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Vacuum as a hyperbolic metamaterial

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As demonstrated by Chernodub, vacuum in a strong magnetic field behaves as a periodic Abrikosov vortex lattice in a type-II superconductor. We investigate electromagnetic behavior of vacuum in this state. Since superconductivity is realized along the axis of magnetic field only, strong anisotropy of the vacuum dielectric tensor is observed. The diagonal components of the tensor are positive in the x and y directions perpendicular to the magnetic field, and negative in the z direction along the field. As a result, vacuum behaves as a hyperbolic metamaterial medium. If the magnetic field is constant, low frequency extraordinary photons experience this medium as a (3+1) Minkowski spacetime in which the role of time is played by the spatial z coordinate. Spatial variations of the magnetic field curve this effective spacetime, and may lead to formation of “event horizons”, which are analogous to electromagnetic black holes in hyperbolic metamaterials. We also note that hyperbolic metamaterials behave as diffractionless “perfect lenses”. Since large enough magnetic fields probably had arisen in the course of evolution of early Universe, the demonstrated hyperbolic behavior of early vacuum may have imprints in the large scale structure of the present-day Universe…..
Read more: http://arxiv.org/ftp/arxiv/papers/1108/1108.2203.pdf

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August 16, 2011 at 8:23 am

Posted in Materials Science

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Physicists take inspiration from spilled milk

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Two Lehigh physicists have developed an imaging technique that makes it possible to directly observe light-emitting excitons as they diffuse in a new material that is being explored for its extraordinary electronic properties. Called rubrene, it is one of a new generation of single-crystal organic semiconductors.

Ivan Biaggio, professor of physics, and Ph.D. candidate Pavel Irkhin have developed an imaging technique that could help overcome a bottleneck impeding the efficient conversion of solar energy.


Excitons, which are created by light, play a central role in the harvesting of solar energy using plastic . The achievement by Ivan Biaggio, professor of physics, and Pavel Irkhin, a Ph.D. candidate, represents the first time that an technique has been used to witness the long-range  of energy-carrying excitons in an organic crystal.
One way to understand the mechanics of excitons, says Biaggio, is to pour a cup of milk on the floor. The milk spreads out in all directions from the point of impact. How far it goes depends on the type of surface on which it lands. Now imagine that the milk has been replaced with particle-like bundles of energy and the floor with an ordered arrangement of organic molecules.
Biaggio’s group used a focused laser beam to create the  – the excitons – in a crystal made of . They tracked the movements of the excitons over distances smaller than the size of a human hair by directly taking pictures of the light that they emit. Unlike the spilled milk, the excitons spread only in a direction corresponding to a particular arrangement of molecules.

Hope for overcoming a solar bottleneck

An understanding of exciton diffusion is critical for plastic solar cell technology, in which the absorption of light creates excitons instead of directly inducing a current, as it does in the most commonly used silicon systems.
After they are created in plastic solar cells, excitons diffuse toward specially designed interfaces where they drive electrons into an external circuit, creating the flow of electrons we know as electric current. This diffusion process is one of the technical challenges limiting the efficiency of plastic solar cells.
“This is the first time that excitons have been directly viewed in a molecular material at room temperature,” said Biaggio. “We believe the technique we have demonstrated will be exploited by other researchers to develop a better understanding of exciton diffusion and the bottleneck it forms in plastic solar cells.”

Top: Crystal facets and locations with exciton diffusion experimentation. A: Micrometer thin crystal on bc facet, with different orientation. PL pattern shows exciton diffusion effect in the thin crystal and below. B: Clean bc facet. C: Crystal facet where b axis is not parallel to the surface, producing asymmetric PL pattern. Credit: Ivan Biaggio, Lehigh University


When will we have cheap and efficient plastic solar cells? It is the goal of researchers around the world to improve exciton diffusion lengths until they become as large as the light absorption—that’s the point when sunlight is most efficiently collected and converted into energy.
An article by Irkhin and Biaggio, titled “Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion Length in Rubrene Single Crystals,” was published July 1 by the journal Physical Review Letters.
The work was supported by a Faculty Innovation Grant from Lehigh, which provides resources to develop novel ideas and demonstrate new approaches to important research questions.
Thanks to the direct imaging of the diffusing excitons, Irkhin and Biaggio were able to obtain precise measurement of their diffusion length. This length was found to be very large in a particular direction, reaching a value several hundreds of times larger than in the plastic solar cells that are presently used. This is the first time that  have been directly viewed in a molecular material at room temperature, and it is believed that the widespread adoption of the technique developed by Irkhin and Biaggio will lead to significant progress in the field.
“It is important that physicists explore the most fundamental phenomena underlying the mechanisms that enable  harvesting with cheap organic materials,” said Biaggio. “Organics have lots of unexplored potential and the very efficient exciton diffusion that we have observed in rubrene may build the basis for new ideas and technologies towards the development of ever more efficient and .”
http://www.physorg.com

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August 13, 2011 at 4:56 pm

Researchers Develop Technique for Dynamically Controlling Plasmonic Airy Beams

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The movie shows the computer-based dynamical control of the trajectory and peak intensity position of plasmonic Airy beams achieved by Berkeley Lab’s Xiang Zhang.

One of the earliest lessons in science that students learn is that a ray or beam of light travels in a straight line. Students also learn that light rays fan out or diffract as they travel. Recently it was discovered that light rays can travel without diffraction in a curved arc in free space. These rays of light were dubbed “Airy beams,” after the English astronomer Sir George Biddell Airy, who studied what appears to be the parabolic trajectory of light in a rainbow.
Now, scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated the first technique that provides dynamic control in real-time of the curved trajectories of Airy beams over metallic surfaces. This development paves the way for fast-as-light, ultra-compact communication systems and optoelectronic devices,and could also stimulate revolutions in chemistry, biology and medicine.

The key to the success of this work was their ability to directly couple free-space Airy beams – using a standard tool of optics called a “grating coupler” – to quasi-particles called surface plasmon polaritons (SPPs). Directing a laser beam of light across the surface of a metal nanostructure generates electronic surface waves – called plasmons – that roll through the metal’s conduction electrons (those loosely attached to molecules and atoms). The resulting interaction between plasmons and photons creates SPPs. By directly coupling Airy beams to SPPs, the researchers are able to manipulate light at an extremely small scale beyond the diffraction limit…… Read the rest of this entry »

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August 12, 2011 at 7:42 am

A Single Molecule of Water Encapsulated in Fullerene C-60

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H2O@C60: A Single Molecule of Water in Fullerene C-60

Kei Kurotobi, Yasujiro Murata
Water normally exists in hydrogen-bonded environments, but a single molecule of H2O without any hydrogen bonds can be completely isolated within the confined subnano space inside fullerene C60. We isolated bulk quantities of such a molecule by first synthesizing an open-cage C60 derivative whose opening can be enlarged in situ at 120°C that quantitatively encapsulated one water molecule under the high-pressure conditions. The relatively simple method was developed to close the cage and encapsulate water. The structure of H2O@C60 was determined by single-crystal x-ray analysis, along with its physical and spectroscopic properties….
Read more: http://www.sciencemag.org/content/333/6042/613

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August 7, 2011 at 1:19 pm

Spinons take the heat

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A sketch of the original apparatus used by Wiedemann and Franz to measure the thermal conductivity of elemental metals, as reported in their seminal paper of 1853.

An international group of researchers has measured, for the first time, the phenomenon of spin–charge separation in bulk in a solid. They also found that the material violates the empirical Wiedemann–Franz law that has held true for more than 150 years.

The Wiedemann–Franz law says that the ratio of thermal to electrical conductivity for a metal is approximately the same for different metals at the same temperature. It has been known indirectly, for some time, that 1D metals – chains just one atom thick – are very different from metals in 2D or 3D.The researchers, led by Nigel Hussey at the University of Bristol, UK, were looking to test a prediction made by physicists Charles Kane and Matthew Fisher in 1996, which suggested a violation of the Wiedemann–Franz law if electrons are confined to individual atomic chains…. Read the rest of this entry »

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July 28, 2011 at 3:15 pm

Posted in Materials Science

Space stuff of the future put to the test

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This may look like a NASA version of the game Connect Four, but in fact it’s a scientific experiment.

Photographed during a spacewalk as part of the current Atlantis shuttle mission, the Materials on International Space Station Experiment 8 (MISSE-8) consists of a series of circular test beds containing solar cells, computing devices and new and experimental materials, some of which, NASA hopes, may have applications on future spacecraft. The panel is attached to the outside of the space station, exposing the objects to the rigours of space, which include radiation, micrometeorites and extreme temperatures.

MISSE-8 is expected to return to Earth in early 2013.

http://www.newscientist.com/blogs/shortsharpscience/2011/07/space-stuff-of-the-future-put.html

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July 18, 2011 at 3:37 pm

Light propagates as if ‘space is missing’

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Schematics of the zero-index metamaterial contained within a Mach-Zehnder interferometer.

Researchers in the UK and the US have crafted an optical nanostructure that allows light to pass through without accumulating a phase change – as if the medium were completely missing in space. The device could find applications in optoelectronics, they say, for instance as a way of transporting signals without allowing information to become distorted.

Whenever light travels through a medium it experiences a phase-shift, as individual oscillations become out of phase with each other. In certain optics applications, including interferometers, these phase variations can introduce an unwanted dispersion of frequencies. This effect can lead to phase distortions, which ultimately reduce the quality of signals….. Read the rest of this entry »

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July 18, 2011 at 2:05 pm

Graphene gives up more of its secrets

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Undoped graphene isn't a metal, semiconductor, or insulator but a semimetal, whose unusual properties include electron-electron interactions between particles widely separated on graphene's honeycomb lattice -- here suggested by an artist's impression of the Feynman diagrams often used to keep track of such interactions. Interactions occur over only very short distances in ordinary metals. Long-range interaction alter the fundamental character of charge carriers in graphene

Graphene, a sheet of carbon only a single atom thick, was an object of theoretical speculation long before it was actually made. Theory predicts extraordinary properties for graphene, but testing the predictions against experimental results is often challenging.

Now researchers using the Advanced  (ALS) at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have taken an important step toward confirming that graphene is every bit as unusual as expected – perhaps even more so.

“Graphene is not a semiconductor, not an insulator, and not a metal,” says David Siegel, the lead author of a paper in the Proceedings of the National Academy of Sciences (PNAS) reporting the research team’s results. “It’s a special kind of semimetal, with electronic properties that are even more interesting than one might suspect at first glance.”…. Read the rest of this entry »

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July 16, 2011 at 9:42 am

Posted in Materials Science, TECHNOLOGY

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