Array of Graphene dots absorbs light perfectly

http://physics.aps.org/articles/v5/12

http://prl.aps.org/abstract/PRL/v108/i4/e047401

The results may be used in e.g. solar cells ! How to broaden the absorption spectrum?

Converting mechnical energy by surface tension

Hey, someday you may charge your phone by just walking around !
[http://www.physicstoday.org/daily_edition/physics_update/a_microfluidics_path_to_harvesting_mechanical_energy]

Imagine a drop of water on a solid, pulled up into a ball by surface tension. The simple application of voltage between liquid and solid alters the interfacial energy and prompts the droplet to spread out, an effect known as electrowetting; the greater the voltage, the greater the spread. In the past decade, researchers exploiting the effect have developed, among other applications, liquid lenses with voltage-tunable focal lengths and microfluidic circuits that store and steer droplets without the need for pumps or mixers. Tom Krupenkin and Ashley Taylor at the University of Wisconsin–Madison have now developed an approach that runs the process in reverse—converting the mechanical energy of liquid motion into electrical current. In one implementation, they pressurized a fluidic channel to force a train of mercury droplets past dielectric-coated electrodes connected to a bias voltage on the order of tens of volts. As the overlap area between the droplets and electrodes changed, so did the charge stored at their interface, giving rise to an alternating current that can drive a load. The researchers measured a few milliwatts from a channel containing 22 droplets. But from their model of the process they calculate that average powers of 1 W or more could easily be generated in a fluidic device with 1000 flowing droplets. The devices are small enough to fit into a pair of shoes; with each step, fluid is squirted back and forth between the heel and toe. (T. Krupenkin, J. A. Taylor, Nat. Commun. 2, 448, 2011.)—R. Mark Wilson

New design of transistors

These are of course seminal to future in electronics industry.

The team has created a two-layer GaAs/AlGaAs quantum well heterostructure, in which the wave function of one layer extends into the second to modulate the tunneling current between the layers. In this design, a voltage on the first quantum well causes that layer to be depleted of carriers, which changes the subband energy level in the well. As the subband energy approaches the top of the quantum well potential, the wave function extends further and further out toward the second layer. When the wave function overlaps the second layer, the tunneling current can increase as much as two orders of magnitude, a substantial degree of gating leverage.

Although the reported design only works at cryogenic temperatures, a different choice of materials, for example, graphene, may allow operation at more technologically relevant temperatures. – David Voss

Noteworthy papers from latest issue of Science

1. Disorder-Enhanced Transport in Photonic Quasicrystals, 332:1541(2011);

Quasicrystals are aperiodic structures with rotational symmetries forbidden to conventional periodic crystals; examples of quasicrystals can be found in aluminum alloys, polymers, and even ancient Islamic art. Here, we present direct experimental observation of disorder-enhanced wave transport in quasicrystals, which contrasts directly with the characteristic suppression of transport by disorder. Our experiments are carried out in photonic quasicrystals, where we find that increasing disorder leads to enhanced expansion of the beam propagating through the medium. By further increasing the disorder, we observe that the beam progresses through a regime of diffusive-like transport until it finally transitions to Anderson localization and the suppression of transport. We study this fundamental phenomenon and elucidate its origins by relating it to the basic properties of quasicrystalline media in the presence of disorder.

2.Carbon-Based Supercapacitors Produced by Activation of Graphene, 332:1537(2011)

Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp²-bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

3. The Limits of Ordinary Matter, 332:1513(2011)

All ordinary matter consists of protons and neutrons, collectively called nucleons, which are bound together in atomic nuclei, and electrons. The elementary constituents of protons and neutrons, the quarks, almost always remain confined inside nucleons (or any other particle made up of quarks, called hadrons). The fundamental force that binds quarks together—the strong, or “color” force—cannot be overcome unless extremely high-energy conditions are created, such as through heavy-particle collisions. Theoretical simulations based on quantum chromodynamics (QCD) predict that the transition temperature for the appearance of free quarks should occur at 2.0 × 10¹² K (an energy of 175 million eV) (1, 2). Since 2000, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has created the necessary conditions to form quark matter in particle collision, but determining the transition temperature under these conditions is challenging. On page 1525 of this issue, Gupta et al. (3) show that the relevant temperature and energy scales can be extracted from recent experimental studies and find that the transition temperature is in remarkable agreement with theory.

4 This paper is not published in Science, but highlighted in it: Nano Lett. 11, 10.1021/nl200928k (2011).

It has long been known from ex situ studies that metal nanoparticles can catalyze reaction of oxygen with graphite surfaces and create grooves or channels. Such reactions could be used for patterning graphene sheets. Booth et al. have studied the dynamics of silver nanoparticles on suspended monolayer and bilayer graphene sheets in a transmission electron microscope. They imaged these samples at temperatures from 600 to 850 K and partial pressures of oxygen over the sample from about 30 to 100 millitorr. The nanoparticles cut channels along <100> crystallographic directions, but some fluctuations of motion normal to the channel direction were also observed. The nanoparticles did not move at a constant speed. Instead, their velocity profile was erratic, and the start-stop motion was better described by a Poisson distribution.

News from this issue of Science Magazine

1. Batteries are going to break out from old forms:

Now he's on his way. Last month, Chiang and his colleagues reported in Advanced Energy Materials that they've created a new battery design called a semisolid flow cell that's like a battery with a fuel tank. Like today's batteries, the device contains lithium ions that shuttle back and forth either storing or releasing electrical charges on demand. But instead of packaging those ions along with the electrodes and other apparatus all together, as in a typical battery, Chiang's semisolid flow cell separates the energy-delivery apparatus from energy storage. In this battery, the storage medium is a pair of gooey, black liquids, the consistency of yogurt, that contain nanoscale particles of materials commonly used as anodes and cathodes in lithium-ion cells. These particles are suspended in an electrolyte and separated by a porous membrane. When power is needed, a bolus of each goo is pumped from external tanks into a network of current collectors that extract electrons while lithium ions shuttle through the membrane from the anode particles to the cathode particles. The spent slurries can then be reenergized, as in a normal rechargeable battery, or pumped out and replaced. Other types of flow batteries have been made in the past, but Chiang says the new setup can store up to 30 times as much energy as previous versions. He has launched another company, called 24M, to commercialize the technology.[http://www.sciencemag.org/content/332/6037/1494.full]

2.

Move over, China. Japan's “K Computer” is now the fastest supercomputer in the world. On 20 June, K was ranked number one in the TOP500 list of the world's supercomputers, performing three times as fast as its Chinese rival and the previous champion, Tianhe-1A.

The TOP500 list is updated twice a year and ranks how quickly computers solve a standard mathematical equation. K, built by Fujitsu and located at the RIKEN Advanced Institute for Computational Science in Kobe, can perform 8.2 quadrillion calculations per second, equivalent to linking about 1 million desktop computers. That performance is still shy of the target kei, or 10 quadrillion, calculations for which the supercomputer was named. This is the first time Japan has topped the list since 2004.[http://www.sciencemag.org/content/332/6037/1488.2.full]

3. Education is not a rece:

In the United States and elsewhere, the competitive pressures placed on young people in school are damaging many otherwise promising lives. In addition to generating debilitating anxiety and encouraging a culture of cheating, this competition takes the joy out of learning. The film Race to Nowhere, which continues to receive attention since its release a year ago, documents the unhealthy consequences of the competitive “teach to the test” climate that many U.S. students experience. The film, in which I was interviewed, puts in clear relief the pressures that youth are under to amass large numbers of Advanced Placement (college-equivalent) classes, win science fairs, excel in the arts and sports, and in other ways distinguish themselves from the competition for admission into a few select universities that parents and schools believe are critical for future success. Research on motivation makes it clear that focusing attention entirely on performance, whether grades or test scores, destroys whatever intrinsic interest the subject matter might have had.* There are certainly students whose passions spur them to realize their full potential in rigorous academic courses and other impressive activities. But how many potential Nobel Prize winners have written off science before the end of high school because their only exposure to the subject had been in test preparation courses rather than in classes that delved into meaningful questions? It doesn't have to be this way, but change will require coordinated efforts at many levels.

Success in life does not require a degree from one of 10 universities. We need to evaluate U.S. high schools (pre-college education) on how well they help students find a college that matches their interests and goals, not on the proportion of students that they send to elite institutions. And the coveted universities need to demonstrate that they are interested in students who have a genuine passion for extending their educational experience, not merely in tallying items on resumés.

Many U.S. teachers also must change their approach to teaching. Extensive research shows that students will become more emotionally engaged (and even passionate) if simple principles are followed: if the subject matter is connected to students' personal lives and interests; if students have opportunities to be actively involved in solving or designing solutions to novel and multidimensional problems, doing experiments, debating the implications of findings, or working collaboratively; if students have multiple opportunities to earn a good grade (by rewriting papers or retaking tests); if attention is drawn to the knowledge and skills that students are developing, not to grades or scores; and if all learning and skill development is celebrated, whatever the level.

Schools must create homework policies to ensure that diligent students aren't kept up late into the night; schedule some spacing between major tests and offer ample opportunities for students to get extra help; make sure that at least one adult is paying attention to every student's emotional needs; provide parent education on the advantages of a broad array of potential colleges; survey students regularly on the sources of their stress and make sure that this feedback informs policies; and offer opportunities for students to pursue academic interests unencumbered by performance concerns, such as in independent studies or clubs.

The world is rapidly changing. Problem-solving skills and critical analysis have become infinitely more important than being able to answer the typical questions given on standardized tests. A valuable science of teaching and learning exists that should guide efforts to improve students' interest, engagement, and intellectual skills, as well as reduce the debilitating stress that is becoming epidemic.** Only by paying attention to what we know can we make the changes that youth need to lead healthy and productive lives. [http://www.sciencemag.org/content/332/6037/1481.full]

Giant Electroresistance

There are many devices that are of fundamental interest. The most latest and famous may be the one invented by Fert et al., which displays giant magnetoresistance. In the past few years, there have appeared a flurry of work exploiting ferroelectrics as the barrier layer. I have mentioned quite a number of them in this blog. Here comes a new one. It is made up by sandwiching a BTO layer with two LSMO at different dopings. All these efforts are clearly directed to manipulate the interweaving properties of spin, charge and orbital degrees.

A giant tunneling electroresistance effect may be achieved in a ferroelectric tunnel junction by exploiting the magnetoelectric effect at the interface between the ferroelectric barrier and a magnetic La1 xSrxMnO3 electrode. Using first-principles density-functional theory we demonstrate that a few magnetic monolayers of La1 xSrxMnO3 near the interface act, in response to ferroelectric polarization
reversal, as an atomic-scale spin valve by filtering spin-dependent current. This produces more than an order of magnitude change in conductance, and thus constitutes a giant resistive switching effect. [PRL 106, 157203 (2011)]

Correction is more inadvertent than prevention: electronics

The statistics say that, the present rate of error occurrence is around 10^{-27}. This is imposed by the fault-tolerant architecture of logical circuits. These workers now prove that, physical fault tolerant prevention is more efficient than architectural correction in suppressing errors. [PRL 106, 176801 (2011)]

The error rate in complementary transistor circuits is suppressed exponentially in electron number, arising from an intrinsic physical implementation of fault-tolerant error correction. Contrariwise, explicit assembly of gates into the most efficient known fault-tolerant architecture is characterized by a subexponential suppression of error rate with electron number, and incurs significant overhead in wiring
and complexity.We conclude that it is more efficient to prevent logical errors with physical fault tolerance than to correct logical errors with fault-tolerant architecture.

Relativity and the Lead-Acid Battery

This relation may seem at first glance quite unusual. For most, relativity needs be concerned only when the considered speed becomes comparable to that of light. The Lead -acid battery seems having no attachment to this speed. However, if you look into the electrons that are busy inside the substance, you may change your mind. Lead is a heavy element, and relativity effects, as you can derive from Dirac's equation, scale as the quartic power of the atomic numbers. That is how relativity plays a role, which proves crucial for your cars to start, in Lead-Acid Battery [http://prl.aps.org/pdf/PRL/v106/i1/e018301].

The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at nonrelativistic, scalar-relativistic, and fully relativistic levels, and using several exchange correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7–1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

Molecules filtering spins

Using STM with a magnetic tip can be used to probe the magnetic feature of a surface. The tunneling current shall be sensitive to the alignment (collimation) between the spin orientation of the surface and that of the tip. If the tunneling, as is usually the case, is non-magnetic, then parallel alignment yield a bigger current. A very valuable aspect of STM is that this device probes the local properties of a material. This makes it especially useful in investigating defects or impurities of a surface. Now these authors [Phys. Rev. Lett. 105, 066601 (2010)] came to examine what will happen to the signal if the electrons tunnel from the Fe surface into the tip through a single organic molecule with Beneze rings. The result is this: this molecule allows more spin-up electrons to pass. So, it works as a selective valve, which may be tailored to specific applications that needs manipulate spin current. This phenomenon was predicted 3 years ago in Ref.[3], where the computation was implemented in the aid of DFT. However, it may prove more elucidating if a simple model description is prescribed.
For convenience, some references are attested on this subject:

1. Atodiresei, N. et al. Phys. Rev. Lett. 105, 066601 (2010).
2. Brede, J. et al. Phys. Rev. Lett. 105, 047204 (2010).
3. Rocha, A. R. & Sanvito, S. J. Appl. Phys. 101, 09B102 (2007).
4. Barraud, C. et al. Nature Phys. 6, 615–620 (2010).
5. Sanvito, S. Nature Phys. 6, 562–564 (2010).
6. Cinchetti, M. et al. Nature Mater. 8, 115–119 (2009).
7. Drew, A. J. et al. Nature Mater. 8, 109–114 (2009).
8. Szulczewski, G., Sanvito, S. & Coey, J. M. D. Nature Mater. 8, 693–695 (2009)

Seeing the image obscured by painted glasses

This is an interesting innovation. Unfortunately, I cant access the original article !

A new laser technique can capture an image of an object obscured behind painted glass.

Sylvain Gigan and his team at ESPCI ParisTech in France have devised a method that traces the scattered path that photons take as they pass through an opaque white material. On one side of a glass slide covered in thick white paint, the researchers projected the image of a flower. They illuminated this set-up with a laser and took a photograph from the slide's other side. After calculating the light's zigzagging journey through the painted glass, they were able to reconstruct the flower image.

With improvements, the technique might one day be used in medical imaging to see through opaque biological tissue such as skin.

Nature Commun. doi:10.1038/ncomms1078 (2010)

Limit on the speed of computers

How fast can a computer process its data ? In the past decades, the cpu simply goes faster and faster, as its size becomes smaller and smaller. However, there seem a limit, since it is not evident whether a sub-atomic circuit can be manufactured and manipulated. An electronic component can hardly be made tinier than an atom. Last year, two physicists argued that, in not more than 75 years, cpu cannot transcend itself anymore. Its speed shall saturate, a fact in spite of how the computer operates, be it classical or quantum.

real-time tracking the motions of electrons

Atto-second spectroscopy enables this !

doi:10.1038/nature09212

Attosecond technology (1 as=10⁻¹⁸ S) promises the tools needed to directly probe electron motion in real time. These authors report attosecond pump–probe measurements that track the movement of valence electrons in krypton ions. This first proof-of-principle demonstration uses a simple system, but the expectation is that attosecond transient absorption spectroscopy will ultimately also reveal the elementary electron motions that underlie the properties of molecules and solid-state materials.

Type-2 superconductors used as tweezers

A just published work (PRA, 81, 063408, 2010) did calculations on how type-2 superconductors might be used to trap atoms or other tiny particles. In comparison with other choices, the scheme there-proposed has less loss of coherences. Their calculations proved type-2 SC as a very versatile device in that direction. What holds the key as tweezers is the formation of vortice, which allows the existence of local magnetic field. Such vortice appear in type-2 SC, but not in type-1.

World's Smallest Superconductor Developed: Sheet of Four Pairs of Molecules Less Than One Nanometer Wide

"Researchers have said that it's almost impossible to make nanoscale interconnects using metallic conductors because the resistance increases as the size of wire becomes smaller. The nanowires become so hot that they can melt and destruct. That issue, Joule heating, has been a major barrier for making nanoscale devices a reality," said lead author Saw-Wai Hla, an associate professor of physics and astronomy with Ohio University's Nanoscale and Quantum Phenomena Institute.

Superconducting materials have an electrical resistance of zero, and so can carry large electrical currents without power dissipation or heat generation. Superconductivity was first discovered in 1911, and until recently, was considered a macroscopic phenomenon. The current finding suggests, however, that it exists at the molecular scale, which opens up a novel route for studying this phenomenon, Hla said. Superconductors currently are used in applications ranging from supercomputers to brain imaging devices.

In the new study, which was funded by the U.S. Department of Energy, Hla's team examined synthesized molecules of a type of organic salt, (BETS)2-GaCl4, placed on a surface of silver. Using scanning tunneling spectroscopy, the scientists observed superconductivity in molecular chains of various lengths. For chains below 50 nanometers in length, superconductivity decreased as the chains became shorter. However, the researchers were still able to observe the phenomenon in chains as small as four pairs of molecules, or 3.5 nanometers in length.

To observe superconductivity at this scale, the scientists needed to cool the molecules to a temperature of 10 Kelvin. Warmer temperatures reduced the activity. In future studies, scientists can test different types of materials that might be able to form nanoscale superconducting wires at higher temperatures, Hla said.

"But we've opened up a new way to understand this phenomenon, which could lead to new materials that could be engineered to work at higher temperatures," he said.

The study also is noteworthy for providing evidence that superconducting organic salts can grow on a substrate material.

"This is also vital if one wants to fabricate nanoscale electronic circuits using organic molecules," Hla added.

Collaborators on the paper include Kandal Clark, a doctoral student in the Russ College of Engineering and Technology at Ohio University; Sajida Khan, a graduate student in the Department of Physics and Astronomy at Ohio University; Abdou Hassanien, a researcher with the Nanotechnology Research Institute, Advanced Industrial Science and Technology (AIST) and the Japan Science and Technology Agency's Core Research of Evolutional Science & Technology (JST-CREST) in Japan who conducted the work as a visiting scientist at Ohio University; Hisashi Tanaka, a scientist at AIST and JST-CREST who synthesized the molecules; and Kai-Felix Braun, a scientist with the Physikalisch Technische Bundesanstalt in Braunschweig, Germany, who conducted the calculations at the Ohio Supercomputing Center.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Ohio University. The original article was written by Andrea Gibson.

This year's Nobel Prize: CCD

Getting a digital camera for Christmas? Before you fire it up to capture Uncle Wally's fateful fifth trip to the punch bowl, take a moment to picture this: You've got a genuine scientific marvel in your mitts. In fact, it took nothing less than two Nobel prizes and a revolution in physics in order for you to point and shoot.

Why? Because to take a filmless picture, your camera or camcorder relies on, um, quantum mechanics. In particular, it exploits the fact -- revealed by Albert Einstein himself -- that a beam of light, which behaves like a wave in some circumstances, acts like a bunch of separate particles in other circumstances. (If that seems infuriatingly contradictory, suck it up. It's just how we do things in this cosmos. Or go complain to the management.)

The individual particles, called photons, come in a wide range of energies. Visible light has enough so that when its photons slam into something, such as a sheet of specially fabricated semiconductor material in a digital camera, they kick electrons right out of the stuff, producing an electrical charge at the crash site. Explaining this phenomenon, known as the photoelectric effect, got Einstein his Nobel.

In most consumer cameras, the photoelectric action takes place back behind the lens, when the light reflected from Uncle Wally hits a "charge-coupled device," or CCD. A typical CCD contains a light-sensitive semiconductor rectangle, usually smaller than a fingernail, crisscrossed by a grid of tiny channels that divide it into several million separate picture elements, or pixels.

Each pixel emits a different number of electrons, depending on how many photons struck it, and it stores those electrons in a gizmo called a capacitor, which functions like a bucket. After the exposure is over, the CCD circuitry empties the millions of pixel buckets one by one, records the amount of charge in each, and transfers the resulting mosaic to a processor that converts it into digital form -- all in a fraction of a second. Not surprisingly, the guys from Bell Labs who invented the CCD won a 2009 Nobel Prize in Physics.

Of course, if that were all that happened, you'd only have a black-and-white picture. But to photograph your gift from Aunt Myrna, who somehow found a sweater so lurid that it can be seen from space, you want color. There are a few ways to get hues you can use, and they all rely on the convenient truth that all the shades we recognize can be represented by various proportions of red, green and blue, the "RGB" of computer monitor fame.

Unless you've got a high-end camcorder, your gear probably has a single CCD whose grid is covered by an exactly matching grid of color filters arranged in a repeating pattern. For every two-by-two set of four pixels, one is covered by a blue filter, one by a red filter and two (at opposite corners) by green filters. Doubling up on green is needed because the human eye evolved to be disproportionately sensitive to that color, which is right in the middle of the sun's visible spectrum.

The CCD records the electron count on each set of four pixels, and then the camera's on-board computer compares the value of each pixel in the foursome to that of its three neighbors to calculate the "true" color of each one. Considering that these are software-generated approximations and not actual measured colors, the accuracy is astonishing. And the range is equally impressive: customarily at least 256 levels of R, G and B in each pixel, for a total of 16.7 million different colors.

If you've got a still camera that cost more than a case of cat food, it probably has 6 million to 25 million pixels, or six to 25 megapixels in photo argot. How does that stack up to film? The finest 35-mm film in the best cameras using incomparable lenses produces images that can "resolve" (that is, show the difference between) somewhere around 90 million separate spots. A lot of that detail, however, would never be noticed by the human eye unless the photo was blown up to drive-in movie dimensions. A reasonable benchmark is that a good film picture is equivalent to about 20 megapixels.

But the whole megapixel mania that is used to market digital cameras can be awfully misleading, especially in the case of the pocket-size models. For one thing, if you don't have a good enough lens or a CCD sophisticated enough to capture fine differences in contrast and tone, it doesn't matter how many megapixels you've supposedly got. You'll just get a more expensive blur.

For another, most people don't enlarge their photos to the point at which the difference between six and 10, or 12 and 16 megapixels is important. And if you pass your pictures around on the Internet, they probably won't display at much over 100 dots per inch anyway -- about one-third the resolution of an ordinary print. For most folks, gross pixel count is more about self-image than photographic image. But who needs an ego boost when you've mastered quantum mechanics?