Interesting news on the web

Researchers Build Artificial Immune System to Solve Computational Problems

2009-12-09T17:10:00.000+05:30

By mimicking the way that a living body acquires immunity to disease through vaccination, researchers have designed an artificial immune system to solve optimization problems more effectively than before. The results show that the biologically motivated approach is better at exploring a greater amount of space and quickly locating the desired local and global optima than previous methods.

The researchers, Kumlachew M. Woldemariam and Gary G. Yen, both from Oklahoma State University, have published their study in a recent issue of IEEE Transactions on Systems, Man, and Cybernetics - Part B: Cybernetics.
As the researchers explain, the field of artificial immune systems (AIS) is an emerging branch of evolutionary computation in which computational systems are based on the structure and behavior of the immune system. By providing methods in pattern recognition, data analysis, and machine learning, AIS has applications in fields including computer science, robotics, and information technology.
Unlike previous artificial immune systems, the system in the new study specifically takes advantage of the way that vaccines can improve the performance of the immune system. The antibody cells in the immune system are constantly trying to recognize foreign or malfunctioning cells (antigens such as bacteria, viruses, and tumors) in the midst of the body’s own healthy cells (mainly other antibodies). The way that the immune system distinguishes between different kinds of cells can be formulated as an optimization problem, in which the antibodies are the points in the decision space and the antigens are the solutions that the immune system looks for.
Although the immune system usually works very well, sometimes it doesn’t identify an antigen quickly enough to prohibit it from causing illness. To accelerate the immune system’s functioning, vaccines can be administered to enable the immune system to detect a new (weakened) antigen and develop an immunological memory so that it can quickly recognize the same antigen in the future.
Drawing inspiration from how such vaccines work, the researchers’ artificial immune system can be enhanced to quickly identify optimum solutions by being injected with certain points in decision space that act as weak antigens, or vaccines. In their study, Woldemariam and Yen explain how to determine which points should be used as vaccines to be “injected” into the algorithm. Once in the algorithm, the vaccines activate the antibody population to incorporate more diverse antibodies and explore new landscapes in decision space, so that the antibodies continually come closer to locating a desired local optimum point.

“The main idea of introducing the notion of vaccines in the evolutionary algorithm is to help enhance the diversity of antibodies,” Yen told.

“The way the vaccines are extracted from the decision space is in such a way that the decision space is explored widely. The implementation of the algorithm in multimodal optimization problem shows how fast and efficiently the antibodies, triggered by the vaccines, learn and locate the local and global optimal solutions of the problem. Therefore the vaccines help antibodies to achieve a speedy learning and diversified exploration.”
When comparing the new algorithm, called Vaccine-AIS, to other artificial immune systems, the researchers found that Vaccine-AIS outperformed the others by locating the global and local optima in a plot in fewer evaluations. The key to the improved performance is that the algorithm can cover a larger portion of the search space, due to help from the vaccines. As Yen added, AIS has the potential for improving computational abilities in many other areas.
“AIS was originally designed for data mining, anomaly detection and the like,” Yen said. “Its use as an optimization tool is a very young research area but its performance is drawing interest from researchers. Recently there has been research on using artificial immune systems in fault estimation and to design a power system stabilizer. The application of AIS in medical image processing for MRI (magnetic resonance imaging) is being studied as well.”

More information: Kumlachew M. Woldemariam and Gary G. Yen. “Vaccine-Enhanced Artificial Immune System for Multimodal Function Optimization.” IEEE Transactions on Systems, Man, and Cybernetics - Part B: Cybernetics, Vol. 40, No. 1, February 2010.

Original Article at http://www.physorg.com/news179060729.html

Scientists make bendable, transparent LEDs—without organics

2009-12-03T21:37:00.000+05:30

Organic LEDs, or OLEDs, promise to bring flexible, transparent displays to the market, but some researchers have found a way to get the same effect by printing microscopic inorganic LEDs onto plastic and glass.

Organic light emitting diodes, or OLEDs, promise to bring a great deal of flexibility to where we can put a display—literally. Because of their organic components, it should be possible to create flexible and transparent displays, opening up a large number of potential uses. But now, just as OLEDs may finally be ready for the consumer market, some engineers have figured out a way to get many of the same properties using inorganic LEDs (ILEDs), using a method that's so simple, even a biologist could understand it. It's a few years away—at least—from commercialization, but it's a significant advance.
The paper that describes the process will be published today in Science. The basic idea is that, since LEDs are so efficient at converting electrical charges to light, the human eye can detect the light of very small LEDs. As a result, it's possible to make a display out of a surface where only a small fraction is occupied by the actual LEDs, which can be small enough to be invisible to the naked eye. Under these conditions, the display will take on the properties of whatever material the LEDs are embedded in: bendable, transparent, etc.
Unfortunately, although we've gotten rather good at depositing the layered structure needed for making a normal ILED, the manufacturing processes we use don't scale down to the size of individual pixels in a typical display, which need to be on the order of 100µm or less. So the researchers came up with a simple solution: make a big one, and then chop it into little pieces.
The researchers started with a permanent substrate topped with aluminum arsenide, and layered on all the typical materials (gallium, indium, phosphorous, etc.) needed to create an LED that would glow red. When that was completed, they used a technique called plasma ion etching to cut a rectangular grid into the slab, leaving behind small squares, approximately 50µm across, held in place by the AlAs substrate. The squares were then anchored in place by a small bit of material in two corners, and the AlAs substrate was etched away with hydrofluoric acid. What was left was a grid of small LEDs held in place by two small posts that could be broken away easily.
This array, however, is packed so tightly that it would completely obscure any surface it was transferred to. So the authors crafted printing devices from a flexible material that only contains slots for a subset of the total LED square (say, every third one). The elastic material can pick up the LEDs, "print" them onto a separate surface, and then return to the original source and pick up the next set over. By adjusting how far apart the LED slots are—every second LED in the grid, or every fifth—it's possible to print out devices with different spacing.
The authors prepared a flexible plastic surface by laying down a grid of wiring, printed the LEDs on it, and then locked them in place with epoxy; a second mesh of wiring completed the circuit. Given an adhesive, the plastic could be applied to just about anything. For demonstration purposes, the authors stuck it to a glass cylinder. They also created a wiring grid that acted as a passive matrix, allowing them to activate individual LEDs in the grid. For these applications, the LEDs only took up about one percent of the total surface, enough to leave it transparent (provided they weren't lit, obviously).
The authors also demonstrated how to create a bendable display. By pre-stretching the flexible plastic substrate before laying down the wiring, the wiring would buckle within the material when tension was relaxed. This provided enough slack to accommodate a fair degree of flexibility in the final material.
If, at this point, you think you're missing something, you're not—it really is that simple.
That said, it's still a long way from being ready for the market. The authors say that none of the LEDs failed during testing, but some of the wiring leading into the device wasn't up to the strains of their test procedures. As a result, most of the individual devices they made had rows of dead pixels. All the devices only worked in red, as well. Still, the process uses well-understood materials and techniques, so there's no reason it can't be rapidly improved on, and the transition to a production environment doesn't seem to face any major stumbling blocks.

Original post is here.

Flexible, self-healing antennae made from liquid metal

2009-12-03T12:39:00.001+05:30

Researchers have embedded a self-healing liquid metal in a flexible substrate to create antennae that are flexible, self-healing, and can actually measure the stresses they're subjected to.

As engineers attempt to integrate electronics into things like clothing and medical devices, they're increasingly running up against the material properties of the substances we use to make the hardware. A lot of the materials that go into a typical electronic device are brittle, inflexible, and prone to damage, and materials scientists are looking at a variety of options for replacing them. A recent paper in Advanced Functional Materials describes a technique for forming an antenna from liquid metal. The resulting (not-so-) hardware is flexible, self-healing, and can change the frequency that it's sensitive to based on the stress it's subjected to.

The idea behind the new work is pretty simple, as liquid metal is obviously going to be pretty flexible. It's just as obvious, however, that containing it is also going to be challenging. There are also a limited number of choices when it comes to metals that are liquid anywhere near room temperatures, and not all of those are viable options—nobody's going to be enthused about bringing anything containing substantial amounts of liquid mercury to the market these days, for example.

The authors focused on an alloy of gallium (which is a liquid at room temperature) and indium, mixed in proportions that minimize the melting temperature. The alloy has a very useful property that came into play during some of the tests performed by the authors: when exposed to the air, it forms a thin oxidized coat that helps keep it from flowing freely.

That oxidized coat isn't very mechanically robust, so the research team started with a common, flexible polymer called polydimethylsiloxane (PDMS), which is used in a lot of materials science work. For most experiments, the PDMS was molded with an internal channel that was then filled with the gallium-indium mixture and then sealed.

The resulting device was pretty much as flexible as the starting material. It could be flexed, twisted, folded in half, and stretched an additional 40 percent beyond its normal length. When the stress was released, the PDMS snapped back into place; the antenna formed by the liquid metal within remained functional throughout.

The authors could even cut through the antenna with a razor blade and, in many cases, the two ends would spontaneously re-form a single, conducting wire once the blade was removed. In the other cases, the authors simply had to press the severed ends together to get it to re-establish a connection. That's where the thin oxide layer was critical, as it helped contain the liquid metal within the PDMS when it was cut, but wasn't robust enough to keep the ends from rejoining.

One of the interesting side-effects of this flexibility is that the physical configuration of the antenna can be changed simply by stretching it, and this will alter the frequencies that it's sensitive to. Simply by stretching the device 8mm, the peak response could be shifted by over 200MHz.

Obviously, a gallium-indium alloy is going to be a bit more expensive than many of the metals that are currently used in antennae. But the fact that the device is resistant to strain and can self-heal to a degree may make it substantially more durable, which can pay off in the long run. It can also eliminate the need for soldering, as simply jabbing a wire into the liquid should be enough to establish an electrical contact. The authors also suggest that the stress-induced changes in its properties could be put to good use for things like enabling remote monitoring of the integrity of machinery and structures like bridges.

Refer to the original post here.
Also see :
http://www.popsci.com/technology/article/2009-12/bendable-stretchable-shape-shifting-antenna?page=
and http://www.physorg.com/news178897908.html

International Workshop on Electronic Structure Calculaitons at my place

2009-12-01T17:38:00.003+05:30

International workshop on Frontiers in Electronic structure calculations : Techniques and Applications

Guide To LED Technology

2009-11-07T13:51:00.001+05:30

LEDs have risen from their original occupation as humble indicator lamps to serving as the light source for some of today’s most advanced TVs. Electronics engineers prize the LED for its brightness and cool-running efficiency. Environmentalists and utility companies tout its low power consumption. Videophiles are warming to it for the performance enhancements it facilitates. And average consumers love the way it has slimmed their new TVs. In this article, we’ll explain how the LED works; how it’s used in current-model TVs and in the latest video projectors; and how it’s likely to be used in future displays. Of course, we can’t say exactly what the LED’s future in video will be, but we can say with a great deal of confidence that within the next 10 years, you’ll own at least one LEDbased TV — if you don’t have one already.
LED BASICS
LED is the acronym for light-emitting diode. A diode is the simplest type of semiconductor. Rather than control the flow of electrons, as a transistor does, a diode just conducts electricity in one direction and blocks it in the other. It’s made from a semiconducting material such as gallium arsenide or indium gallium nitride, combined with another substance that changes its electrical properties to suit the task at hand.
About a century ago, scientists discovered that diodes emit infrared light as an electrical current passes through them. In the 1960s, several companies developed diodes that produced visible light, and the LED was born. Early LEDs were dim and mostly limited in color to red, green, and amber. Despite these restraints, LEDs quickly replaced incandescent light bulbs for use as indicator lights, primarily because they last so much longer. Almost all LEDs have lifetimes specified in tens of thousands of hours, and some are even rated to last 100,000 hours or longer. In the 1980s and 1990s, brighter LEDs emerged, along with LEDs in white, blue, and other colors. These breakthroughs caught the attention of video engineers. As display technologies evolved away from light-emitting cathode-ray tubes toward “light valve” technologies such as LCD and DLP, engineers needed a cool-running, efficient, reliable light source. LED delivers on all three.
Interestingly, most white LEDs are actually blue LEDs coated with a yellow phosphor. Some of the photons emerging from the blue LED excite the yellow phosphor, thus producing yellow photons, which combine with the blue photons to produce white light. By fine-tuning the underlying blue LEDs’ color and the phosphor formulation, LED makers are able to deliver white light pure enough to drive high-quality video displays. In fact, white LED light is even broader in spectrum than the light from the coldcathode fluorescent lamps (CCFLs) used as backlights in most LCD TVs. LEDs can therefore produce a wider range of colors.
LED MEETS TV
In video displays, LEDs are now used in several different ways. For the most part, they serve merely as a light source rather than as a way to reproduce individual pixels of video. But they can also produce images directly, rather than just working in tandem with other display technologies such as LCD.
For now, the video industry uses LEDs primarily as a CCFL backlight alternative for LCD panels. LED-driven LCD sets first appeared about 2 years ago, and they have since taken over much of the high-end LCD TV market. However, the cost is still high and overall market penetration is low; they currently account for only about 3 percent of total LCD TV sales.
LEDs have three general advantages over CCFLs: They’re more energy-efficient, they allow for a slimmer chassis, and they deliver a wider color gamut (or range of available colors). They can have other advantages, too, depending on how the TV is designed.
Some confusion has occurred in the labeling of these LED-driven TVs, which could fairly be called LED/LCD TVs. Samsung has heightened the confusion by labeling these displays LED TV, which most video experts consider a misleading moniker. Generally, a true LED TV is defined as one in which the pixels are formed from individual LEDs. Each pixel is self-illuminating and requires no backlight.
True LED TVs do exist — most of the large-format displays you see in ballparks and used as digital signage are made from arrays of thousands of LEDs, which are often similar to the 5-mm LEDs your local RadioShack stocks in the “dork drawers” at the back of the store. The new Organic Light-Emitting Diode (OLED) TVs are also true LED TVs.
FULL-ARRAY LED: THE STATE OF THE ART?
LED/LCD TVs come in two basic varieties: fullarray (which some manufacturers simply refer to as backlit) and edge-lit.
In a full-array TV, LEDs are positioned directly behind the LCD panel in rows, with the LEDs typically spaced 1 to 3 inches apart. A diffuser panel between the LED array and the LCD panel spreads out the light so that the screen gets a smooth, consistent field of illumination.
This arrangement offers the potential for gigantic performance advantages. The LEDs can be dimmed individually or in small groups. This process, called “local dimming,” allows for LEDs behind the dark parts of a picture to be run at a lower intensity, so the blacks and dark grays look darker while the brighter parts of the picture stay the same. The effect is a huge increase in contrast, which has historically been a weak point for LCD TVs. (Local dimming is impossible with CCFLs because they run the entire length of the screen.) Consequently, the newest full-array LED/ LCD models match or even surpass the contrast of plasma TVs.
However, local dimming has some limitations. A large LED/LCD TV might have an array of roughly 1,000 LEDs. That means each LED backlights about 2,000 pixels in the LCD display panel on a 1080p-rez TV. Furthermore, many sets control the LEDs not individually but in blocks of perhaps five or 10 LEDs. Obviously, with so many pixels being illuminated by so few LEDs, it’s impossible to achieve precise transitions between high-brightness and low-brightness areas. This imprecision can result in an artifact called “blooming”: white halos that appear around the edges of bright onscreen objects silhouetted against a dark background — a white rocket floating through black space, for example. Manufacturers can combat blooming by increasing the number of LEDs in their sets’ backlight array, decreasing the number of LEDs in each control block, refining the drive electronics for the LEDs, and increasing the native contrast of their LCD panels. Newer full-array TVs show less of this artifact, but it still exists.
Another advantage of full-array LED/LCD TVs is improved picture uniformity. Because the screen is lit by hundreds or thousands of LEDs instead of 20 or so CCFLs, you don’t see the gaps that are sometimes visible between CCFLs. (This artifact, which is often referred to as screen “clouding,” crops up regularly in our reviews of standard, non- LED-based LCD TVs.)
Most manufacturers use white LEDs in their arrays. However, for some of its LCD TVs Sony instead uses groups of four closely spaced color LEDs: two green, one red, one blue. (The green, red, and blue light combine to make white.) Sony has trademarked this technology Triluminos. The advantage is that the exact colors of red, green, and blue can be chosen independently to give a potentially wider color gamut than a TV using white LEDs. However, Triluminos is more expensive to implement than a white LED array, and other manufacturers have been able to meet or exceed the HDTV color gamut specifications using just white LEDs

MIT team finds way to combine Silicon and Gallium Nitride for Microprocessors

2009-10-03T12:06:00.000+05:30

Silicon and Gallium nitride have been used to create a single hybrid microchip. This will allow transistors to be made smaller and sets of several chips made of different material in a cellphone can be combined into a single chip This is also an advance towards photonics on a chip which are needed for high speed interchip communication and for zettaflop computers. It could take a couple of years to get to the point where it could be commercialized.

Results: An MIT team led by Tomás Palacios, assistant professor in the Department of Electrical Engineering and Computer Science, has succeeded in combining two semiconductor materials, silicon and gallium nitride, that have different and potentially complementary characteristics, into a single hybrid microchip. This is something researchers have been attempting to do for decades.

Why it matters: This advance could point to a way of overcoming fundamental barriers of size and speed facing today's silicon chips. "We won't be able to continue improving silicon by scaling it down for long," Palacios says, so it's crucial to find other approaches. Besides microprocessor chips, the new integrated technology can be used for other applications such as hybrid chips that combine lasers and electronic components on a single chip, and energy-harvesting devices that can harness the pressure and vibrations from the environment to produce enough power to run the silicon components. It could also lead to more efficient cell phone manufacturing, replacing four or five separate chips made from different semiconductor materials. "With this technology, you could potentially integrate all these functions on a single chip," Palacios says.

New energy-saving transistor to eliminate the need for AC adapters in laptops

2009-06-25T13:57:00.000+05:30

Fujitsu Laboratories announced Tuesday that the company has developed a new energy-saving transistor.

The new transistor developed by the company, based in Kawasaki, Kanagawa Prefecture, can reduce electricity loss that occurs in the power supply units of computers and other devices to one-third or less of the current level. By downsizing the transistor, the power supply unit can be integrated into the body of laptop computers, eliminating the need for AC adapters. The company is aiming for practical application of the new transistor by 2011.

In computers, the power supply unit converts alternating current into direct current, during which about 30 percent of the electric power is lost as the transistor produces heat.

While traditional transistors are normally made from silicon, Fujitsu's key technology research team succeeded in developing a transistor using gallium nitride, a material commonly used in blue LEDs. With gallium nitride's resistance to high-voltage current being 10 times as high as silicon's, transistors made from gallium nitride are more robust. At the same time, such transistors can reduce the electricity loss to below one-third of the level of silicon transistors, while a shorter inter-electrode distance enables a downsizing of the transistor.

Fujitsu hopes to start bulk production of the new transistor by 2011, and implement the replacement of older models at the company's data center. At the same time, the company's research team will engage in the development of small-sized transistors to replace the AC adapters of laptop computers.

"In five years, we will be able to develop technology to remove AC adapters from computers," says Toshihide Kikkawa, a chief researcher in charge of the development of the transistor. "By offering our expertise to other corporations, our new transistor can be further applied in electric vehicles and home appliances."

Reference Page

Nanotechnology builds battery on a virus framework

2009-04-18T11:32:00.003+05:30

For the first time, MIT researchers have shown they can genetically engineer viruses to build both the positively and negatively charged ends of a lithium-ion battery.

The new virus-produced batteries have the same energy capacity and power performance as state-of-the-art rechargeable batteries being considered to power plug-in hybrid cars, and they could also be used to power a range of personal electronic devices, said Angela Belcher, the MIT materials scientist who led the research team.

The new batteries, described in the April 2 online edition of Science [abstract], could be manufactured with a cheap and environmentally benign process: The synthesis takes place at and below room temperature and requires no harmful organic solvents, and the materials that go into the battery are non-toxic.

In a traditional lithium-ion battery, lithium ions flow between a negatively charged anode, usually graphite, and the positively charged cathode, usually cobalt oxide or lithium iron phosphate. Three years ago, an MIT team led by Belcher reported that it had engineered viruses that could build an anode by coating themselves with cobalt oxide and gold and self-assembling to form a nanowire.

In the latest work, the team focused on building a highly powerful cathode to pair up with the anode, said Belcher, the Germeshausen Professor of Materials Science and Engineering and Biological Engineering. Cathodes are more difficult to build than anodes because they must be highly conducting to be a fast electrode, however, most candidate materials for cathodes are highly insulating (non-conductive).

To achieve that, the researchers, including MIT Professor Gerbrand Ceder of materials science and Associate Professor Michael Strano of chemical engineering, genetically engineered viruses that first coat themselves with iron phosphate, then grab hold of carbon nanotubes to create a network of highly conductive material.

Because the viruses recognize and bind specifically to certain materials (carbon nanotubes in this case), each iron phosphate nanowire can be electrically “wired” to conducting carbon nanotube networks. Electrons can travel along the carbon nanotube networks, percolating throughout the electrodes to the iron phosphate and transferring energy in a very short time.

The viruses are a common bacteriophage, which infect bacteria but are harmless to humans.

…Now that the researchers have demonstrated they can wire virus batteries at the nanoscale, they intend to pursue even better batteries using materials with higher voltage and capacitance, such as manganese phosphate and nickel phosphate, said Belcher. Once that next generation is ready, the technology could go into commercial production, she said

Bugs Build Batteries

By Lauren Cahoon
ScienceNOW Daily News
3 April 2009

Green technology just went viral. Researchers have used viruses to create rechargeable batteries similar to those found in hybrid cars and laptops. Until now, batteries like this were made in chemically intensive, high-heat processes. The results could herald a low-energy, environmentally friendly alternative.

Most commercial rechargeable batteries pass a lithium ion from the anode, the negatively charged terminal, to the positively charged cathode and have to be made at high temperatures. Materials chemist Angela Belcher of the Massachusetts Institute of Technology in Cambridge and her colleagues decided to try making a better battery by using biological processes. This approach is logical, Belcher says, because some of the materials in batteries, such as phosphate and iron, are present in living systems and can be easily manipulated by organisms.

The team first created an anode by genetically engineering the M13 virus, a common parasite of bacteria, to attract cobalt oxide and gold to its outer shell and then assemble into films and sheets (Science, 12 May 2006, p. 885).

The next step was to tackle the positively charged cathode, which is more challenging because it needs to be highly conductive. The team engineered the M13 viruses to accumulate ions of iron phosphate and to latch onto a highly conductive network of carbon nanotubes. Electrons could travel quickly through this system and boost the cathode's capacity. In fact, Belcher's battery had the same power performance as commercially available lithium ion batteries and could be charged and discharged at least 100 times without wearing out, the team reported online yesterday in Science.

The virus-built technology may provide the first biological method for producing batteries. Belcher notes that the entire system, with the exception of the carbon nanotubes, is created at room temperature and uses only water as a solvent. And when the batteries die and degrade, they don't leave behind toxic chemicals. "This is definitely a very clean approach," Belcher says.

However, she cautions that the technology isn't yet useful as a commercial application. Because the virus battery only matches the capacity and power performance of those available on the market, "we're not going to scale up this material," says Belcher. "It wouldn't gain us anything in terms of performance."

Other experts agree that Belcher's discovery isn't going to change the face of the battery industry overnight. "It is of scientific curiosity," says M. Stanley Whittingham, a chemist at the Institute for Materials Research at Binghamton University in New York state. To change battery technology, he says, the researchers need to build a higher capacity cathode.

References : http://sciencenow.sciencemag.org/cgi/content/full/sciencenow;2009/403/1

http://www.foresight.org/nanodot/?p=3013

Symmetry boosts spin’s lifetime

2009-04-03T12:35:00.002+05:30

Spinning in a helix

Physicists in the US have invented a new way to increase the lifetime of electron spins flowing in a semiconductor device. Their technique involves fine–tuning the properties of a tiny gallium-arsenide structure and could make it possible to create logic circuits that use electron spin.

Electron spins can assume one of two values — up or down — and this property is already used to store data in computer hard disks and magnetic memories. In the future, more advanced "spintronic" devices could exploit both the spin and charge of the electron to create a wider range of digital devices that are faster are more energy efficient than conventional silicon chips.

However, such devices would rely on electrons maintaining their spin as they travel around a circuit. This has proven difficult because each time such an electron scatters from a defect or lattice vibration in a metal or semiconductor there is a small chance that its spin could flip direction. This occurs because a change in the motion of the electron affects the direction of its spin thanks to an effect called spin-orbit (SO) coupling.

Random flipping

In most materials electrons experience many collisions and this flipping occurs randomly and very rapidly. This can be minimized by using defect–free materials kept at very low temperatures — but this is not really possible in practical devices.

Now Jake Koralek of the Lawrence Berkeley National Lab; David Awschalom of the University of California Santa Barbara; and colleagues have tackled the problem by working out a way to tune the SO interaction in a tiny gallium arsenide structure called a quantum well ( Nature 458 610).

The team focussed on two aspects of how spatial symmetries within a semiconductor affect SO coupling — the Dresselhaus and Rashba effects. The former is related to the “inversion asymmetry” that occurs in gallium arsenide crystals and was adjusted by changing the width of the quantum well. The Rashba effect is caused by the application of an electric field to the quantum well and the team controlled this by adding impurities (dopants) to certain regions of the quantum well.

Rashba and Dresselhaus on equal terms

“We tuned the Rashba and Dresselhaus terms to be equal”, explained Koralek. This meant that the resultant SO interaction has a much higher spatial symmetry than in a typical semiconductor or metal. Although individual spins are still affected by spin-orbit coupling, they are able to rotate in unison in a long-lasting collective state called a “persistent spin helix” (PSH).

By using a laser technique called transient spin-grating spectroscopy, the team measured how long a PSH persisted in the quantum well. Two “pump” laser pulses are fired at the quantum well where the resulting interference pattern of light creates alternating stripes of spin up and spin down electrons — called a spin-grating. The wavelength of the spin-grating can be adjusted by simply changing the angle between the two pump pulses.

The amplitude and wavelength of the spin–grating are then measured by firing a third “probe” laser pulse at the sample and observing the resulting diffraction pattern. By varying the time between the pump and probe pulses, the team were able to measure how long the spin-grating endured.

100-times longer

They found that when the wavelength of the spin-grating matched the expected wavelength of the persistent spin helix, the spin-grating endured for hundreds of picoseconds — compared to just a few picoseconds when the wavelengths did not match.

This 100-times improvement was seen at the relatively low temperature of 5 K and the team found that it dropped off rapidly as the temperature increased to room temperature. This strong temperature dependence was not expected and could mean that the technique is not appropriate for use in practical spintronic devices.

Although a few hundred picoseconds doesn’t sound like a long time, Awschalom told physicsworld.com that such “spin engineered” materials could someday be used in devices that perform large numbers of spin operations on electrons before their spins decayed.

Koralek added that the Rashba interaction can also be controlled by applying a voltage to the quantum well — and this could lead to a spin transistor that could turn a spin current on and off.

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Article taken from : http://physicsworld.com/cws/article/news/38527

Microscopic magnetic mimic of bacterial motor

2009-03-04T12:44:00.006+05:30

Basics of Bacterial motion (The trivial):

One of the most fascinating areas of research is biomimetics, where researchers attempt to replicate nature's creative accomplishments in the lab. For pure inspiration, look no further than to bacteria. Bacteria seem to go everywhere and do pretty much everything. The natural tendency of those of us with a inclination towards geekiness is to think of micro-robots. There are a number of natural roles that micro-robots could play, such as cleaning plaques from artery walls or defouling pipelines.

One of the problems with micro-robots has been making them small enough. One millimeter might seem small to us, but not to a bacteria. One of the primary constraints on shrinking these devices is the method of locomotion, which, because it requires moving parts, tends to be on the bulky side. This is about to change as some recent results published in Applied Physics Letters show researchers building their very own artificial bacterial flagella. In the publication, controlled motion and the simple task of separating cell-sized polystyrene beads were both demonstrated.

The bacteria flagellum is an interesting little piece of nature. Apart from providing endless amusement and artificial controversy, they also self-assemble—put the right proteins and some fuel in a test tube of water and the result is a bunch of little flagella propelling themselves about at random. Physicists are interested in them for two reasons: they are one of the few mechanisms for transporting micro-robots that might be replicable using inorganic materials, and, given the size of the micro-robot and the viscosity of relevant mediums (close to water in the human body), flagella are very close to an optimal propulsion system.

Research **

Researchers from ETH Zurich have recently published the results from their successful attempts to replicate flagella at the same size scale as natural flagella. To make a flagellum, they deposited layers of aluminium gallium arsenide, indium gallium arsenide, gallium arsenide, and chromium on top of a gallium arsenide crystal. They then use an ion beam to mill away most of the layers, leaving a long, thin rectangle of material.

At one end of this, they deposited a small square of chromium-nickel-gold to provide a magnetic head. The rectangle of material is separated from the gallium arsenide substrate by dissolving the aluminium gallium arsenide. The remaining strip of material is stressed because of the layering, so it curls up into a helix. This process is so well-controlled that the researchers can choose any helix they desire simply by changing the materials and thickness of the different layers.

This tiny scroll was then dropped into a swimming pool sitting under a microscope, where the researchers could observe and control it. The magnetic head of the scroll responds to any magnetic field—changing the field causes the orientation of the scroll to change and induces swimming motions. As a result, the flagella swims in controlled curves, allowing researchers to direct it to any location with any orientation. This was demonstrated by swimming the flagella to a polystyrene sphere, after which they were directed to rotate and push the sphere. The researchers also showed that two spheres could be separated by drilling a flagella between them. Videos of the swimmers in action can be found here.

This is, of course, a very cool piece of research, but it probably won't be used for any sort of remote activity. Although external fields are used to power and direct the swimmer, we still require sensors to see where it is. This probably eliminates any chance of putting these deep inside the human body to scrape plaque from artery walls or doing the equivalent inside mechanical systems. So there is certainly more research required in that direction. On the other hand, the rotary motion can certainly be used as a power source for micro-robots, and that doesn't require being able to sense where the micro-robot is.

Introduction:

A number of robotic swimming methods have been proposed at relatively small scales. Because many of these methods rely on reciprocating motions, they do not scale downwards. Yet over three billion years ago bacteria evolved a swimming strategy at micrometer dimensions that nature has had difficulty improving upon. Just over thirty-five years ago their swimming technique using rotating flagella was first described by H.C. Berg and R.A. Anderson (Nature vol. 245, pp. 380-382, 1973). Inspired by the flagellar motion of bacteria such as Escherichia Coli (see video 4), we have recently developed artificial bacterial flagella (ABF). Our ABF represent the first demonstration of wireless swimming microrobots similar in size and geometry to natural bacterial flagella, and are many orders of magnitude smaller than existing artificial helical swimmers.

Manufacturing:

Helical swimming robot consists of two parts: a helical tail and a soft-magnetic metal head. The tails are 27 to 42 nm thick, less than 2um wide, and coil into diameters smaller than three microns. The fabrication of ABF is based on a self-scrolling technique. The helical tail is patterned in 2D as an InGaAs/GaAs bilayer nanoribbon or an InGaAs/GaAs/Cr trilayer nanoribbon. The metal head is fabricated from a Cr/Ni/Au thin film using a lift-off process. The 2D films detach from the GaAs wafer and self-organize to form tethered helical robots. To untether the helical swimming microrobots from the substrate, micromanipulation is performed to cut, pick, and release them in water. After that, the helical swimming microrobots are propelled and steered precisely in water by a low-strength (1-2 mT), rotating magnetic field. Details of the experimental process can be found in (APL, 94, 064107, 2009).

By adjusting the rotating speed and direction of the magnetic field, the velocity and direction of motion of the helical swimmer can be tuned in a controlled fashion. The figure below shows an example where a 74µm long helical swimmer is driven to reach a target. The average velocity is approximately 5µm/s at 470 rpm. By inverting the rotating magnetic field, the swimmer turns in the opposite direction, and the linear motion is reversed.

Ref : here.

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** Please visit here for more information and videos and pictures.

http://www.iris.ethz.ch/msrl/research/micro/helical_swimmers/

http://www.rowland.harvard.edu/labs/bacteria/projects_filament.html

The PZ10 processor

2008-12-27T13:44:00.001+05:30

Researchers from IBM and Georgia Institute of Technology designed and built a computer processor that broke the world speed record reaching an astonishing 500GHz speed, more than one hundred times faster than the fastest commonly available computer chip.

Most processors are made from silicon, but
in recent years several discoveries were made that revealed that there are other materials better suited for high processing speeds than silicon. There are a number of time critical systems, like collision-warning systems, where the silicon based processor is already being replaced by CPUs made from a layer of gallium arsenide, even if the materials needed are expensive and more difficult to produce. Because an extensive transition to another base material would be very costly and would take huge amount of time to complete, the computer hardware industry is searching for ways to improve existing silicon based CPUs. Such a way is to add small amount of germanium inside the silicon-based chips that are designed for mobile phones in order to make them more efficient.

Germanium allows chips to reach higher clock speeds and use less power and such processors can be fabricated using the existing production lines. Even so, reaching the 500GHz frequency was no easy feat, as the IBM researchers super cooled the processor prototype to -268.5 degrees Celsius, using liquid helium. The extremely low temperature, just above the minimum theoretical possible one known as "absolute zero", enabled the processor perform half a trillion calculations every second, which translates into a speed of 500GHz. "A decade ago we couldn't even envisage being able to run at these speeds," said Professor David Ahlgren of IBM.

The extreme speed of the prototype processor was greater than the conventional design even at room temperature where it reached 350 billion calculations per second and there are hopes that the mark can be pushed further. "We observe effects in these devices at cryogenic temperatures which potentially make them faster than simple theory would suggest," said Professor John Cressler of the Georgia Institute of Technology, who was cited by the news site related to BBC.

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Original article can be found here.

Light powered Motor

2008-12-09T13:47:00.001+05:30

Research team at Tokyo institute of technology has developed a motor which entirely runes on light, rather than photovoltaic cells which convert light to electricity this motor has special elastomer whose molecular structure expands or contracts when when light falls on it, this is how this motor can convert light energy directly into the mechanical energy.

Team was working on this project since 2003 when they found that plastic compound containing azobenze can expand in ultraviolet light and regain its original shape in normal visible light since then they had improved very much. They performed the test by rotating a pair of wheels measuring 3 millimeters and 10 millimeters in diameter looped with shape shifting plastic coated 0.08 millimeter belt, when ultraviolet light was shine on small wheels and visible light on small wheel they starts rotating with a top speed of 1 rpm.

The coated film measured 4 times elastic than human muscle, this thing is not very efficient but it can be improved to convert light energy into mechanical energy.

Brief History of Light Emitting Diode (LED)

2008-11-27T11:10:00.002+05:30

LED lamp History

The phenomenon of solid state junctions producing light was discovered in the crystal detector era. In the 1960s commercial red LED’s became available, and by the 1970s these were in widespread use as indicators in a very wide range of equipment. These early LED’s had much too small an output to be useful as lighting. They replaced the previously widely used indicator types of filament lamps and neon. Compared to neon, indicator LED’s have longer lifetimes and run on lower voltage; compared to miniature filament lamps, indicator LED’s have much longer lifetimes, such that they do not require replacement, and consume less power. The lack of need for replacement also eliminates the need for bulb sockets and a user access port.

Commercial amber (yellow) and orange LED’s followed, and were used where differentiation of multiple LEDs was required. For many years LED’s came in infra-red, red, orange, yellow, and green. Blue, cyan, and violet LEDs finally appeared in the 1990s.

To produce a white SSL device, a blue LED was needed. In 1993, Shuji Nakamura of Nichia Corporation came up with a blue LED using gallium nitride (GaN). With this invention, it was now possible to create white light by combining the light of separate LED’s (red, green, and blue), or by placing a blue LED in a package with an internal light converting phosphor. With the phosphor type, some of the blue output becomes either yellow or red and green with the result that the LED light emission appears white to the human eye.

In 2008, SSL technology advanced to the point that Sentry Equipment Corporation in Oconomowoc, Wis. was able to light its new factory almost entirely with LEDs, both interior and exterior. Although the initial cost was three times more than a traditional mixture of incandescent and fluorescent bulbs, the extra cost will be repaid within two years from electricity savings, and the bulbs should not need replacement for 20 years.