much-needed resource for educators, managers and policymakers, the Journal of Women and Minorities in Science and Engineering publishes original, peer-reviewed papers that report innovative ideas and programs for classroom teachers, scientific studies and formulation of concepts related to the education, recruitment and retention of underrepresented groups in science and engineering.
Access now available to NSBP members at www.nsbp.org.
Hubble finds source of Magellanic Stream
Hubble European Space Agency Information Centre
Astronomers using the Cosmic Origins Spectrograph on-board the Hubble Space Telescope as well as the Very Large Telescope in Chile have solved the 40-year-old mystery of the origin of the Magellanic Stream, a long ribbon of gas stretching nearly halfway around the Milky Way. New Hubble observations reveal that most of this stream was stripped from the Small Magellanic Cloud some two billion years ago, with a smaller portion originating more recently from its larger neighbor, the Large Magellanic Cloud. These results are presented in a set of two papers, both published the Astrophysical Journal.
Theorists sharpen tests of fundamental theory in high energy experiments
Stanford Linear Accelerator
An important goal in high energy physics is to make perturbative QCD predictions as precise as possible, not only to test QCD itself, but also to expose new physics beyond the standard model. In a paper published in Physical Review Letters, three theorists have presented a method that will help physicists automatically eliminate an important theoretical ambiguity of QCD predictions. Virtual particles give rise to infinite terms in QCD calculations — a big problem for theorists, who must remove the uncertainty in their calculations caused by these infinities without introducing new ambiguities.
The new advance in this problem comes via a novel generalization of a technique that many theorists employ to remove infinities, called modified minimal subtraction. This, along with the Principle of Maximum Conformality, leads to predictions for physical processes that are independent of the choice of renormalization scheme and the initial choice of renormalization scale. The resulting scales also determine the effective number of flavors at each order of perturbation theory. The method can be applied to processes with multiple physical scales. The new method satisfies all of the principles of the renormalization group, eliminates an unnecessary source of systematic error, and makes the calculations easier to reduce to a form that can be used to make testable, ambiguity-free predictions.
Disorder in nanostructure is decisive in determining material's resistance to corrosion
Max-Planck-Institut für Eisenforschung GmbH
An international team of scientists has analyzed an amorphous steel comprising iron, chromium, molybdenum, boron and carbon. Using spatially resolved atomic probe tomography, they found that the more ordered a material's structure is, and the more uneven the distribution of its atoms, the more easily it is corroded by rust. If the elements of the alloy don't form a regular crystal lattice and are distributed completely uniformly across the material, then, under corrosive conditions, a passivation layer forms on its surface and protects it from rusting. If, in contrast, ordered nanocrystals form that sometimes contain more chromium and sometimes more molybdenum, the corrosion quickly eats away the material because no protective passivation layer forms. The results are presented in Science.
Journey through the center of the Earth
How long does it take to fall down a tunnel through the center of the Earth to the other side? A 1966 analysis published in the American Journal of Physics figured that it would take 42 minutes assuming a uniformly dense Earth. A paper recently posted on arXiv explores this question when the uniformly dense Earth condition is relaxed; instead the internal structure of the Earth, as ascertained by seismic data, is used. The numerically solved equation describing the dynamics indicates that time taken to fall along the diameter is 38 rather than 42 minutes. Furthermore, the time taken to fall along a straight line between any two points is no longer independent of distance, but interpolates between 42 minutes for short trips and 38 minutes for long trips.
A record pressure for solid iron
Lawrence Livermore National Laboratory
Using a series of compressive shocks rather than a single shock, a team of researchers compressed iron up to 5.6 million atmospheres (5.6 million times the pressure at the Earth's surface), a record pressure for solid iron. Using multiple series of shocks keeps the entropy low while compressing the material, which is the key to keeping the temperature lower than the melting point and allowing the iron to remain solid. The team then employed in situ extended x-ray absorption fine structure (EXAFS) to study the atomic structure of the compressed iron. The EXAFS data show that the close-packed structure of iron is stable in the regime explored, confirming simulation predictions and other experimental studies by x-ray diffraction up to 3 million atmospheres. They also found that with fast compression, the strength of iron is enhanced, leading to more plastic work and elevated sample temperatures; an unexpected result. The results are reported in Physical Review Letters.
Nanoscale agent optimizes mammography
Medical Physics Web
The x-ray contrast between a cancerous lesion and surrounding normal tissue can be enhanced by using an exogenous contrast agent. Existing small-molecule agents used for contrast-enhanced digital mammography (CEDM), however, have limited tumor specificity and a short effective working time. To address these limitations, researchers in Canada are developing slightly larger contrast agents that selectively extravasate from leaky vasculature. One prime candidate is perfluoro-octylbromide (PFOB), nanoscale droplets of which offers strong x-ray attenuation at CEDM energies. In experiments reported in Physics in Medicine and Biology, the team labeled the droplets with fluorescent quantum dots (QDs). They measured the fluorescence emission spectra and x-ray attenuation of the materials. The quantum dots do not effect x-ray attenuation, but do enhance fluorescent emission. The results indicate that fluorescence of QD-PFOB systems can be used to evaluate the biological targeting potential of PFOB droplet formulations in vitro, at a cellular level, without requiring CT imaging. The next step is to try the experiments in live animals, and the team's ultimate goal is to engineer droplets with appropriate x-ray attenuation characteristics and size for use in CEDM.
12th National Festival on Popular Astronomy
2-5 October 2013
Since 2001 the Sirius Astronomy Association in Algeria has hosted National Festivals in Popular Astronomy for the benefit of the public at large. The theme for the 12th edition of this festival is Mars… From Viking to Pathfinder to Opportunity to Curiosity.
The Sirius Astronomy Association especially invites participation by African astronomers and astronomy associations. The organization will provide for the full local accommodation and waive the participation fees for African associations and take full care of Africa participants when reaching Constantine till departing.
National Society of Black Physicists jobs board postings
Visiting Faculty Position - Amherst College
Director of Mount Laguna Observatory (San Diego State University)
Scientific Director -SESAME (Synchrotron-light for Experimental Science and Applications in the Middle East)
Staff Scientist -SLAC National Accelerator Laboratory
Department Chairperson - Howard University Physics & Astronomy
Scholar Scientist in Experimental Condensed Matter Physics and Materials Science
Director of DC Magnet Program Experimental Condensed Matter Physics and Materials Science
Stanford Synchrotron Radiation Lightsource Director
Director of SLAC Linac Coherent Light Source
BNL Postdoctoral Research Associate Positions
Physics Faculty, Tenure Track, Bard College
IAM Architect and Programmer (2 positions available at LIGO UW-Milwaukee)
7701 Las Colinas Ridge, Ste. 800, Irving, TX 75063