April 18, 2012
Physics News Highlights of the American Institute of Physics (AIP) contains summaries of interesting research from the AIP journals, notices of upcoming meetings, and other information from the AIP Member Societies. Copies of papers are available to journalists upon request.
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TOPICS IN THIS ISSUE
1. Bioreactor Redesign Dramatically Improves Yield: Scientists explain why a microalgae bioreactor redesign provides an order-of-magnitude improvement over conventional cultivation methods.
The Ben-Gurion team created a physics model that explains some of the principal observations obtained in novel bioreactors that are being designed and built by a separate group at the university, led by Amos Richmond. These bioreactors are essentially flat containers with transparent walls that can be illuminated by sunshine or artificial light. Air bubbles fed from the bottom mix the water so the algae cells move back and forth between the thin illuminated regions near the walls and the dark interior of the reactor, which results in cells being exposed to short light flashes.
These bioreactors produce biomass yields an order-of-magnitude greater than conventional cultivation methods. In their research the scientists explain that it’s critical to account for the unique interplay between physics and biology: Intrinsic time scales characteristic of photosynthesis can be synchronized with the flow patterns and illumination of the bioreactors in which the algae are grown. The accompanying dramatic improvement in biomass yield may one day turn microalgae into an economically viable source of renewable energy.
Article: “Physics of Ultra-high Bioproductivity in Algal Photobioreactors” is published in Applied Physics Letters.
Authors: Efrat Greewald (1), Jeffrey M. Gordon (1), and Yair Zarmi (1).
2. Graphene Lenses: 2-D electron shepherds
Graphene, the one-atom-thick “wonder material” made of carbon, has another potential use in the world of high-speed electronics – as a tool that can focus a stream of electrons similar to the way an optical lens focuses light. A new prototype reveals that a layer of graphene, when strained through stretching, can act as a two-dimensional lens for electrons. The research, which is published in the American Institute of Physics’ (AIP) journal Applied Physics Letters, was produced by an international group of researchers from the Karlsruhe Institute of Technology in Germany and the French National Center for Scientific Research (CRNS).
Graphene is an excellent conductor: electrons flow freely across its surface in straight lines. According to a previously proposed theory, highly strained graphene impedes the flow of electrons, slowing them down and altering their trajectory. Scientists believed this effect could be used to focus electrons to a fine point – similar to the way an optical lens creates areas of refraction, or bending, to shepherd light to a point.
To create the prototype lens, the team of French and German researchers built a “deformed graphene carpet” that smoothly covers a series of hexagonal nano-holes in a silicon-carbide wafer. Areas of the graphene were strained as they adopted the shape of the holes in the wafer. The researchers found that they could control the focal length of a graphene lens by changing its geometry. Practical applications of this work include uses in high-speed electronics, where strained graphene could act as a transport medium for information exchange between different parts of a circuit. Unlike traditional information exchange, in which electrons flow through cables whose paths cannot cross without a short, the new method would allow electrons an unprecedented freedom of movement, similar to that of light in a vacuum.
Article: “A graphene electron lens” is published in Applied Physics Letters.
Authors: Lukas Gerhard (1), Eric Moyen (2), Timofey Balashov (1), Igor Ozerov (2), Marc Portail (3), Houda Sahaf (2), Laurence Masson (2), Wulf Wulfhekel (1), Margrit Hanbücken (2).
3. Raising the Prospects for Quantum Levitation
More than half-a-century ago, the Dutch theoretical physicist Hendrik Casimir calculated that two mirrors placed facing each other in a vacuum would attract. The mysterious force arises from the energy of virtual particles flitting into and out of existence, as described by quantum theory. Now Norio Inui, a scientist from the University of Hyogo in Japan, has predicted that in certain circumstances a reversal in the direction of the so-called Casimir force would be enough to levitate an extremely thin plate. His calculations are published in the American Institute of Physics’ (AIP) Journal of Applied Physics.
The Casimir force pushes identical plates together, but changes in the geometry and material properties of one of the plates can reverse the direction of the force. Inui calculated that a nanometer-thick plate made from a material called yttrium iron garnet (YIG) could hover half a micrometer above a gold plate. One key finding is that the repulsive force increases as the YIG plate gets thinner. This is convenient since the weight of the plate, and hence the magnitude of the force needed to levitate it, shrinks in tandem with the thickness. Right now the levitating plates exist solely in the theoretical realm. As a next step, many key assumptions in the calculations will need to be experimentally tested. If the models stand up to further scrutiny, possible applications could include levitating the gyroscopes in micro-electro-mechanical systems (MEMS) and keeping the various components of nanomachines from sticking together.
Article: “Quantum Levitation of a Thin Magnetodielectric Plate on a Metallic Plate Using the Repulsive Casimir Force” is published in Journal of Applied Physics.
Authors: Norio Inui (1).
4. Nanodot-based Memory Sets New World Speed Record
A team of researchers from Taiwan and the University of California, Berkeley, has harnessed nanodots to create a new electronic memory technology that can write and erase data 10-100 times faster than today’s mainstream charge-storage memory products. The new system uses a layer of non-conducting material embedded with discrete (non-overlapping) silicon nanodots, each approximately 3 nanometers across. Each nanodot functions as a single memory bit. To control the memory operation, this layer is then covered with a thin metallic layer, which functions as a “metal gate.” The metal gate controls the “on” and “off” states of the transistor. The results are published in the American Institute of Physics’ (AIP) journal Applied Physics Letters.
The researchers were able to achieve this new milestone in speed by using ultra-short bursts of green laser light to selectively anneal (activate) specific regions around the metal layer of the metal gate of the memory. Since the sub-millisecond bursts of laser light are so brief and so precise, they are able to accurately create gates over each of the nanodots. This method of memory storage is particularly robust, the researchers explain, because if an individual charge in one of the nano-sites failed, it would barely influence the others. This enables a stable and long-lived data storage platform.
“The materials and the processes used for the devices are also compatible with current main-stream integrated circuit technologies,” explains Shieh. “This technology not only meets the current CMOS process line, but can also be applied to other advanced-structure devices.”
Article: “Fast Programming Metal-Gate Si Quantum Dot Nonvolatile Memory Using Green Nanosecond Laser Spike Annealing” is published in Applied Physics Letters.
Authors: Yu-Chung Lien (1), Jia-Min Shieh (1,2), Wen-Hsien Huang (1), Cheng-Hui Tu (2), Chieh Wang (2), Chang-Hong Shen (1), Bau-Tong Dai (1), Ci-Ling Pan (3), Chenming Hu (4), and Fu-Liang Yang (1).
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