Waves & Packets
May. 5, 2012

Acoustic analogue to graphene announced
Physics World
Two physicists in Spain have constructed a honeycomb-patterned plastic sheet that yields an acoustic analogue to a Dirac cone — a key feature in the electronic structure of graphene. Their results, reported in Physical Review Letters, start from calculations of surface acoustic waves on a sheet of Plexiglass with holes drilled in it in honeycombed pattern. Those calculations revealed the existence of an acoustic wave that would propagate through the material without scattering. Experiments then confirmed that such a wave did exist at 22 kHz as predicted by the calculations. Furthermore they found a linear relationship between momentum and energy near 22 kHz (the Dirac point), which is a hallmark of a Dirac cone. It may not be long before we see such systems as acoustic lenses, or as a simpler test bench for studying physics near Dirac cones.More

Ultrafast pulses, gold nanotips renew classical view of the photoelectric effect
Laser Focus World
Scientists at the University of Göttingen have demonstrated that in the strong-field regime classical dynamics may indeed prevail in photoemission from metal nanostructures. In experiments reported in Nature, the team found that when IR light was focused on a gold nanotip some electrons oscillated classically in the field to escape confinement. They also found that classical motion dominates at high light intensities, while the quantum picture rules in the low-light regime.More

Lightning signature could help reveal the solar system's origins
NASA
Every second, lightning flashes some 50 times on Earth. Together these discharges coalesce and get stronger, creating electromagnetic waves circling around Earth, to create a beating pulse between the ground and the lower ionosphere, about 60 miles up in the atmosphere. This electromagnetic signature, known as Schumann Resonance, had only been observed from Earth's surface until, in 2011, scientists discovered they could also detect it using NASA's Vector Electric Field Instrument aboard the U.S. Air Force's Communications/Navigation Outage Forecast System satellite. In a paper published in the Astrophysical Journal, researchers describe how this new technique could be used to study other planets in the solar system as well, and even shed light on how the solar system formed. The frequency of Schumann Resonance depends not only on the size of the planet, but also on chemical composition of the atmosphere. The chemical composition of the atmosphere offers clues as to nature of the planetary formation process and of original nebula from which it evolved. More

Searching for seismo-ionospheric earthquakes precursors
Murmansk State Technical University
During earthquakes preparation periods significant disturbances in the ionospheric plasma density are often observed. The problem of anomalous electric field penetration from the near-ground surface into the ionosphere has no generally accepted solution. One of the main hypotheses is ionization of the ground atmospheric gases by radon injected from the cracks of the Earth's crust. Another hypothesis is related with defect electrons also known as "positive holes" activated when rocks are subjected to ever-increasing stress, then positive holes charge clouds arrive at the Earth's surface and generate a positive ground potential. Reporting in the Proceedings of the Murmansk State Technical University, researchers there have analyzed Global Position System distribution global ionospheric Total Electron Content maps for earthquakes of years 2005-2006 and found that 65 percent of the cases studied showed TEC disturbances several hours to days before the earthquake near the epicenter. More

Pioneer anomaly explained with no new physics
Space Ref
About two decades ago scientists noticed that two spacecraft, Pioneer 10 and 11, possess an anomalous acceleration of unknown origin, which is 8.74 x 10-10 m/s2 directed toward the sun. This slowing down of both spacecraft is known as the Pioneer anomaly and it could not be explained by conventional gravity (general relativity). In a paper to be published in Physical Review Letters, it has now been shown that the recoil force associated with an anisotropic emission of thermal radiation explains 80 percent of the anomalous acceleration. The authors were also able to show that the remaining 20 percent does not represent a statistically significant acceleration anomaly. Thus, the observed anomalous acceleration of both spacecraft is consistent with known physics. More

Giant black hole is seen gobbling up a star
Los Angeles Times
Astronomers at Johns Hopkins University reported in Nature that they have watched a black hole devour a star over 15 months starting in 2010. This is the first time such an event had been witnessed in great detail from start to finish. The observations allowed the team to reconstruct properties of both the black hole and the star. Using a wide selection of ground- and space-based telescopes, including NASA's Galaxy Evolution Explorer (GALEX) — a space-based UV telescope, and the Pan-STARRS 1 optical telescope at University of Hawaii's Institute for Astronomy, the team was able identify the star as having been a red giant. Both telescopes spotted a telltale flare in June 2010. Continuous monitoring allowed them to rule out that the flare was from a supernova. Observations at the Multiple Meter Telescope Observatory in Arizona revealed that lots of helium was streaming into the black hole indicating that the star had to have been the helium-rich core of hydrogen-depleted star. And subsequent observations by NASA's Chandra X-ray Observatory ruled out the prospect that the object was an active galactic nucleus.More

Physics of how thin strips curl revealed
American Physics Society
If you twist a rod or pluck on a string, the restoring force is like that of a spring — it is proportional to the displacement. But the curling of a plant tendril or the unfurling of a whip is much harder to analyze because the forces are no longer directly proportional to a spatial coordinate. Only a few of these nonlinear dynamics problems have been solved, and understanding curling would be useful for several fields. A study in Physical Review Letters looks at the simple case of a curved metal strip that is straightened and then released. Relying on the conservation of energy, and using a combination of experiments, numerical simulations, and mathematical analysis, the research team has performed a complete study on the shape and speed of the strip as it curls. The work provides a basic framework for explaining curling in future micromachines or in the splitting open of a red blood cell.More

Researchers develop ultrasensitive chemical sensor using carbon nanotubes
Azom.com
Physicists at that University of Pennsylvania have developed all-electronic DNA-carbon nanotube vapor sensors to discriminate between very similar classes of molecules. As reported in AIP Advances, the team fabricated carbon nanotubes coated with a uniform, nanoscale layer of DNA of a desired base sequence. The DNA decoration is capable of highly specific binding and can discriminate between molecules differing in only one methyl group, and even between molecules that are even enantiomers, i.e., molecules that are nonsuperimposable mirror images of each other, and thus rotate plane-polarized light in opposite directions. When a target chemical interacts with these DNA strands, it generates a sizeable electrical signal across the nanotube even at ultralow concentrations. The team plans to construct DNA nanotube sensors that will "smell" a key volatile compound related to skin cancer, but one can imagine a raft of olfactory receptors for explosive detection, forensic and product quality assurance applications.More

Citizen scientists find new purpose in pulsar search
International Science Grid This Week
A project that lets citizen volunteers contribute to scientists' search for gravitational waves, theoretical ripples in the fabric of space-time, has expanded its efforts — with impressive results. The Einstein@Home project began in 2005 as a way for people to donate idle computing time to the Laser Interferometer Gravitational Wave Observatory. No one has yet found a gravitational wave, but in less than two years Einstein@Home users have found 27 new pulsars, including more than a dozen in the last six months. Pulsar timing is a fascinating area of science in its own right. Pulsars have secrets to reveal about magnetism, nuclear physics, as well as gravitation.More

New finding of magnetic pairing could lead to new improved superconductors
Brookhaven National Laboratory
By measuring how strongly electrons are bound together to form Cooper pairs in an iron-based superconductor, scientists at Brookhaven National Laboratory, Cornell University, St. Andrews University and their collaborators have shown direct evidence supporting theories in which magnetism holds the key to this material's superconductivity. Physicists have long hypothesized that if these electrons have their magnetic moments pointing in opposite directions, they could overcome their mutual repulsion to form Cooper pairs. In the current work on lithium iron arsenide, reported in Science, the team used a new technique, coined as multiband Bogoliubov quasiparticle scattering interference, to measure the anisotropic energy gap, i.e., how strongly bound together the electrons are in a pair as a function of the orientation of the electron's trajectory with respect to its band. The results are the first experimental evidence direct from the electronic structure in support of the theories that the mechanism for superconductivity in iron-based superconductors is due primarily to magnetic interactions.More

NIST researchers develop new way to generate superluminal pulses
National Institute of Standards and Technology
Researchers at the National Institute of Standards and Technology have developed a novel way of producing light pulses that are "superluminal" — in some sense they travel faster than the speed of light. That is, when a pulse is sent forward, the leading edge still does not go faster than the speed of light. But the pulse peak is bent forward such that it arrives at a point faster than it otherwise normally would. The technique, called four-wave mixing, is reported in Physical Review Letters. It reshapes parts of light pulses and advances them ahead of where they would have been had they been left to travel unaltered through a vacuum. The new method could be used to improve the timing of communications signals and to investigate the propagation of quantum correlations. More

Rogue stars ejected from the galaxy found in intergalactic space
Vanderbilt University
Astronomers at Vanderbilt University report in the Astronomical Journal that they have used the Sloan Digital Sky Survey to identify a group of more than 675 stars on the outskirts of the Milky Way that they argue are hypervelocity stars that have been ejected from the galactic core. The stars are red giants with high metallicity, i.e., contain high amounts of elements other than hydrogen and helium. High metallicity in stars is a signature that indicates an inner galactic origin. The researchers' next step is to determine if any of their candidates are unusually red brown dwarfs instead of red giants. Because brown dwarfs produce a lot less light than red giants, they would have to be much closer to appear equally bright. More

National Society of Black Physicists jobs board postings
NSBP
Kenyon College One-year Visiting Assistant Professor of Physics
Faculty Positions in Science, Technology, and Innovation
NASA Postdoctoral Fellowships
Entrepreneurship for Scientists and Engineers in East Africa
National Astrophysics and Space Science Program
Postdoctoral Research Associate PositionsMore

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IOP Journal
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SPIE Reviews
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