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New observations of gamma-ray flashes associated with lightening
Physicists have known for some time that gamma rays are sometimes produced when lightning strikes. Now a team in Japan has made the best study yet of the gamma rays that are produced in the minutes leading up to a lightning flash. In addition, the team also observed for the first time emissions that ended abruptly less than a second before the exact moment the flash occurs. In these measurements, which are reported in Physical Review Letters, the gamma ray emissions apparently started and steadily increased some 3 minutes prior lightning strikes. The gamma detections then abruptly stopped over an interval of 800 ms before optical detections of the lightning. The results indicate that the sources of the gamma emissions are spatially separated from the sources of optical emissions. Earlier this year researchers at Florida Institute of Technology suggested a relativistic feedback mechanism to explain the mechanism of terrestrial gamma-ray flashes associated with lightning. But the mechanism connecting the gamma and optical emissions still has not been definitely identified.
Improving measurements by reducing quantum noise
Vienna University of Technology
Physicists have built a new interferometer for trapped, ultra-cold atomic gases that should yield more precise measurements. Their Mach-Zehnder interferometer contains a Bose-Einstein condensate of about a thousand atoms trapped by an atom chip. The team was able to curb the effect of atomic interactions and increase the interrogation time of their interferometer by strongly suppressing the quantum noise, which ultimately limits the performance of interferometers. In usual atom interferometers, the atoms move freely, and the measurement time is limited by the time of flight. In this new interferometer, described in detail in Nature Communications, the atoms lose their individual behavior in favor of the collective behavior of a Bose-Einstein condensate. The condensate can be optically controlled in a trap during the whole sequence, which in principle would set no limit to the interrogation time.
Laser-cooled Bose-Einstein condensate is a first
The first Bose–Einstein condensate (BEC) to be cooled using just lasers has been made by a team in Austria and reported in Physical Review Letters. Making a BEC traditionally involves the two-step cooling (laser and evaporative) of a cloud of atoms contained in a magnetic trap. This new all-laser cooling process is much simpler, faster and more efficient. To achieve this feat the researchers combined three techniques that had been previously investigated. First, in a sample of strontium atoms they introduced a central region of the sample, in which atoms are rendered transparent to laser cooling photons. The atoms naturally have a higher density around this central region, and subsequently will be at a higher temperature. A second laser focused on the dimple region makes that region transparent to the cooling laser. The atoms in the lower density region away from the central region are cooled, and that leads to heat flowing from high density area to the low density area. Ultimately, a BEC forms in the central region. Variations of this result may lead to a continuous atomic laser if the reservoir of cooled atoms can be replenished, and if the BEC atoms can be removed from the trap without interaction with the cooling laser.
Optical transistor switches states by trapping a single photon
Researchers have reported, via a paper in Science, an all-optical transistor that can be switched between on and off states using a single photon. The transistor consists of a cold gas of cesium atoms in an optical cavity. The cesium cloud has a ground state (G1), a higher but nearby state (G2), and an even higher excited state (E). A single photon can excite the cesium cloud form G1 to E. A control laser manipulates the sample from E to G2. During the G1 to E transitions, the sample is transparent to photons corresponding to the G2 to E transitions. But when G2 is occupied, incoming photons that are at the G2 to E wavelength are reflected by the sample. So in the former case the photon gate is open, in the latter it is closed. The gate position is controlled by the E to G2 laser. Of course, clouds of super-cooled atoms are not practical for most, if any, transistor applications. But this proof-of-principle demonstration could be used as a photon detector and has implications in quantum computing.
Radio bursts discovered from beyond our galaxy
Max Planck Institute of Radio Astronomy
A team of astronomers has detected the first population of radio bursts known to originate from galaxies beyond our own Milky Way. The radio luminosity profiles of these bursts, described in Science, are not unlike those of individual pulses seen from pulsars. But these bursts do not appear to be periodic. Also they must be much more energetic than pulsars because they have been detected at cosmological distances. The radio bursts last for just a few milliseconds and the farthest one that has been detected is 11 billion light-years away. They also have optical bursts associated with them. The progenitor of these bursts are unknown. Gamma ray bursts are known to be associated with stellar collapses into black holes. Perhaps these radio bursts are also caused by cataclysmic events, such as colliding neutron stars or black holes, evaporating black holes, or stellar explosions. However, the data do not fit nicely with any of these scenarios. Follow-up studies are ongoing to localize the bursts using a network of radio telescopes, including Parks Observatory and NRAO's VLBA.
Current distribution in 2-D layered material measure for 1st time
Although scientists continue to discover the remarkable electronic properties of nanomaterials such as graphene and transition metal dichalcogenides, the way that electric current is distributed at this scale is not well understood. In a new study, scientists for the first time have investigated exactly how current moves through multilayer 2-D materials, and found that current in these materials is distributed very differently than in 3-D materials. The observations, reported in Nano Letters, cannot be explained with conventional models. In their study, the scientists experimentally evaluated the current flow and distribution in a transistor made of 2-D MoS2, which was about 8 nm thick and consisted of approximately 13 layers. They were able to identify high-current spots that changed location as a function of applied gate voltage. At high gate voltages the high current spot was at the top-layer. At low voltages the spot migrated down to the bottom layer. Presumably this exemplifies some sort of "interlayer resistance," which is not found in 3-D materials. The researchers also preformed similar experiments using graphene layers and found the opposite behavior from that in MoS2. The charge screening length in these two materials are distinctly different, (λMoS2 = 7 nm, λgraphene = 0.6 nm).
New type of light wave extends possibilities in photonic control in biology, physics and nanotechnology
Max Planck Institute for the Science of Light
Researchers are now able to use a laser to cause tiny particles to rotate around an axis perpendicular to the light beam – a particle thus rotates like the wheel of a bicycle in its direction of motion. As described in the Journal of the European Optical Society, the researchers achieved this by creating a photonic wheel, i.e., light with purely transverse angular momentum.
The possibility that light can have purely transverse angular momentum when averaged over the complete cross-section of the beam had not been realized before. The researchers want to use the rotating light field to cause a nanoparticle to rotate about itself. The new way of controlling light waves makes optical tweezers, which can be used to grip and maneuver cells and other micro-objects and nano-objects, more versatile. For instance, this result can provide biologists with new experimental possibilities for rotating cells under the microscope in three spatial directions, and material scientists with nanomixers or other nanomachines. The researchers are now developing this idea using circularly polarized light.
Astronomers identify link between stars' ages and their orbits
Astronomers using NASA's Hubble Space Telescope’s Advanced Camera for Surveys have for the first time linked two distinct populations of stars in an ancient globular star cluster to their unique orbital dynamics, offering proof that the stars do not share the same birth date. Recent observations and archived data of the positions and luminosities of more than 30,000 stars in cluster 47 Tucanae revealed two populations of stars. The first consists of redder stars, which are older, less chemically enriched, and in random, circularized orbits. The second population comprises bluer stars, which are younger, more chemically enhanced, and in more elliptical orbits. The redder generation reflects the initial motion of the gas that formed the cluster and they have retained a memory of their original motion. The younger generation was formed by gas expelled from the older stars combined with other gases in the cluster. Conservation of momentum meant that their orbits became elliptical. This research is reported in the Astrophysical Journal Letters.
Imperfect graphene renders 'electrical highways'
Graphene has well-known electrical and mechanical properties, but to be useful, it needs to be a semiconductor. Researchers at Cornell University have made a step in this direction by showing that graphene can be a semiconductor simply by mechanical ripples in layered structures. When grown in layers, graphene ripples like wall-to-wall carpet exceeding room dimensions. Previously scientists had thought that layered graphene would remain flat but staggered. It turns out that the ripples carry electrons in conductivity solitons, while the flat areas are semiconducting. This result is reported in the Proceedings of the National Academy of Sciences of the United States of America. In a related paper in Physical Review X, a separate group described the theory behind the electrical properties of the solitons and how they fit into the bilayer graphene picture.
National Society of Black Physicists jobs board postings
Scholar Scientist in Experimental Condensed Matter Physics and Materials Science
Director of DC Magnet Program Experimental Condensed Matter Physics and Materials Science
Visiting Assistant Professor of Physics & Astronomy, non-tenure track
Accelerator Research Division Director
Accelerator Research Division Director
Stanford Synchrotron Radiation Lightsource Director
Linac Coherent Light Source
Postdoctoral position in Astrophysics/Astroparticle Physics
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