Scientists Reach Milestone In Study Of Emergent Magnetism

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ScienceDaily (June 28, 2009) — Scientists at the U.S. Department of Energy's Argonne National Laboratory and the University of Chicago have reached a milestone in the study of emergent magnetism.

Studying simple metallic chromium, the joint UC-Argonne team has discovered a pressure-driven quantum critical regime and has achieved the first direct measurement of a "naked" quantum singularity in an elemental magnet. The team was led by University of Chicago scientist Rafael Jaramillo, working in the group of Thomas Rosenbaum, and Argonne scientist Yejun Feng of the Advanced Photon Source.
The sophisticated spin and charge order in chromium is often used as a stand-in for understanding similar phenomena in more complex materials, such as correlated oxides proximate to a quantum critical point.
"Chromium is a simple metallic crystal that exhibits a sophisticated form of antiferromagnetism," said Jaramillo. "The goal was to find a simple system."
Quantum criticality describes a continuous phase transition that is driven by quantum mechanical fluctuations, and is thought to underlie several enigmatic problems in condensed matter physics including high-temperature superconductivity. However, achieving a continuous quantum phase transition in a simple magnet has proved to be a challenging goal, as the critical behavior in all systems studied to date has been obscured by competing phenomena. The discovery of a "naked" transition in simple chromium metal therefore paves the way for a more detailed understanding of magnetic quantum criticality
Like many elements, chromium has been extensively studied for decades and a discovery of this magnitude in an element is particularly important.
"It's not often that you find out something new in an element," Feng said.
The pressure scale and experimental techniques required to measure quantum criticality in chromium necessitated extensive technical development at both Argonne and the University of Chicago. The resulting techniques for high precision measurement of condensed matter systems at high pressure, developed for use at Sector 4 of the Advanced Photon Source, now approach a level of precision and control comparable to more conventional techniques such as magnetic varying field and temperature.
This work is reported in the May 21 issue of the journal Nature.
Funding for this research was provided by the National Science Foundation Division of Materials Research and the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
Adapted from materials provided by DOE/Argonne National Laboratory.

'Look Mom No Electricity': Transmitting Information with Chemistry

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Burning an infofuse transmits a sequence of pulses of light, in which information is encoded using different wavelengths (determined by various metallic salts) and the order of the pattern. Image credit: Samuel W. Thomas III, et al. ©2009 PNAS.

(PhysOrg.com) -- While information technology is generally thought to require electrons or photons for transmitting information, scientists have recently demonstrated a third method of transmission: chemical reactions. Based on a flammable “infofuse,” the new system combines information technology and chemistry into a new area the researchers call "infochemistry."

In the study, led by George Whitesides of Harvard University, with other coauthors from Harvard, Tufts University, and DARPA, the scientists explain that their system transmits information in the form of coded pulses of light generated entirely by chemical reactions, without electricity. The system is self-powered, with power being generated by combustion. The power density of the system is higher than that of electrochemical batteries, and has the advantage of not discharging over time.
As Whitesides explained to PhysOrg.com, the significance of the study is that it “demonstrates direct chemical to binary encoding, and transmission of information at a useful bit rate, without batteries.” The researchers hope that their prototype will one day make it possible to make systems that transmit useful information in circumstances in which electronics and batteries do not work, such as harsh environments and under water.
As the scientists explain, the system consists of a strip or fuse of combustible material (nitrocellulose) about 1 mm long. When ignited, a yellow-orange flame moves along the infofuse. To encode information, the scientists patterned the fuse with various metallic salts, which could be done using a desktop inkjet printer or a micropipettor. With their different emission wavelengths, the salts created distinct emission lines in different regions of the electromagnetic spectrum, similar to how the colors of fireworks are made: blue (copper), green (barium), yellow (sodium), red (lithium, strontium, calcium), or near-infrared (potassium, rubidium, cesium).
The infofuse, which burns at about 3-4 cm/sec depending on thickness and pattern spacing, is then read by a detector, such as a color CCD camera or fiber optic cable coupled to a spectrometer. The distance between the detector and burning infofuse was typically 2 m, but the detector could still detect a signal up to 30 m away in daylight.

By coding letters of the alphabet using patterns of metallic salts, the scientists transmitted the phrase, “LOOK MOM NO ELECTRICITY” on a single infofuse using the new technique. As the scientists explain, light pulses have several controllable variables that can be used to represent different letters and symbols. In addition to emission wavelength, other variables include pulse duration, time between pulses, and emission intensity. Using combinations of three alkali metals, the researchers demonstrated how to encode 40 different characters by varying some of these parameters.
“It needs a flame, but it does not need additional batteries or power, or auxiliary devices, to convert a chemical signal to a digital one,” Whitesides said. “The power needed to generate the light is produced by chemistry directly, not by drawing power from a battery.”
Although the current infofuses convert energy into light with only 1% of the efficiency of a battery-operated LED, the infofuses generate 10 times more energy per weight than an alkaline battery generates. In general, integrating information technology and chemistry could have certain advantages, possibly leading to systems that operate beyond binary schemes by using a variety of parameters that allow each information unit to carry more information than a bit. Also, since infochemistry is not bound by the principles of electronics (such as fixed circuitry), but rather the principles of chemistry, new systems could lead to novel architectures.
The scientists hope that further improvements to their system could lead to lightweight, portable, self-powered systems that can transmit information and integrate with modern information technologies. Applications could include environmental sensing and transmitting the data optically over a distance. The system could also be used for message transmission in search-and-rescue type applications.
More information: “Infochemistry and infofuses for the chemical storage and transmission of coded information.” Samuel W. Thomas III, et al. Proceedings of the National Academy of Sciences. vol. 106, no. 23, 9147-9150.

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)

Ion trap quantum computing

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(PhysOrg.com) -- “Right now, classical computers are faster than quantum computers,” René Stock tells PhysOrg.com. “The goal of quantum computing is to eventually speed up the time scale of solving certain important problems, such as factoring and data search, so that quantum computing can not only compete with, but far outperform, classical computing on large scale problems. One of the most promising ways to possibly do this is with ion traps.”

Stock, a post-doc at the University of Toronto, points out that ion trap quantum computing has made a lot of progress in the last 10 years. “Ions in traps have been one of most successful physical implementation of quantum computing in physical systems.” Stock believes that it is possible to use ion-trap quantum computing to create measurement-based quantum computers that could compete with classical computers for very large and complex problems - and even on smaller scale problems. His work on the subject, done with Daniel James, appears in Physical Review Letters: “Scalable, High-Speed Measurement-Based Quantum Computer Using Trapped Ions.”
“One of the most important considerations in quantum computing is the fact that quantum computing scales polynomially, rather than exponentially, as classical computing does.” This polynomial scaling is what makes quantum computing so useful for breaking data encryption. In order to make data encryption more secure, one usually increases the number of bits used. “Because of the exponential scaling, breaking data encryptions quickly becomes impossible using standard classical computers or even networks of computers,” Stock explains. “The improved scaling with quantum computers could be one a biggest threads to data encryption and security.”
While this sounds promising, Stock points this out that there are still problems with quantum information processing: “While scaling would be better with quantum computing, current operation of quantum information processing is too slow to even compete with classical computers on large factoring problems that take 5 months to solve.”
The way ion-trap quantum computing works now - or at least is envisioned to work - requires that ions be shuttled back and forth around the trap architecture. Stock explains that this takes time. “As the complexity of problems and the size of the quantum computing to be implemented increases, the time issue becomes even more important. We wanted to figure out how we could change the time scale,” Stock explains. “We found that we could speed up the processing by using an array of trapped ions and by parallelizing entangling operations.”

“Instead of moving ions around,” Stock continues, “you apply a two-ion operation between all neighboring ions at the same time. The created multipartite ‘entangled’ array of ions is a resource for quantum computing.” Actual computing is then based on measurement of ions in the array in a prescribed order and using a slightly different measurement basis for each ion. “In this scheme, it is the time required to read out information from the ions that critically determines the operational time scale of the quantum computer,” Stock says.
Stock describes the measurement component as vital to this model of quantum computing. Instead of exciting the ions and getting them to emit a photon and measuring the photon, Stock and his colleague instead devised a different way in which they were able to measure the quantum bit encoded in a calcium ion. “You can use an ionization process to speed up measurement, since the electron can be extracted faster from the atom than you can get a photon out of an atom. The extracted electron is then guided onto a detector by the ion trap itself.” All of this takes place on a nanosecond time scale. “By speeding up the measurement,” Stock insists, “we can speed up the operation capability of the quantum computer.”
Stock points out that this quantum computing scheme would be impractical as far as taking over common use from classical computers. “The lattice would have thousands of ions, which would need to be controlled, and carefully stored and protected. It means that the computer would be relatively large and impractical.”
Uses for such a quantum computer are not limited to breaking data encryption. “This process would allow us to take problems of great complexity and still solve them on a humanly possible timescale. This could provide the key to modeling complex systems - especially perhaps in biology - that we can’t solve now. This would be a tremendous advantage over classical computing.”
More information: Stock, René and James, Daniel. “Scalable, High-Speed Measurement-Based Quantum Computer Using Trapped Ions.” Physical Review Letters (2009). Available online: http://link.aps.org/doi/10.1103/PhysRevLett.102.170501 .
Copyright 2009 PhysOrg.com. All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Post-Quantum Correlations: Exploring the Limits of Quantum Nonlocality

This figure shows levels of nonlocality as measured by the CHSH Bell inequality. Classical nonlocal correlations (green) are at 2 and below; quantum nonlocal correlations (red) are above 2 but below Tsirelson’s bound (BQ); and post-quantum nonlocal correlations (light blue) are above and, in some cases, below Tsirelson’s bound. BCC marks the “bound of triviality,” above which correlations are unlikely to exist. In the current study, scientists found that post-quantum correlated nonlocal boxes (dark blue line) are also unlikely to exist, despite some boxes being arbitrarily close to being classical. Image credit: Brunner and Skrzypczyk. ©2009 APS.

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(PhysOrg.com) -- When it comes to nonlocal correlations, some correlations are more nonlocal than others. As the subject of study for several decades, nonlocal correlations (for example, quantum entanglement) exist between two objects when they can somehow directly influence each other even when separated by a large distance. Because these correlations require “passion-at-a-distance” (a term coined by physicist Abner Shimony), they violate the principle of locality, which states that nothing can travel faster than the speed of light (even though quantum correlations cannot be used to communicate faster than the speed of light). Besides being a fascinating phenomenon, nonlocality can also lead to powerful techniques in computing, cryptography, and information processing.

Quantum Limits
Despite advances in quantum research, physicists still don’t fully understand the fundamental nature of nonlocality. In 1980, mathematician Boris Tsirelson found that quantum correlations are bounded by an upper limit; quantum nonlocality is only so strong. Later, in 1994, physicists Sandu Popescu and Daniel Rohrlich made another surprising discovery: a particular kind of correlation might exist above the “Tsirelson bound,” as well as below the bound, in a certain range (see image). These so-called post-quantum correlations are therefore “more nonlocal” than quantum correlations.
“Tsirelson's bound represents the most nonlocal ‘boxes’ that can be created with quantum mechanics,” Nicolas Brunner, a physicist at the University of Bristol, told PhysOrg.com. “Nonlocality here is measured by the degree of violation of a Bell inequality. So, quantum non-locality appears to be limited. The big question is why. That is, is there a good physical reason why post-quantum correlations don’t seem to exist in nature?”
In a recent study, Brunner and coauthor Paul Skrzypczyk, also of the University of Bristol, propose an explanation for why post-quantum correlations are unlikely to exist, which may reveal insight into why quantum nonlocality is bounded, as well as into the underlying difference between quantum and post-quantum correlations.
In their study, Brunner and Skrzypczyk have shown that a certain class of post-quantum correlations is unlikely to exist due to the fact that it makes communication complexity trivial. This triviality occurs due to the fact that the nonlocality of these correlations can be enhanced beyond a critical limit, and - surprisingly - in spite of the fact that some of these correlations are arbitrarily close to classical correlations (they give an arbitrarily small violation of Bell’s inequality). As previous research has suggested, any theory in which communication complexity is trivial is very unlikely to exist.
Beyond Quantum
“’Post-quantum’ means beyond quantum,” Brunner explained. “This term applies to correlations, which are conveniently - and probably most simply - described by ‘black boxes.’ The basic idea is the following: imagine a black box shared by two distant parties Alice and Bob; each party is allowed to ask a question to the box (or make a measurement on the box, if you prefer) and then gets an answer (a measurement outcome). By repeating this procedure many times, and at the end comparing their respective results, Alice and Bob can identify what their box is doing. For instance, it could be that the outcomes are always the same whenever Alice and Bob choose the same questions. This kind of behavior is a correlation; knowing one outcome, it is possible to deduce the other one, since both outcomes are correlated.
“Now, it happens that there exist different types of correlations; basically those that can be understood with classical physics (where correlations originate from a common cause), and those that cannot. This second type of correlation is called nonlocal, in the sense that it cannot be explained by a common cause. A priori it is not obvious to tell whether some correlations are local or not. The way physicists can tell this is by testing a Bell inequality; when a Bell inequality is violated, then the correlations cannot be local; that is, there cannot exist a common cause to these correlations.
“Now, an amazing thing about quantum mechanics is that it allows one to construct boxes that are non-local. This is quantum nonlocality. Now, it happens that not all nonlocal boxes can be constructed in quantum mechanics. Thus there exist correlations which are unobtainable in quantum mechanics. These are called post-quantum correlations. In general, post-quantum correlations can be above Tsirelson’s bound, but in some very specific cases, they can also be below.”
‘Distilling’ Post-Quantum Nonlocality
To demonstrate that post-quantum correlations cannot exist in nature, Brunner and Skrzypczyk developed a protocol for deterministically distilling nonlocality in post-quantum states. That is, the technique refines weakly nonlocal states into states with greater nonlocality. In this context, “distillation” can also be thought of as “purifying,” “amplifying,” or “maximizing” the nonlocality of post-quantum correlations. Since nonlocal correlations are more useful if they are stronger, maximizing nonlocality has significant implications for quantum information protocols. The physicists’ protocol works specifically with “correlated nonlocal boxes,” which are a particular class of post-quantum boxes.
Brunner and Skrzypczyk’s distillation protocol builds on a recent breakthrough by another team (Forster et al.), who presented the first nonlocality distillation protocol just a few months ago. However, the Forster protocol can distill correlated nonlocal boxes only up to a certain point, violating a Bell inequality called the Clauser-Horne-Shimony-Holt (CHSH) inequality only up to CHSH = 3. While this value is greater than Tsirelson’s bound of 2.82, it does not reach the bound of 3.26, which marks the point at which communication complexity becomes trivial.
Taking a step forward, Brunner and Skrzypczyk’s protocol can distill nonlocality all the way up to the maximum nonlocality of the Popescu-Rohrlich box, which is 4. In passing the 3.26 bound of triviality, they show that these post-quantum correlated nonlocal boxes do indeed collapse communication complexity.
The distillation protocol is executed by two distant parties that share two weakly correlated nonlocal boxes. Each party can input one bit into a box to receive one output bit, simulating a binary input/binary output system with local operations. As the scientists explain, a distillation protocol can be viewed as a way of classically wiring the two boxes together. The protocol is a choice of four wirings, one for each input of Alice and Bob. The wiring (algorithm) that determines the outbit bits of the boxes will transform the two nonlocal boxes into a single correlated nonlocal box, which has stronger nonlocality than the two individual boxes.
Importantly, this protocol can distill any correlated nonlocal box that violates the CHSH inequality by less than a limit of 3.26 to more than 3.26. In other words, any correlated nonlocal box that has not previously made communication complexity trivial can be made to do so. Surprisingly, some of these boxes can even be arbitrarily close to being classical (below or equal to 2), and yet, since they can be distilled beyond the “bound of triviality,” they still collapse communication complexity. According to previous studies of triviality, such boxes are very unlikely to exist - even those below Tsirelson’s bound.
Trivial Complexity
Theoretically, when communication complexity is trivial, even the most complex problems can be solved with a minimum amount of communication. In the following example, Brunner explains what would happen in real life if a single bit of information could solve any problem.
“Communication complexity is an information processing task,” Brunner said. “Here is an example. Suppose you and I would like to meet during the next year; so given our respective agendas, we would like to know whether there is a day where both of us are free or whether there is not; doesn’t matter what that day is, we just want to know whether there is such a day or not.
Since we are in distant locations, we must send each other some information to solve the problem. For instance, if I send you the whole information about my agenda, then you could find out whether a meeting is possible or not (and so solve the problem). But indeed that implies that I should send you a significant quantity of information (many bits). It turns out that in classical physics (or, if you prefer, in everyday life), there is no better strategy; I really have to send you all that information. In quantum physics, though there exist stronger correlations than in classical physics (quantum nonlocal correlations), I would still have to send you an enormous amount of communication.
“Now, the really astonishing thing is that, if you have access to certain post-quantum correlations (post-quantum boxes), a single bit of communication is enough to solve this problem! In other words, communication complexity becomes trivial in these theories, since one bit of communication is enough to solve any problem like this one. Importantly, in classical or quantum physics, communication complexity is not trivial. More generally, for computer scientists, a world in which communication complexity becomes trivial is highly unlikely to exist. Previously, it was known that post-quantum boxes with a very high degree of violation of a Bell inequality make communication complexity trivial; now, the astonishing thing about our result is that we show that some correlations with a very small degree of violation of a Bell inequality - but indeed not accessible with quantum mechanics - can also make communication complexity trivial.”
Post-Quantum Future
In the future, Brunner and Skrzypczyk hope to find improved distillation protocols that might work for a wider variety of post-quantum nonlocal boxes, not only correlated nonlocal boxes. More research is also needed to explain why quantum correlations cannot exist in the gap between Tsirelson’s bound and the bound of triviality. Ultimately, this line of research could help make a distinction between quantum and post-quantum correlations, with important theoretic implications.
“The greatest implications of our results are the following,” Brunner said. “First, they give new evidence that certain post-quantum theories allow for a dramatic increase in communication power compared to quantum mechanics, and therefore appear very unlikely to exist in nature. The nice thing, in particular, is that some of these theories allow only for little nonlocality (as measured by the degree of violation of a Bell inequality). Thus our result is a striking demonstration that we still have no clue on how to correctly measure nonlocality. Finally, it is one step further towards an information-theoretic axiom for quantum mechanics.”
More information: Nicolas Brunner and Paul Skrzypczyk. “Nonlocality Distillation and Postquantum Theories with Trivial Communication Complexity.” Physical Review Letters 102, 160403 (2009).
Copyright 2009 PhysOrg.com. All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

A Light Touch

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Magnetically encoded information is at the core of much modern technology, and researchers are always looking for better ways to manipulate it. In the 8 May Physical Review Letters, a German team shows that a surprisingly feeble light beam can flip zeros to ones and vice versa, in a special magnetic layer. Although currently limited to very low temperatures, the apparently new effect might one day be extended to improve data storage.
Most familiar magnets are metals. They contain atoms that host tiny magnetic bar magnets, or moments, that can point up or down, and the atoms are surrounded by a sea of electrons. But researchers have long been interested in a different type of magnet, one consisting of widely separated magnetic ions embedded in a semiconductor. Unlike a metal, the number of free electrons in a semiconductor changes when it's exposed to electric current or light, so these materials should provide new ways to influence the magnetic properties, via the electrons. Light can flip the magnetization--the total magnetic moment of atoms in a region--from up to down, for example. But until now, experimenters needed very bright light to weaken the magnetization enough to reorient it.
In the new research, a team led by Laurens Molenkamp of the University of Würzburg in Germany grew a thin, crystalline layer of the common semiconductor gallium arsenide but replaced about one percent of the gallium atoms with the magnetic atom manganese. At temperatures below about 25 Kelvin, this layer acts as a ferromagnet: the magnetic moments on different manganese atoms point in the same direction, either up from the surface or down into it. The magnetization direction persists even when the researchers apply an opposing magnetic field, as long as the field does not exceed a threshold called the coercive field.
But when the team focused the light from a standard red laser on the film, the magnetization in the illuminated spot changed direction to match the magnetic field-- opposite to the rest of the film. The light didn’t change the strength of the magnetization, the team found, but instead reduced the field strength needed to flip it.
A mundane explanation would be that the light simply heats the film, which decreases the coercive field. To check this possibility, the team monitored the size of the magnetically flipped spot over many seconds of illumination. "It grows rather slowly," notes Würzburg team member Georgy Astakhov. "Heat diffusion occurs much, much faster." Instead, the team proposes that electrons liberated by the light (and the "holes" they leave behind) affect the magnetization directly. These charge carriers, they suggest, are quickly trapped in regions with high or low manganese concentration. These trapped charges effectively grease the motion of the "domain wall" that separates regions of opposite magnetization, by smoothing out local variations that would otherwise impede its motion. When the domain wall can move smoothly, a region with magnetization pointing up can more easily spread at the expense of a neighboring region having oppositely-directed magnetization.
Theo Rasing, of Radboud University in Nijmegen, Netherlands, says that more work is needed to confirm this non-thermal explanation, including extending the experiment to more than one sample. He also notes that because the dim light is on for a long time, the energy needed to flip the magnetization is not so different from other experiments that use very bright but short light pulses. Nonetheless, Rasing says that seeing a non-thermal change in magnetism with such a dim beam expands such "opto-magnetic" effects to new materials and mechanisms and should inspire further experiments by others.--Don Monroe Don Monroe is a freelance science writer in Murray Hill, New Jersey.
Related Information:
Physics Viewpoint essay by Molenkamp: Convincing a Magnetic Semiconductor to Work at Room Temperature (December, 2008)
Focus story on another optical magnetization flipping technique: Flipping Atoms Fast (1999)
Nonthermal Photocoercivity Effect in a Low-Doped (Ga,Mn)As Ferromagnetic Semiconductor G. V. Astakhov, H. Hoffmann, V. L. Korenev, T. Kiessling, J. Schwittek, G. M. Schott, C. Gould, W. Ossau, K. Brunne, and L. W. Molenkamp Phys. Rev. Lett. 102, 187401 (issue of 8 May 2009)

Carbon Nanotubes: Innovative Technology Or Risk To Health Or Environment?

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ScienceDaily (May 10, 2009) — Carbon nanotubes have made a meteoric career in the past 15 years, even if their applications are still limited. Recent research results show that – apart from their favorable mechanical and electrical properties – they also have disadvantageous characteristics.

One aspect which has rarely been considered so far is now addressed by researchers of the research center Forschungszentrum Dresden-Rossendorf. “If the application of products and commodities containing carbon nanotubes will increase in the future, then there will be a higher probability for the tubes to get into the environment during their production, usage or disposal, to be distributed there, and to bind pollutants such as heavy metals on their way trough the environment”, says Harald Zaenker, scientist at the FZD.
Via water into the environment
An important way for carbon nanotubes of getting into the environment is the way via the water. In their original state, the flimsy carbon fibers with a diameter of less than 50 nanometers (1 nanometer = 1 millionth of a millimeter) are hardly water-soluble. At first glance, they should therefore not be mobile in groundwater, lakes etc., i.e. they should rapidly settle or deposit. However, carbon nanotubes are able to form colloidal solutions if their surface structure is changed. Changes in the surface structure can be brought about deliberately during the production of the tubes or can be induced by natural processes if the tubes are released into the environment.
A colloidal solution, unlike a true solution of water-soluble substances, is a solution in which the apparently dissolved substance is finely dispersed in the solvent forming tiny particles. These particles are still much bigger than the molecules of a dissolved substance in a true solution. As colloids, carbon nanotubes might be transported anywhere in environmental waters. It is known meanwhile that the tubes can even penetrate cell walls and, thus, might theoretically be able to enter also animal or human cells. In addition, changes in the surface structure of carbon nanotubes cause another effect: their capability to bind heavy metals is increased.
Tubes with changed surface
The scientists investigated carbon nanotubes both in their original state and in a state changed by oxidizing acids (such as a mixture of nitric and sulfuric acid). They found out that solutions of treated carbon nanotubes scatter light more strongly. “This is an indication that colloids have formed which do not settle”, Harald Zaenker says.
The researchers provided evidence for the first time that the heavy metal uranium, which is ubiquitous in the environment and, hence, also in the water, is particularly attached to the surface of treated carbon nanotubes. The scientists found out that the uranium uptake capacity is increased by an order of magnitude in comparison to untreated carbon nanotubes. “Therefore, it is plausible to assume that carbon nanotubes, if released to the environment, influence the transport of uranium in environmental waters and even in biological systems. The possible impact on the environment and on human health has in general been considered too little”, Harald Zaenker says.
On the other hand, the high bonding capacity of carbon nanotubes for uranium and other heavy metals also suggests using them for the removal of heavy metals from waters. However, they are not yet a cost-efficient alternative to classic water purifiers, Zaenker says. “Eventually, it is important to further study the behavior of carbon nanotubes in waters”, the scientist says. “Only then can the positive and negative aspects of carbon nanotubes be better assessed.”
Journal reference:
Schierz et al. Aqueous suspensions of carbon nanotubes: Surface oxidation, colloidal stability and uranium sorption. Environmental Pollution, 2009; 157 (4): 1088 DOI: 10.1016/j.envpol.2008.09.045
Adapted from materials provided by Forschungszentrum Dresden Rossendorf.

Quantum Cascade Laser Nanoantenna Created

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http://www.sciencedaily.com/releases/2007/10/071022122223.htm

ScienceDaily (Oct. 25, 2007) — In a major feat of nanotechnology engineering researchers from Harvard University have demonstrated a laser with a wide-range of potential applications in chemistry, biology and medicine. Called a quantum cascade (QC) laser nanoantenna, the device is capable of resolving the chemical composition of samples, such as the interior of a cell, with unprecedented detail.

Spearheaded by graduate students Nanfang Yu, Ertugrul Cubukcu, and Federico Capasso, Robert L. Wallace Professor of Applied Physics, all of Harvard's School of Engineering and Applied Sciences, the findings will be published as a cover feature of the October 22 issue of Applied Physics Letters. The researchers have also filed for U.S. patents covering this new class of photonic devices.
The laser's design consists of two gold rods separated by a nanometer gap (a device known as an optical antenna) built on the facet of a quantum cascade laser, which emits invisible light in the region of the spectrum where most molecules have their tell tale absorption fingerprints. The nanoantenna creates a light spot of nanometric size about fifty to hundred times smaller than the laser wavelength; the spot can be scanned across a specimen to provide chemical images of the surface with superior spatial resolution.
"There's currently a major push to develop powerful tabletop microscopes with spatial resolution much smaller than the wavelength that can provide images of materials, and in particular biological specimens, with chemical information on a nanometric scale," says Federico Capasso.
While infrared microscopes, based on the detection of molecular absorption fingerprints, are commercially available and widely used to map the chemical composition of materials, their spatial resolution is limited by the range of available light sources and optics to well above the wavelength. Likewise the so-called near field infrared microscopes, which rely on an ultra sharp metallic tip scanned across the sample surface at nanometric distances, can provide ultrahigh spatial resolution but applications are so far strongly limited by the use of bulky lasers with very limited tunability and wavelength coverage.
"By combining Quantum Cascade Lasers with optical antenna nanotechnology we have created for the first time an extremely compact device that will enable the realization of new ultrahigh spatial resolution microscopes for chemical imaging on a nanometric scale of a wide range of materials and biological specimens," says Capasso.
Quantum cascade (QC) lasers were invented and first demonstrated by Capasso and his group at Bell Labs in 1994. These compact millimeter length semiconductor lasers, which are now commercially available, are made by stacking nanometer thick layers of semiconductor materials on top of each other. By varying the thickness of the layers one can select the wavelength of the QC laser across essentially the entire infrared spectrum where molecules absorb, thus custom designing it for a specific application.
In addition by suitable design the wavelength of a particular QCL can be made widely tunable. The range of applications of QC laser based chemical sensors is very broad, including pollution monitoring, chemical sensing, medical diagnostics such as breath analysis, and homeland security.
The teams co-authors are Kenneth Crozier, Assistant Professor of Electrical Engineering, and research associates Mikhail Belkin and Laurent Diehl, all of Harvard's School of Engineering and Applied Sciences; David Bour, Scott Corzine, and Gloria Höfler, all formerly with Agilent Technologies. The research was supported by the Air Force Office of Scientific Research and the National Science Foundation. The authors also acknowledge the support of two Harvard-based centers, the Nanoscale Science and Engineering Center and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network.
Adapted from materials provided by Harvard University.

Fausto Intilla

www.oloscience.com

Computer Memory May Leap With Solution To Chemical Mystery

Source:

http://www.sciencedaily.com/releases/2007/10/071019123619.htm

ScienceDaily (Oct. 22, 2007) — A Florida State University researcher has helped solve a scientific mystery that stumped chemists for nearly seven decades. In so doing, his team's findings may lead to the development of more-powerful computer memories and lasers.

Naresh S. Dalal, the Dirac Professor of Chemistry and Biochemistry at FSU, recently collaborated with three colleagues, Jorge Lasave, Sergio Koval and Ricardo Migoni, all of the Universidad Nacional de Rosario in Argentina, to determine why a certain type of crystal known as ammonium dihydrogen phosphate, or ADP, behaves the way it does.
"ADP was discovered in 1938," Dalal said. "It was observed to have some unusual electrical properties that weren't fully understood -- and for nearly 70 years, scientists have been perplexed by these properties. Using the supercomputer at SCRI (FSU's Supercomputer Computations Research Institute), we were able to perform in-depth computational analyses that explained for the very first time what causes ADP to have these unusual properties."
ADP, like many crystals, exhibits an electrical phenomenon known as ferroelectricity. Ferroelectric materials are analogous to magnets in that they maintain a positively charged and a negatively charged pole below a certain temperature that is characteristic for each compound.
"Ferroelectric materials can stay in a given state of charge for a long time -- they retain their charge after the external electrical source is removed," Dalal said. "This has made ADP and other materials like it very useful for storing and transmitting data.
ADP is commonly used in computer memory devices, fiber optic technology, lasers and other electro-optic applications."
What researchers found perplexing about ADP was that it often displays a very different electrical phase -- one known as antiferroelectricity.
"With antiferroelectricity, one layer of molecules in a crystal has a plus and a minus pole, but in the next layer, the charges are reversed," Dalal said. "You see this reversal of charges, layer by layer, throughout the crystal."
Using the supercomputer at SCRI enabled Dalal and his colleagues to perform numerous highly complex calculations that couldn't be duplicated in a laboratory environment. For example, they were able to theoretically alter the angles of ADP's ammonium ions and then measure the effects on the crystal's electrical charge. That approach ultimately led to their solution to the seven-decade mystery.
"We found that the position of the ammonium ions in the compound, as well as the presence of stresses or defects in the crystal, determine whether it behaves in a ferroelectric or antiferroelectric manner," Dalal said.
The team's research is important for two main reasons, Dalal said: "First, this allows us to further understand how to design new materials with both ferroelectric and antiferroelectric properties. Doing so could open new doors for computer memory technology -- and possibly play a role in the development of quantum computers.
"Second, our research opens up new ways of testing materials," Dalal said. "Using supercomputers, we can quickly perform tests to see how materials would react under a variety of conditions. Many such tests can't even be performed in the lab."
A paper "Origin of Antiferroelectricity in NH4H2PO4 from First Principles,"describing Dalal, Lasave and Migoni's findings was published recently in Physical Review Letters.
Adapted from materials provided by Florida State University.

Fausto Intilla

www.oloscience.com

Storing Data On Atomic Roundabouts

Source:

http://www.sciencedaily.com/releases/2007/10/071010132322.htm

Science Daily — There are right-handed and left-handed yoghurts, right-handed and left-handed snail shells, and right-handed and (occasionally) left-handed screws. Scientists at the University of Bonn have now demonstrated the existence of right-handed and left-handed "magnetic vortices". Through their research, in collaboration with colleagues from Berlin and Geneva, they believe that this physical phenomenon could eventually lead to the construction of faster and more reliable hard disks. The physicists have reported their discovery in Nature.
The magnetic vortex can be pictured like a traffic roundabout. But instead of cars circulating, it consists of an arrangement of magnetised atoms. They form a pattern rather like a ring of tiny bar magnets, so nothing actually moves around the atomic roundabout, but the direction can change: when the "north poles" are all pointing clockwise the magnetic vortex is "right-handed", otherwise the vortex is "left-handed".
"The existence of a circular atomic traffic system of this sort has been presumed for several years," explains the Bonn physics professor Dr. Manfred Fiebig. "In the Nature study we have actually discovered this kind of vortex field in a substance called lithium cobalt phosphate and employed laser-optics to determine its direction." Borrowing from the term "ferromagnetism", the authors -- who include, alongside Manfred Fiebig, the Dutch scientist Bas Van Aken and the Geneva-based physicists Hans Schmid and Jean-Pierre Rivera -- have called the phenomenon "ferrotoroidicity".
This finding is extremely interesting from a fundamental research standpoint. But there could also be very practical consequences in terms of technological applications. This is because magnetic vortices could be used to store information: when the atomic roundabout "traffic" goes right, it could be made to stand for the binary number "0"; going left, it could designate the "1" -- a physical principle that might be introduced one day into the design of computer hard disks.
Slow magnetic fields
"We now store data by magnetically poling the surface coating of a hard disk," explains Manfred Fiebig. "Today's data storage device contains many billions of polable zones, ordered in rows. To write information onto them or read from them you have to have magnetic fields." The current technology has two problems: on the one hand, to produce the necessary fields there must be a flow of electricity for which electrical charge carriers are actuated, and this is a relatively slow process. On the other hand, with ever greater densities of data the danger is that the magnetic fields to be read can destroy the stored information.
The atomic roundabouts do not have these drawbacks. Here, information is also "magnetically" stored but, as Manfred Fiebig points out, the "direction of rotation of the vortices can be changed by electrical fields." Moreover, "The reading process does not require a magnet field that might overwrite the stored data by mistake." Another advantage is that no electricity has to flow to generate the electrical fields so, in principle, storage can run much faster.
Next goal: learning to write
Professor Fiebig came to Bonn University from the Max Born Institute in Berlin over a year ago. The measurements made in Berlin were then analysed in Bonn, and this analysis of the data has produced the proof of the vortices. "We haven't yet succeeded in reading the direction of rotation of the magnetic vortex," the physicist adds. One of the next steps for him and his team is to find out how to write information reliably. They are also looking for other material that may prove suitable for future mass storage media.
However, Fiebig and his team certainly won't be building the hard disk drives of the future as he himself makes clear: "Our primary interest centres on the principles at work behind this phenomenon and what they reveal about the nature of magnetism. But if this research does result one day in a technological application, that'll obviously be quite a bonus."
This study is published in Nature, 11.10.2007, doi: 10.1038/nature06139
Note: This story has been adapted from material provided by University of Bonn.

Fausto Intilla
www.oloscience.com