A scientist of Indian origin has created new ’superatoms’ with magnetic properties for the first time.

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London, June 27 (ANI): A team of researchers led by a scientist of Indian origin has created new ’superatoms’ with magnetic properties for the first time, a breakthrough that could be used to make “spintronic devices”, faster computer processors and denser memory storage.
According to a report in New Scientist, the research was led by Shiv Khanna from Virginia Commonwealth University.
Superatoms were discovered in the 1980s when Walter Knight and colleagues at the University of California, Berkeley, found that groups of sodium atoms can share electrons amongst themselves.
The electrons form a collective “supershell” that coats the cluster.
Until now, clusters that copy the magnetic properties of other elements have proved more difficult to design.
Magnetism is caused by the spin of an atom’s electrons, which are arranged in shells, or orbitals, around the atom’s nucleus.
Their net spin determines the strength of the atom’s magnetic “moment,” and because they tend to occur in pairs that cancel each other out, it is the atom’s unpaired electrons that contribute to its magnetic moment.
Unpaired electrons, however, will make an atom, or a superatom, more likely to react with others in an attempt to fill its orbitals and become stable.
As a result, stability and magnetism have long been thought to be mutually exclusive.
A team led by Shiv Khanna at Virginia Commonwealth University has come up with a way around the problem.
Khanna’s team worked out that encapsulating an atom of vanadium in a cage of eight caesium atoms would create a stable supershell of electrons around the entire cluster.
This would prevent the vanadium atom’s unpaired electrons from reacting with other atoms, maintaining its magnetism.
The arrangement would yield a magnetic moment of five Bohr magnetons, which is the same as an atom of manganese.
“What we have done is expand the range of possible magnetic materials,” said Khanna.
Khanna’s magnetic superatoms are only calculations at this point, but he has funding from the Department of Energy to make them a reality.
He hopes the clusters can be used to give researchers a new dimension of control in designing new materials.
For example, stable magnetic clusters could one day be used in new “spintronic” devices, which compute or store information using magnetic moments rather than simply electrical charge.
Encoding data in this way means the devices can be far smaller than those used to make conventional electronic components, potentially providing an overall boost in computing power. (ANI)

A Higgs boson without the mess.

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Particle physicists at CERN’s Large Hadron Collider (LHC) hope to discover the Higgs boson amid the froth of particles born from proton-proton collisions. Results in the 19 June Physical Review Letters show that there may be a way to cut through some of that froth. An experiment at Fermilab’s proton-antiproton collider in Illinois has identified a rare process that produces matter from the intense field of the strong nuclear force but leaves the proton and antiproton intact. There’s a chance the same basic interaction could give LHC physicists a cleaner look at the Higgs.
A proton is always surrounded by a swarm of ghostly virtual photons and gluons associated with the fields of the electromagnetic and strong nuclear forces. Researchers have predicted that when two protons (or a proton and an antiproton) fly past one another at close range, within about a proton’s diameter, these virtual particle clouds may occasionally interact to create new, real (not virtual) particles. The original protons would merely lose some momentum and separate from the beam. Such an “exclusive” reaction–where the original particles don’t break apart–gives unusually clean data because there are so few particles to detect.
In the new experiment, researchers were looking for signs that the interaction of virtual gluons had generated short-lived particles including the Χc (”Chi-c”) and J/ψ mesons, which are charm-anticharm quark pairs that decay into muons and antimuons. The Χc reaction would be especially rare because it requires protons to donate two gluons each, a requirement that also makes detailed predictions challenging, says Fermilab’s Mike Albrow, a member of the Collider Detector at Fermilab (CDF) collaboration.
In 2007, CDF researchers observed hints of exclusive, virtual gluon reactions in the form of high-energy photons radiating from colliding protons and antiprotons. Now the team has sifted through nearly 500 muon-antimuon pairs, identifying 65 that must have come from the decay of the Χc–very close to the rate predicted in 2005 by a team at Durham University in England. Because the Χc has similar particle properties to the much heavier Higgs boson, the same basic reaction should produce the Higgs at the higher collision energies provided by the LHC, says Albrow. “It’s the strongest evidence that the Higgs boson must be produced this way, if it does exist.”
Based on the rate of Χc production, Albrow estimates LHC collisions could produce 100 to 1000 Higgs bosons per year in each of the accelerator’s two largest particle detectors, ATLAS and CMS. “Even a few dozen events per year would enable you to measure the [Higgs's] mass, spin, and other properties,” he says. That’s why ATLAS and CMS teams are reviewing proposals to add detectors to look for exclusive Higgs events.
But not everyone is so optimistic that these events would be detectable in significant numbers. “It looks hard, but one should never say never,” says Joseph Incandela of the University of California, Santa Barbara, deputy physics coordinator for CMS. Incandela points out that once the LHC is operating at full capacity, every crossing of its twin proton beams is expected to yield about 20 collisions, throwing up other particles that may obscure exclusive reactions. But he says there are scenarios such as supersymmetry, a proposed extension to the standard model (the textbook theory of particle physics) in which there could be multiple Higgs bosons. In those situations, Albrow adds, exclusive reactions might be the only ones clean enough to distinguish the different Higgs particles.
by JR Minkel
JR Minkel is a freelance science writer in Nashville, Tennessee. His first book, Instant Egghead Guide: The Universe, comes out in July.
This story was first published in Physical Review Focus and is copyright American Physical Society. Reprinted with permission.
For more information on exclusive events, see the CERN Courier.

Chemists Form World's Smallest Droplet Of Acid

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ScienceDaily (June 22, 2009) — Exactly four water molecules and one hydrogen chloride molecule are necessary to form the smallest droplet of acid. This was the result of work by the groups of Prof. Dr. Martina Havenith (physical chemistry) and Prof. Dr. Dominik Marx (theoretical chemistry) within the research group FOR 618. They have carried out experiments at ultracold temperatures close to absolute zero temperature using infrared laser spectroscopy to monitor the molecules.

This has been accompanied by theoretical ab initio simulations. According to their calculations, the reaction at these extremely cold temperatures is only possible if the molecules are aggregating one after the other.
Chemistry at ultracold temperatures in space
If you put a classical acid, for example hydrogen chloride in water, the acid molecules will preferentially lose a proton (H+). Thereby the pH-value of the solution is decreased and the solution becomes acidic. In particular, so-called hydronium ions (H3O+), are formed by protonated water molecules. This hydronium ion is an important ingredient in many chemical reactions. Despite of the fact that this is one of the most fundamental reactions, it was not clear until now how many water molecules are actually required in order to form a charge separated negative Cl- ion and a positive H3O+ ion. “Whereas we all know acids from our daily life, we have now been able to observe for the first time acid formation on a molecular level.” "We will need this knowledge in order to understant chemical processes on nanoscopic structures, on small particles and on surfaces” explains Prof. Havenith-Newen. This indicates that there is a rich chemistry even at very low temperatures; a fundamental basis for reactions within stratospheric clouds or in interstellar media. Previously, it had been unclear whether reactions with only a few water molecules can take place at theses ultracold temperatures.
Ultracold trap
For their experiments, the researchers have successively embedded hydrogen chloride as well as single water molecules in a special ultracold trap. They used nanodroplets of suprafluid helium which have a temperature of less than -272,8 °C. Molecules will first be cooled down before they have a chance to aggregate. “Suprafluid” is a special property of the helium which implies that the embedded molecules are still free to rotate before they are frozen, thereby allowing monitoring with unsurpassed precision. Captured in such a way, it is possible to obtain the chemical fingerprint of the acid – its infrared spectrum. By combining trapping with high resolution IR laser spectroscopy and theoretical calculations, the chemists demonstrated that exactly four water molecules are required to form the smallest droplet of acid: (H3O)+(H2O)3Cl-.
Important: One molecule after the other
After these results, the researchers were left with the question of how this reaction can take place at ultracold temperatures near absolute zero. “Usually, activation of chemical reactions requires the input of energy, just like for cooking at home you need a cooking plate or a gas flame” explains Prof. Marx. “However, how should this be possible at a few Kelvin (close to absolute zero)?” The calculations, in combination with experiment, showed that the reaction is only possible by a successive aggregation process. Instead of putting together 4 water molecules and an HCl molecule simultanesously at the beginning and the waiting for a dissociation process to occur, they found in their simulations that when adding the water molecules step by step, a proton is transferred exactly when adding the fourth water molecule. Then, a hydronium ion will immediately form with one of the four added water molecules. This unusual mechanism is called “aggregation induced dissociation”. “We suspect that such aggregation induced reactions, can explain chemical transformations at ultracold conditions, such as can be found at small ice particles in clouds and in interstellar media”, explains Prof. Marx.
The work which described here is part of the research unit FOR 618 “Understanding the Aggregation of Small Molecules with Precise Methods - Interplay between Experiment and Theory”( Co-ordinator: Prof. Dr. Wolfram Sander (Faculty of Chemistry and Biochemistry) which has been funded by the Germany Science Foundation and which has just been extended for three more years after successful evaluation.
Journal reference:
Anna Gutberlet, et al. Below 1 K: The Smallest Droplet of Acid Aggregation-Induced Dissociation of HCl(H2O)4. Science, 324, 1545 (2009) DOI: 10.1126/science.1171753
Adapted from materials provided by Ruhr-Universitaet-Bochum, via AlphaGalileo.

ATLAS (LHC,CERN) e-News: Category 1, on shift

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To get any physics out of ATLAS, we must manage the data that will course through its cables, from the shifts in the control room to data distribution and software. Christophe Clement, Run Coordinator of ATLAS, describes this important work as less visible since it doesn’t directly result in papers. “And there’s a lot of it,” he adds. “Nevertheless, this is the work that really makes you feel you are carrying out an experiment which has to do with reality.”Control room tasks make up only about 13 per cent of the operation activities, according to Steinar Stapnes, who deals with Operation Task overall planning in ATLAS almost daily, yet they are essential. “Any failure in coverage can have bad consequences, perhaps for hardware and certainly for data taking,” he says.Each institute needs to take its turn on shift; this critical work cannot be compensated for with other contributions that are easier to accomplish remotely or require less diligent attention. For this reason, the Operation Task Planning group has split the operation tasks into two categories: 1 and 2.Category 1 tasks are the real-time operation and monitoring of detector performance, and first line of defence when problems arise, carried out by shifters in the control room and the experts who are called in at anytime of the day or night, should something go wrong. “We make sure that these very important tasks are well-covered. Everybody should feel responsible for them,” says Steinar.He also emphasizes the tradition in particle physics of making sure that graduate students and post-docs get time in the driver’s seat. “For most young people, it’s incredibly interesting, educational, and rewarding for them to get the experience of being part of the team that operates the detector,” says Steinar.Anything beyond the shifts will be Category 2. This includes data acquisition and core software development and maintenance, databases, calibrations, managing data distribution through the Grid, recalibration of data as the detectors are better understood, and software tuning. Category 2 also comprises other tasks associated with processing the data for analysis and those related to longer-term hardware and software maintenance at Point 1.Along with the special designation for shifts and on-call time, the scheduling system has changed. Run Coordination wants to foster a team spirit among the shifters, bringing groups together multiple times over the course of a week. Christophe explained that in this team-based system, shifters: “Get to know the other crew members better, make new contacts, and become more confident with the operation of their sub-detector. Basically work more as a crew.” “It’s not something new in some sense; other experiments have done similar things,” says Steinar, “but it is different with ATLAS because of a larger crew and a large collaboration.”Since collaborators may only work shifts for a maximum of six consecutive days, Run Coordination tried to make a schedule with eight-day blocks, each shifter taking one day off during the block. However, this was in the end deemed too rigid, both for people travelling to CERN to do shifts and for CERN residents. This resulted in a spontaneously generated version with three- and four-day blocks. An unintended consequence was that visiting physicists were inclined to take two blocks in succession, resulting in seven consecutive shifts. CERN safety regulations must be respected, so Run Coordination adjusted the system to allow shifters to choose two consecutive three-day shifts.

This system is also designed to respect the experts who are on-call. In the pattern above, those who work over the weekend have at least a Friday’s worth of experience. This way, the teams are more likely to be able to handle problems without calling in an expert. Christophe notes that on the first day, shifters tend to do some re-learning, but during the rest of a block: “you’ve done this yesterday, and you know what is the problem and how to fix it.”Also, those who take night shift must have recent experience on a day or evening shift, to avoid exhausting the on-call experts with midnight questions and visits to the Control Room. Between the category definitions and block scheduling for shifts, the running of the ATLAS detector should be smooth and effective, with each institution carrying its weight on the front lines.The system starts up Week 26 (28 June to 4 July). For more information or to book a shift, check the webpage.

Katie McAlpine
ATLAS e-News

'Colossal' Magnetic Effect Under Pressure

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The structure models for F-type and A-type magnetic ordering in manganite in response to pressure. The arrows inside orbitals indicate the spin direction of d electrons.

(PhysOrg.com) -- Millions of people today carry around pocket-sized music players capable of holding thousands of songs, thanks to the discovery 20 years ago of a phenomenon known as the “giant magnetoresistance effect,” which made it possible to pack more data onto smaller and smaller hard drives. Now scientists are on the trail of another phenomenon, called the “colossal magnetoresistance effect” (CMR) which is up to a thousand times more powerful and could trigger another revolution in computing technology.

Understanding, and ultimately controlling, this effect and the intricate coupling between electrical conductivity and magnetism in these materials remains a challenge, however, because of competing interactions in manganites, the materials in which CMR was discovered. In the June 12, 2009, issue of the journal Physical Review Letters, a team of researchers report new progress in using high pressure techniques to unravel the subtleties of this coupling.
To study the magnetic properties of manganites, a form of manganese oxide, the research team, led by Yang Ding of the Carnegie Institution’s High Pressure Synergetic Center (HPSync), applied techniques called x-ray magnetic circular dichroism (XMCD) and angular-dispersive diffraction at the Advanced Photon Source (APS) of Argonne National Laboratory in Illinois. High pressure XMCD is a newly developed technique that uses high-brilliance circularly polarized x-rays to probe the magnetic state of a material under pressures of many hundreds of thousands of atmospheres inside a diamond anvil cell.
The discovery of CMR in manganite compounds has already made manganites invaluable components in technological applications. An example is magnetic tunneling junctions in soon-to-be marketed magnetic random access memory (MRAM), where the tunneling of electrical current between two thin layers of manganite material separated by an electrical insulator depends on the relative orientation of magnetization in the manganite layers. Unlike conventional RAM, MRAM could yield instant-on computers. However, no current theories can fully explain the rich physics, including CMR effects, seen in manganites.
“The challenge is that there are competing interactions in manganites among the electrons that determine magnetic properties,” said Ding. “And the properties are also affected by external stimuli, such as, temperature, pressure, magnetic field, and chemical doping.”
“Pressure has a unique ability to tune the electron interactions in a clean and theoretically transparent manner,” he added. “It is a direct and effective means for manipulating the behavior of electrons and could provide valuable information on the magnetic and electronic properties of manganite systems. But of all the effects, pressure effects have been the least explored.”

The researchers found that when a manganite was subjected to conditions above 230,000 times atmospheric pressure it underwent a transition in which its magnetic ordering changed from a ferromagnetic type (electron spins aligned) to an antiferromagnetic type (electron spins opposed). This transition was accompanied by a non-uniform structural distortion called the Jahn-Teller effect.
“It is quite interesting to observe that uniform compression leads to a non-uniform structural change in a manganite, which was not predicted by theory,” said Ding, “Working with Michel van Veenendaal’s theoretical group at APS, we found that the predominant effect of pressure on this material is to increase the strength of an interaction known as superexchange relative to another known as the double exchange interaction. A consequence of this is that the overall ferromagnetic interactions in the system occur in a plane (two dimensions) rather than in three dimensions, which produces a non-uniform redistribution of electrons. This leads to the structural distortion.”
Another intriguing response of manganite to high pressure revealed by the experiments is that the magnetic transition did not occur throughout the sample at the same time. Instead, it spread incrementally.
“The results imply that even at ambient conditions, the manganite might already have two separate magnetic phases at the nanometer scale, with pressure favoring the growth of the antiferro-magnetic phase at the expense of the ferromagnetic phase,” said coauthor Daniel Haskel, a physicist at Argonne’s APS. “Manipulating phase separation at the nanoscale level is at the very core of nanotechnology and manganites provide an excellent playground to pursue this objective”.
“This work not only displays another interesting emergent phenomenon arising from the interplay between charge, spin, orbital and lattice in a strongly correlated electron system,” commented coauthor Dr. Ho-kwang Mao of Carnegie’s Geophysical Laboratory, Director of HPSync,” but it also manifests the role of pressure in magnetism studies of dense matter.”
More information: Pressure-induced magnetic transition in manganite (La0.75Ca0.25MnO3) Yang Ding, Daniel Haskel, Yuan-Chieh Tseng, Eiji Kaneshita, Michel van Veenendaal, John Mitchell, Stanislav V. Sinogeikin, Vitali Prakapenka, and Ho-kwang Mao, Physical Review Letters, June 2009.

Provided by Carnegie Institution

Scientists Demonstrate All-fiber Quantum Logic

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ScienceDaily (June 4, 2009) — A team of physicists and engineers have demonstrated all-fiber quantum logic, where single photons are generated and used to perform the contolled-NOT quantum logic gate in optical fibers with high fidelity.

The only quantum technology in practical use today is quantum cryptography and is currently limited in the distance over which secure communication may occur.
More sophisticated quantum networks will require multiple nodes with the ability to implement small-scale quantum processing in order to increase the range of quantum communications. Such networks will rely on optical fiber links, making fiber-based photon generation and information processing of key technological importance.
Jeremy O’Brien, Professor of Physics and Electrical Engineering at Bristol University and colleagues, have shown it is possible for a high-fidelity fiber controlled-NOT gate to operate with fiber heralded single-photon sources.
Professor O’Brien speaking about the research, said: “On the basis of a simple model we are able to conclude that imperfections are primarily due to the photon sources, meaning that the gate itself works with very high fidelity.”
“Such all fiber quantum information processing will likely have important applications in future quantum networks.”
All-fiber quantum information processing could be used in less mature quantum technologies such as computing, communication and advanced measurement, as well as in the fundamental science of quantum optics.
The team reported its results in the March 2009 issue of Physical Review A (Vol 79, No 3).
Journal reference:
Alex S. Clark, Jérémie Fulconis, John G. Rarity, William J. Wadsworth, and Jeremy L. O%u2018Brien. All-optical-fiber polarization-based quantum logic gate. Physical Review A, 2009; 79 (3): 030303 DOI: 10.1103/PhysRevA.79.030303
Adapted from materials provided by University of Bristol.

Regular Light Bulbs Made Super-Efficient with Ultra-Fast Laser

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Chunlei Guo stands in front of his femtosecond laser, which can double the efficiency of a regular incandescent light bulb. Credit: University of Rochester
(PhysOrg.com) -- An ultra-powerful laser can turn regular incandescent light bulbs into power-sippers, say optics researchers at the University of Rochester. The process could make a light as bright as a 100-watt bulb consume less electricity than a 60-watt bulb while remaining far cheaper and radiating a more pleasant light than a fluorescent bulb can.

The laser process creates a unique array of nano- and micro-scale structures on the surface of a regular tungsten filament—the tiny wire inside a light bulb—and theses structures make the tungsten become far more effective at radiating light.
The findings will be published in an upcoming issue of the journal Physical Review Letters.
"We've been experimenting with the way ultra-fast lasers change metals, and we wondered what would happen if we trained the laser on a filament," says Chunlei Guo, associate professor of optics at the University of Rochester. "We fired the laser beam right through the glass of the bulb and altered a small area on the filament. When we lit the bulb, we could actually see this one patch was clearly brighter than the rest of the filament, but there was no change in the bulb's energy usage."
The key to creating the super-filament is an ultra-brief, ultra-intense beam of light called a femtosecond laser pulse. The laser burst lasts only a few quadrillionths of a second. To get a grasp of that kind of speed, consider that a femtosecond is to a second what a second is to about 32 million years. During its brief burst, Guo's laser unleashes as much power as the entire grid of North America onto a spot the size of a needle point. That intense blast forces the surface of the metal to form nanostructures and microstructures that dramatically alter how efficiently can radiate from the filament.
In 2006, Guo and his assistant, Anatoliy Vorobeyv, used a similar laser process to turn any metal pitch black. The surface structures created on the metal were incredibly effective at capturing incoming radiation, such as light.
"There is a very interesting 'take more, give more' law in nature governing the amount of light going in and coming out of a material," says Guo. Since the black metal was extremely good at absorbing light, he and Vorobyev set out to study the reverse process—that the blackened filament would radiate light more effectively as well.

"We knew it should work in theory," says Guo, "but we were still surprised when we turned up the power on this bulb and saw just how much brighter the processed spot was."
In addition to increasing the brightness of a bulb, Guo's process can be used to tune the color of the light as well. In 2008, his team used a similar process to change the color of nearly any metal to blue, golden, and gray, in addition to the black he'd already accomplished. Guo and Vorobeyv used that knowledge of how to control the size and shape of the nanostructures—and thus what colors of light those structures absorb and radiate—to change the amount of each wavelength of light the tungsten filament radiates. Though Guo cannot yet make a simple bulb shine pure blue, for instance, he can change the overall radiated spectrum so that the tungsten, which normally radiates a yellowish light, could radiate a more purely white light.
Guo's team has even been able to make a filament radiate partially polarized light, which until now has been impossible to do without special filters that reduce the bulb's efficiency. By creating nanostructures in tight, parallel rows, some light that emits from the filament becomes polarized.
The team is now working to discover what other aspects of a common light bulb they might be able to control. Fortunately, despite the incredible intensity involved, the femtosecond laser can be powered by a simple wall outlet, meaning that when the process is refined, implementing it to augment regular light bulbs should be relatively simple.
Guo is also announcing this month in Applied Physics Letters a technique using a similar femtosecond laser process to make a piece of metal automatically move liquid around its surface, even lifting a liquid up against gravity.
Source: University of Rochester (news : web)

Super-efficient Transistor Material Predicted

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(PhysOrg.com) -- New work by condensed-matter theorists at the Stanford Institute for Materials and Energy Science at SLAC National Accelerator Laboratory points to a material that could one day be used to make faster, more efficient computer processors.

In a paper published online Sunday in Nature Physics, SIMES researchers Xiao-Liang Qi and Shou-Cheng Zhang, with colleagues from the Chinese Academy of Sciences and Tsinghua University in Beijing, predict that a room temperature material will exhibit the quantum spin Hall effect. In this exotic state of matter, electrons flow without dissipating heat, meaning a transistor made of the material would be drastically more efficient than anything available today. This effect was previously thought to occur only at extremely low temperatures. Now the race is on to confirm the room-temperature prediction experimentally.
Zhang has been one of the leading physicists working on the quantum spin Hall effect; in 2006 he predicted its existence in mercury telluride, which experimentalists confirmed a year later. However, the mercury telluride had to be cooled by liquid helium to a frigid 30 millikelvins, much too cold for real-world applications.
In their hunt for a material that exhibited the quantum spin Hall effect, Zhang and Qi knew they were looking for a solid with a highly unusual energy landscape. In a normal semiconductor, the outermost electrons of an atom prefer to stay in the valence band, where they are orbiting atoms, rather than the higher-energy conduction band, where they move freely through the material. Think of the conduction band as a flat plain pitted with small valence-band valleys. Electrons naturally "roll" down into these valleys and stay there, unless pushed out. But in a material that exhibits the quantum spin Hall effect, this picture inverts; the valence-band valleys rise to become hills, and the electrons roll down to roam the now lower-energy conduction band plain. In mercury telluride, this inversion did occur, but just barely; the hills were so slight that a tiny amount of energy was enough to push the electrons back up, meaning the material had to be kept extremely cold.
When Zhang, Qi and their colleagues calculated this energy landscape for four promising materials, three showed the hoped-for inversion. In one, bismuth selenide, the theoretical conduction band plain is so much lower than the valence band hills that even room temperature energy can't push the electrons back up. In physics terms, the conduction band and valence band are now inverted, with a sizeable difference between them.

"The difference [from mercury telluride] is that the gap is much larger, so we believe the effect could happen at room temperature," Zhang explained.
Materials that exhibit the quantum spin Hall effect are called topological insulators; a chunk of this material acts like an empty metal box that's completely insulating on the inside, but conducting on the surface. Additionally, the direction of each electron's movement on the surface decides its spin, an intrinsic property of electrons. This leads to surprising consequences.
Qi likens electrons traveling through a metal to cars driving along a busy road. When an electron encounters an impurity, it acts like a frustrated driver in a traffic jam, and makes a U-turn, dissipating heat. But in a topological insulator, Qi said, "Nature gives us a no U-turn rule." Instead of reversing their trajectories, electrons cruise coolly around impurities. This means the quantum spin Hall effect, like superconductivity, enables current to flow without dissipating energy, but unlike superconductivity, the effect doesn't rely on interactions between electrons.
Qi points out that, because current only flows on their surfaces, topological insulators shouldn't be seen as a way to make more efficient power lines. Instead, these novel compounds would be ideal for fabricating tinier and tinier transistors that transport information via electron spin.
"Usually you need magnets to inject spins, manipulate them, and read them out," Qi said. "Because the current and spin are always locked [in a topological insulator], you can control the spin by the current. This may lead to a new way of designing devices like transistors."
These tantalizing characteristics arise from underlying physics that seems to marry relativity and condensed matter science. Zhang and Qi's paper reveals that electrons on the surface of a topological insulator are governed by a so-called "Dirac cone," meaning that their momentum and energy are related according to the laws of relativity rather than the quantum mechanical rules that are usually used to describe electrons in a solid.
"On this surface, the electrons behave like a relativistic, massless particle," Qi said. "We are living in a low speed world here, where nothing is relativistic, but on this boundary, relativity emerges."
"What are the two greatest physics discoveries of the last century? Relativity and quantum mechanics." Zhang said. "In the semiconductor industry in the last 50 years, we've only used quantum mechanics, but to solve all these interesting frontier problems, we need to use both in a very essential way."
Zhang and Qi's new predictions are already spurring a surge of experiments to test whether these promising materials will indeed act as room-temperature topological insulators.
"The best feedback you can get is that there are lots of experiments going on," he said.
More information: http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1270.html
Provided by SLAC National Accelerator Laboratory (news : web)

Researchers develop new method for producing transparent conductors

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(PhysOrg.com) -- Researchers at UCLA have developed a new method for producing a hybrid graphene-carbon nanotube, or G-CNT, for potential use as a transparent conductor in solar cells and consumer electronic devices. These G-CNTs could provide a cheaper and much more flexible alternative to materials currently used in these and similar applications.

Yang Yang, a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science and a member of UCLA's California NanoSystems Institute (CNSI), and Richard Kaner, a UCLA professor of chemistry and biochemistry and a CNSI member, outline their new processing method in research published today in Nano Letters, a journal of the American Chemical Society.
Transparent conductors are an integral part of many electronic devices, including flat-panel televisions, plasma displays and touch panels, as well as solar cells. The current gold standard for transparent conductors is indium tin oxide (ITO), which has several limitations. ITO is expensive, both because of its production costs and a relative scarcity of indium, and it is rigid and fragile.
The G-CNT hybrid, the researchers say, provides an ideal high-performance alternative to ITO in electronics with moving parts. Graphene is an excellent electrical conductor, and carbon nanotubes are good candidates for transparent conductors because they provide conduction of electricity using very little material. Yang and Kaner's new single-step method for combining the two is easy, inexpensive, scalable and compatible with flexible applications. G-CNTs produced this way already provide comparable performance to current ITOs used in flexible applications.
The new method builds on Yang and Kaner's previous research, published online in November 2009, which introduced a method for producing graphene, a single layer of carbon atoms, by soaking graphite oxide in a hydrazine solution. The researchers have now found that placing both graphite oxide and carbon nanotubes in a hydrazine solution produces not only graphene but a hybrid layer of graphene and carbon nanotubes.
"To our knowledge this is the first report of dispersing CNTs in anhydrous hydrazine," Yang said. "This is important because our method does not require the use of surfactants, which have traditionally been used in these solution processes and can degrade intrinsic electronic and mechanical properties."
G-CNTs are also ideal candidates for use as electrodes in polymer solar cells, one of Yang's main research projects. One of the benefits of polymer, or plastic, solar cells is that plastic is flexible. But until an alternative to ITOs, which lose efficiency upon flexing, can be found, this potential cannot be exploited. G-CNTs retain efficiency when flexed and also are compatible with plastics. Flexible solar cells could be used in a variety of materials, including the drapes of homes.
"The potential of this material (G-CNT) is not limited to improvements in the physical arrangements of the components," said Vincent Tung, a doctoral student working jointly in Yang's and Kaner's labs and the first author of the study. "With further work, G-CNTs have the potential to provide the building blocks of tomorrow's optical electronics."
Source: University of California - Los Angeles

New element found to be a superconductor

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(PhysOrg.com) -- Of the 92 naturally occurring elements, add another to the list of those that are superconductors. James S. Schilling, Ph.D., professor of physics in Arts & Sciences at Washington University in St. Louis, and Mathew Debessai — his doctoral student at the time — discovered that europium becomes superconducting at 1.8 K (-456 °F) and 80 GPa (790,000 atmospheres) of pressure, making it the 53rd known elemental superconductor and the 23rd at high pressure.

Debessai, who receives his doctorate in physics at Washington University's Commencement May 15, 2009, is now a postdoctoral research associate at Washington State University.
"It has been seven years since someone discovered a new elemental superconductor," Schilling said. "It gets harder and harder because there are fewer elements left in the periodic table."
This discovery adds data to help improve scientists' theoretical understanding of superconductivity, which could lead to the design of room-temperature superconductors that could be used for efficient energy transport and storage.
The results are published in the May 15, 2009, issue of Physical Review Letters in an article titled "Pressure-induced Superconducting State of Europium Metal at Low Temperatures."
Schilling's research is supported by a four-year $500,000 grant from the National Science Foundation, Division of Materials Research.
Europium belongs to a group of elements called the rare earth elements. These elements are magnetic; therefore, they are not superconductors.
"Superconductivity and magnetism hate each other. To get superconductivity, you have to kill the magnetism," Schilling explained.
Of the rare earths, europium is most likely to lose its magnetism under high pressures due to its electronic structure. In an elemental solid almost all rare earths are trivalent, which means that each atom releases three electrons to conduct electricity.
"However, when europium atoms condense to form a solid, only two electrons per atom are released and europium remains magnetic. Applying sufficient pressure squeezes a third electron out and europium metal becomes trivalent. Trivalent europium is nonmagnetic, thus opening the possibility for it to become superconducting under the right conditions," Schilling said.
Schilling uses a diamond anvil cell to generate such high pressures on a sample. A circular metal gasket separates two opposing 0.17-carat diamond anvils with faces (culets) 0.18 mm in diameter. The sample is placed in a small hole in the gasket, flanked by the faces of the diamond anvils.

Pressure is applied to the sample space by inflating a doughnut-like bellow with helium gas. Much like a woman in stilettos exerts more pressure on the ground than an elephant does because the woman's force is spread over a smaller area, a small amount of helium gas pressure (60 atmospheres) creates a large force (1.5 tons) on the tiny sample space, thus generating extremely high pressures on the sample.
Unique electrical, magnetic properties
Superconducting materials have unique electrical and magnetic properties. They have no electrical resistance, so current will flow through them forever, and they are diamagnetic, meaning that a magnet held above them will levitate.
These properties can be exploited to create powerful magnets for medical imaging, make power lines that transport electricity efficiently or make efficient power generators.
However, there are no known materials that are superconductors at room temperature and pressure. All known superconducting materials have to be cooled to extreme temperatures and/or compressed at high pressure.
"At ambient pressure, the highest temperature at which a material becomes superconducting is 134 K (-218 °F). This material is complex because it is a mixture of five different elements. We do not understand why it is such a good superconductor," Schilling said.
Scientists do not have enough theoretical understanding to be able to design a combination of elements that will be superconductors at room temperature and pressure. Schilling's result provides more data to help refine current theoretical models of superconductivity.
"Theoretically, the elemental solids are relatively easy to understand because they only contain one kind of atom," Schilling said. "By applying pressure, however, we can bring the elemental solids into new regimes, where theory has difficulty understanding things.
"When we understand the element's behavior in these new regimes, we might be able to duplicate it by combining the elements into different compounds that superconduct at higher temperatures."
Schilling will present his findings at the 22nd biennial International Conference on High Pressure Science and Technology in July 2009 in Tokyo, Japan.
Provided by Washington University in St. Louis (news : web)

Too much entanglement can destroy the power of quantum computers!

SOURCE

Computers that exploit quantum effects appear capable of outperforming their classical brethren. For example, a quantum computer can efficiently factor a whole number, while there is no known algorithm for our modern classical computers to efficiently perform this task [1]. Given this extra computational punch, a natural question to ask is “What gives quantum computers their added computational power?” This question is intrinsically hard—try asking yourself where the power of a traditional classical computer comes from and you will find yourself pondering questions at the heart of the vast and challenging field known as computational complexity. In spite of this, considerable success has been made in answering the question of when a quantum system is not capable of offering a computational speedup. A particularly compelling story has emerged from the study of entanglement—a peculiar quantum mechanical quality describing the interdependence of measurements made between parts of a quantum system. This work has shown that a quantum system without enough entanglement existing at some point in the process of a computation cannot be used to build a quantum computer that outperforms a classical computer [2]. Since entangled quantum systems cannot be replicated by local classical theories, the idea that entanglement is required for speedup seems very natural. But now two groups [3, 4] have published papers in Physical Review Letters that put forth a surprising result: sometimes too much entanglement can destroy the power of quantum computers!
Both papers focus on a model called the “one-way quantum computer,” which was invented by Hans Briegel and Robert Raussendorf in 2001 [5]. A one-way quantum computation begins with a special quantum state entangled across many quantum subsystems, and the computation proceeds as a measurement is made on each subsystem. The actual form of each of the measurements in the sequence of measurements is determined by the outcome of previous measurements (Fig. 1), and one can think of the measurements as an adaptive program executed on the substrate of the entangled quantum state. A particularly nice property of the one-way quantum computing model is that it separates quantum computing into two processes—the preparation of a special initial quantum state and a series of adaptive measurements. In this way we may view the initial quantum state as a resource that can boost localized measurements and classical computation up into quantum realms. Investigations have revealed numerous quantum states that can be used as the special initial state to build a fully functioning quantum computer. But how special is this initial quantum state? Will any entangled quantum state do?
The two papers approach this problem from slightly different perspectives, but both arrive at convincing answers to these questions. David Gross at Technische Universität Braunschweig in Germany, Steven Flammia at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and Jen Eisert at the University of Potsdam, Germany, pursue this question directly in terms of entanglement [3]. They first show that if a certain quantification of entanglement—known as the geometric measure of entanglement—is too large, then any scheme that mimics the one-way quantum computation model cannot outperform classical computers. In fact, they show that the measurements in this case could be replaced by randomly flipping a coin, without significantly changing the effect of the computation. Thus while these states have a large amount of entanglement, they cannot be used to build a one-way quantum computer. Gross, Flammia, and Eisert also show that if one picks a random quantum state, it will, with near certainty, be a state that has a high value of geometric entanglement. The random states they consider are drawn via a probability distribution known as the Haar measure, which is the probability distribution that arises naturally when one insists that the probability of drawing a particular state not depend in any way on the basis of states one uses to describe a quantum system. Gross et al.’s findings show that not only do states that are too entangled to allow one-way quantum computation exist, they are actually generic among all quantum states.
Michael J. Bremner and Andreas Winter of the University of Bristol in the UK and Caterina Mora at the University of Waterloo in Canada take a slightly different route to finding states that are not useful for one-way quantum computation [4]. They begin by showing that a random quantum state (again drawn from the Haar measure) is not useful for one-way quantum computation with high probability, confirming the result of Gross et al. But they also show it is possible to choose a random quantum state from an even smaller class of states than the completely random quantum states and still end up with a state not useful for one-way quantum computation. This more limited class of states has even less entanglement (though still quite a lot) than those considered by Gross et al., but they can still be useless for one-way quantum computation.
The bottom line is that entanglement, like most good things in life, must be consumed in moderation. For the one-way quantum computation model, a randomly chosen initial state followed by adaptive measurements is not going to give you a quantum computer. Part of the reason for this, as revealed by Gross et al., is that a randomly chosen initial state has too much geometric entanglement. But even states with less entanglement may be useless for one-way quantum computation. All is according to the color of the crystal through which you look, however, one may naturally ask: What do all of these statements about the power of initial random quantum states have to do with the real world? It is thought, for example, that perfectly random quantum states (drawn from the Haar measure) cannot be produced efficiently on a quantum computer. So, while it may be that a perfectly random quantum state isn’t useful for one-way quantum computation, maybe the states that exist in nature, which can be constructed efficiently, actually are useful. It is known, for example, that the ground states of certain chains of interacting spins can be used for one-way quantum computation. A recent preprint by Richard Low [6] hints, however, that even states that exist in nature might also be in the class of useless states considered by Gross et al. and Bremner et al. In particular, Low has shown that there is a way to efficiently construct a class of entangled random quantum states that are not useful for one-way quantum computation. Thus the kinds of generic situations that both groups consider should not be ruled out because there is no physical model that efficiently prepares these states: quantum states that are impotent for one-way quantum computation may be the norm and not the exception. The implications for this on the viability of one-way quantum computation are probably not dire, but it does point out how special the states that can be useful for this model need to be—as well as the clever thinking needed to think this model up in the first place.
Finally, one can take a step back and ask “What are the implications of these results for understanding the source of the power of quantum computation?” Entanglement, in quite a real sense, is not the full answer to this question. The results of these two papers drill a deeper hole into the view of those who believe that the largeness of entanglement, and of entanglement alone, should be the useful discriminating factor between quantum and classical computation. From the perspective of theoretical computer science, this is not too surprising. One of the big open questions in this field is whether what is efficiently computable on a classical computer is the same as what is efficiently computable on a computer that operates according to different laws of the universe—a universe where a computer can nondeterministically branch (in computer science, this is known as the P versus NP question). This latter nondeterminism isn’t the kind a physicist normally thinks about. Instead it is a nondeterminism in which one can select out which of the nondeterministic branches of a universe one wishes to live in. This nondeterminism is not the way in which our universe appears to work, but it is one way the world could work (i.e., a possible set of laws of physics).
Trying to understand why our classical computers cannot efficiently compute what could be efficiently computed in these nondeterministic worlds is the holy grail of computer science research. The failure to solve this problem is similar to saying there is no known way to write down a quantity that succinctly quantifies why modern computers are different from computers that exist in the nondeterministic world. We should not be surprised, then, if there is no way to write down a quantity that quantifies why a quantum computer is powerful. After all, quantum physics is just another set of laws that operate differently than classical laws. While it is easy to view this through a negative lens, in actuality it should provide the wind behind research into quantum algorithms: there is still much to be discovered about where quantum computers might offer computational advantages over classical computers. Just be aware that creating too much entanglement followed by a series of measurements may not be the best way to get the answer.

Sodium Loses Its Luster: A Liquid Metal That's Not Really Metallic

Source:

http://www.sciencedaily.com/releases/2007/09/070926155621.htm

Science Daily — When melting sodium at high pressures, the material goes through a transition in which its electrical conductivity drops threefold.

In a series of new calculations, Lawrence Livermore National Laboratory scientists describe the unusual melting behavior of dense sodium.
"We found that molten sodium undergoes a series of pressure-induced structural and electronic transitions similar to those observed in solid sodium but beginning at a much lower pressure," said LLNL's Eric Schwegler.
Schwegler and former colleagues Stanimir Bonev, now at Dalhousie University in Nova Scotia, and Jeans-Yves Raty at FNRS-University of Liège in Belgium report the new findings in the Sept. 27 edition of the journal, Nature.
Earlier experimental measurements of sodium's melting curve have shown an unprecedented pressure-induced drop in melting temperature from 1,000 K at 30 GPa (30,000 atmospheres of pressure) down to room temperature at 120 GPa (120 million atmospheres of pressure).
Usually when a solid melts, its volume increases. In addition, when pressure is increased, it becomes increasingly difficult to melt a material.
However, sodium tells a different story.
As pressure is increased, liquid sodium initially evolves into a more compact local structure. In addition, a transition takes place at about 65 GPa that is associated with a threefold drop in electrical conductivity.
The researchers carried out a series of first-principle molecular dynamic simulations between 5 and 120 GPa and up to 1,500 K to investigate the structural and electronic changes in compressed sodium that are responsible for the shape of its unusual melting curve.
The team discovered that in addition to a rearrangement of the sodium atoms in the liquid under pressure, the electrons were transformed as well. The electronic cloud gets modified; the electrons sometimes get trapped in interstitial voids of the liquid and the bonds between atoms adopt specific directions.
"This behavior is totally new in a liquid as we usually expect that metals get more compact with pressure," Raty said.
Note: This story has been adapted from material provided by DOE/Lawrence Livermore National Laboratory.

Fausto Intilla

www.oloscience.com

Quantum Device Traps, Detects And Manipulates The Spin Of Single Electrons

Source:

http://www.sciencedaily.com/releases/2007/09/070927135543.htm

Science Daily — A novel device, developed by a team led by University at Buffalo engineers, simply and conveniently traps, detects and manipulates the single spin of an electron, overcoming some major obstacles that have prevented progress toward spintronics and spin-based quantum computing.

Published online recently in Physical Review Letters, the research paper brings closer to reality electronic devices based on the use of single spins and their promise of low-power/high-performance computing.
"The task of manipulating the spin of single electrons is a hugely daunting technological challenge that has the potential, if overcome, to open up new paradigms of nanoelectronics," said Jonathan P. Bird, Ph.D., professor of electrical engineering in the UB School of Engineering and Applied Sciences and principal investigator on the project. "In this paper, we demonstrate a novel approach that allows us to easily trap, manipulate and detect single-electron spins, in a scheme that has the potential to be scaled up in the future into dense, integrated circuits."
While several groups have recently reported the trapping of a single spin, they all have done so using quantum dots, nanoscale semiconductors that can only demonstrate spin trapping in extremely cold temperatures, below 1 degree Kelvin.
The cooling of devices or computers to that temperature is not routinely achievable, Bird said, and it makes systems far more sensitive to interference.
The UB group, by contrast, has trapped and detected spin at temperatures of about 20 degrees Kelvin, a level that Bird says should allow for the development of a viable technology, based on this approach.
In addition, the system they developed requires relatively few logic gates, the components in semiconductors that control electron flow, making scalability to complex integrated circuits very feasible.
The UB researchers achieved success through their innovative use of quantum point contacts: narrow, nanoscale constrictions that control the flow of electrical charge between two conducting regions of a semiconductor.
"It was recently predicted that it should be possible to use these constrictions to trap single spins," said Bird. "In this paper, we provide evidence that such trapping can, indeed, be achieved with quantum point contacts and that it may also be manipulated electrically."
The system they developed steers the electrical current in a semiconductor by selectively applying voltage to metallic gates that are fabricated on its surface.
These gates have a nanoscale gap between them, Bird explained, and it is in this gap where the quantum point contact forms when voltage is applied to them.
By varying the voltage applied to the gates, the width of this constriction can be squeezed continuously, until it eventually closes completely, he said.
"As we increase the charge on the gates, this begins to close that gap," explained Bird, "allowing fewer and fewer electrons to pass through until eventually they all stop going through. As we squeeze off the channel, just before the gap closes completely, we can detect the trapping of the last electron in the channel and its spin."
The trapping of spin in that instant is detected as a change in the electrical current flowing through the other half of the device, he explained.
"One region of the device is sensitive to what happens in the other region," he said.
Now that the UB researchers have trapped and detected single spin, the next step is to work on trapping and detecting two or more spins that can communicate with each other, a prerequisite for spintronics and quantum computing.
Co-authors on the paper are Youngsoo Yoon, Ph.D., a UB doctoral student in electrical engineering; L. Mourokh of Queens College and the College of Staten Island of the City University of New York; T. Morimoto, N. Aoki and Y. Ochiai of Chiba University in Japan; and J. L. Reno of Sandia National Laboratories.
The research was funded by the U.S. Department of Energy. Bird, who also has received funding from the UB Office of the Vice President for Research, was recruited to UB with a faculty recruitment grant from the New York State Office of Science, Technology and Academic Outreach (NYSTAR).
Note: This story has been adapted from material provided by University at Buffalo.

Fausto Intilla

www.oloscience.com