Two theoretical physicists from Rice University are reporting initial success in that area in a paper published in the Proceedings of the National Academy of Science. Their new conceptual model, which was created to learn more about the quantum quirks of high-temperature superconductors and other high-tech materials, has also proven useful in describing the origins of ferromagnetism — the everyday “magnetism” of compass needles and refrigerator magnets.

“As a theorist, you strive to have exact solutions, and even though our new model is purely theoretical, it does produce results that match what’s observed in the real world,” said Rice physicist Qimiao Si, the lead author of the paper. “In that sense, it is reassuring to have designed a model system in which ferromagnetism is allowed.”

Ferromagnets are what most people think of as magnets. They’re the permanently magnetic materials that keep notes stuck to refrigerators the world over. Scientists have long understood the large-scale workings of ferromagnets, which can be described theoretically from a coarse-grained perspective. But at a deeper, fine-grained level — down at the scale of atoms and electrons — the origins of ferromagnetism remain fuzzy.

“When we started on this project, we were aware of the surprising lack of theoretical progress that had been made on metallic ferromagnetism,” Si said. “Even a seemingly simple question, like why an everyday refrigerator magnet forms out of electrons that interact with each other, has no rigorous answer.”

Si and graduate student Seiji Yamamoto’s interest in the foundations of ferromagnetism stemmed from the study of materials that were far from ordinary. Si’s specialty is an area of condensed matter physics that grew out of the discovery more than 20 years ago of high-temperature superconductivity. In 2001, Si offered a new theory to explain the behaviour of the class of materials that includes high-temperature superconductors. This class of materials — known as “quantum correlated matter” — also includes more than ten known types of ferromagnetic composites.

Si’s 2001 theory and his subsequent work have aimed to explain the experimentally observed behaviour of quantum-correlated materials based upon the strangely correlated interplay between electrons that goes on inside them. In particular, he focuses on the correlated electron effect that occur as the materials approach a “quantum critical point,” a tipping point that’s the quantum equivalent of the abrupt solid-to-liquid change that occurs when ice melts.

The quantum critical point that plays a key role in high-temperature superconductivity is the tipping point that marks a shift to antiferromagnetism, a magnetic state that has markedly different subatomic characteristics from ferromagnetism. Because of the key role in high-temperature superconductivity, most studies in the field have focused on antiferromagnetism. In contrast, ferromagnetism — the more familiar, everyday form of magnetism — has received much less attention theoretically in quantum-correlated materials.

“So our initial theoretical question was, ‘What would happen, in terms of correlated electron effects, when a ferromagnetic material moves through one of these quantum tipping points?” said Yamamoto, who is now a postdoctoral researcher at the National High Magnetic Field Laboratory in Tallahassee, Fla.

To carry out this thought experiment, Si and Yamamoto created a model system that idealizes what exists in nature. Their jumping off point was a well-studied phenomenon known as the Kondo effect — which also has its roots in quantum magnetic effects. Based on what they knew of this effect, they created a model of a “Kondo lattice,” a fine-grained mesh of electrons that behaved like those that had been observed in Kondo studies of real-world materials.

Si and Yamamoto were able to use the model to provide a rigorous answer about the fine-grained origins of metallic ferromagnetism. Furthermore, the ferromagnetic state that was predicted by the model turned out to have quantum properties that closely resemble those observed experimentally in heavy fermion ferromagnets.

“The model is useful because it allows us to predict how real-world materials might behave under a specific set of circumstances,” Yamamoto said. “And, in fact, we have been able to use it to explain experimental observations on heavy fermion metals, including both the antiferromagnets as well as the less well understood ferromagnetic materials.”

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Invited speakers

• Pulickel Ajayan (Rice University, USA)
• Eva Andrei (Rutgers, USA)
• Sarah Burke (British Columbia, Canada)
• Millie Dresselhaus (MIT, USA)
• Toshiaki Enoki (Tokyo Inst. of Tech., Japan)
• Klaus Ensslin (ETH, Switzerland)
• Andrea C. Ferrari (Cambridge, UK)
• Andre Geim (Manchester, UK) ***
• Paco Guinea (Madrid, Spain)
• Tony Heinz (Columbia, USA)
• Pablo Jarillo-Herrero (MIT, USA) ***
• Francesco Mauri (Paris, France)
• Kostya Novoselov (Manchester, UK)
• Francois M. Peeters (Antwerpen, Belgium)
• Alain Penicaud (Bordeaux, France)
• Vitor Manuel Pereira (Boston University, USA)
• Nuno Peres (Minho, Portugal)
• Rod Ruoff (Austin, USA)
• Lieven Vandersypen (Delft, Netherlands) ***
• Amir Yacoby (Harvard, USA) ***

*** To be confirmed

Please visit the Conference Web site for more information. Registration and abstract submission are already open.

Organizers

Marcos A. Pimenta
Antonio H. Castro Neto
Luiz Gustavo Cancado

An international team of researchers has made a further step towards this vision and demonstrated a novel analytical sensor which mimics our olfaction system. The difference between this and similar prior e-noses is that the active element of this new device is an individual wedge-like nanowire (nanobelt) made of tin dioxide. The required diversity of the sensing elements is encoded in the nanobelt morphology via longitudinal width variations of the nanobelt realized during its growth and via functionalization of some of the segments with palladium catalyst. “Our approach demonstrates the potential of combining bottom-up nanowire fabrication protocols with state-of-the art microfabrication methods to design prospective simple sensing arrays which, in principle, might be scaled down to the size of few micrometers and thus become the smallest analytical instrument,” tells Andrei Kolmakov, an associate professor in the physics department at Southern Illinois University at Carbondale.

Kolmakov and a team of researchers from Karlsruhe Institute of Technology, Rensselaer Polytechnic Institute, Sincrotrone Trieste, and first author Victor V. Sysoev from Saratov State Technical University, have published their findings in ACS Nano. In what probably is the simplest and yet fully functioning e-nose, the device is made of an individual single-crystal metal oxide quasi-1D nanobelt. The nanobelt was indexed with a number of platinum electrodes in a way that each segment of the nanobelt between two electrodes defines an individual sensing elemental receptor of the array.

“We contacted this individual nanobelt with dozen of electrodes and obtained an array of sensors” explains Kolmakov. “Due to this variation of the resistance along the length on the nanobelt the same gas will produce slightly different responses, which leads to the nanobelt’s recognition ability. In fact, this is similar to our own human olfactory receptors which are not very gas selective individually but slightly differ from each other.” Previous designs of nanowire-based electronic noses were using different nanowires to obtain different responses. That meant that they were more complex, expensive, fragile etc. Compared to this, nothing can be simpler, cheaper, and more robust than just a single individual nanowire.

“Our fabrication advance can be compared with development of integrated electronics versus circuits based on individual electronic components” says Kolmakov. “In our case the main sensor functionality is integrated into one single nanostructure.” The research team based their device on tin oxide. This is a well studied semiconducting oxide which traditionally serves as a testbench material to implement novel sensing principles and platforms in gas sensorics. Using a conventional vapor-solid method they grew tin oxide nanobelts and selectively doped segments of the nanobelt’s surface with palladium nanoparticles in order to enhance the resistivity of these segments.

The real-world applications of e-nose instruments would have to deal with complex odors or aromas, for example, ones widespread in the food industry. To test the performance of their e-nose against such complex environments, the researchers exposed a 10-segment single nanobelt sensor to the vapors from the headspace of four alcoholic beverages (glühwein, sparkling wine, vermouth, brandy). To eliminate the strong influence of the different ethanol content to the sensor signal, all beverages were diluted in distilled water to contain the same amount (10 rel. %) of ethanol. The resistance of all 10 segments of the tested nanobelt were plotted in a radar diagram and the results showed that the results are clearly analyte specific.

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In 2009, the U.S Department of Energy (DOE) proposed on-board hydrogen storage system performance targets that have become widely accepted. So far, researchers haven’t been able to successfully demonstrate a material that is capable of simultaneously meeting all of the requirements and criteria set out by the DOE.

A European research team has now reported on a new concept for hydrogen storage using nanoconfined reversible chemical reactions. They demonstrate that nanoconfined hydride has a significant hydrogen storage potential. Research at the Nano Energy-Materials research group at Interdisciplinary Nanoscience Center (iNANO) at Aarhus University in Denmark, led by Flemming Besenbacher and Torben R. Jensen, focuses on the utilization of nanoporous materials as scaffolds for preparation and confinement of nanosized metal hydrides. This bottom-up approach limits the particle size of the hydride to the average pore size of the scaffold material, which allows for the direct production of smaller particles than obtainable mechanically. Furthermore, particle growth and agglomeration may be hindered by the compartmentalization of the nanoparticles within the scaffold material. Nanoconfinement may also mediate improved re-hydrogenation properties of complex metal hydrides.

“Nanoconfinement of metal hydrides is receiving increasing interest in the field of hydrogen storage and this principle has already been applied to a number of promising hydrogen storage materials,” the researchers explain. In their work published in the online edition of ACS Nano, the team introduces an alternative bottom-up approach where nanoparticles of hydrides are synthesized or melt infiltrated in a nanoporous inert scaffold material, which has several advantages:

1. increased surface area of the reactants
2. nanoscale diffusion distances
3. increased number of grain boundaries, which facilitate release and uptake of hydrogen and enhance reaction kinetics

Lithium borohydride (LiBH4) and magnesium hydride (MgH2) nanoparticles are embedded in a nanoporous carbon aerogel scaffold with a maximum pore size of 21 nm and react during release of hydrogen and form magnesium diboride. The hydrogen desorption kinetics is significantly improved compared to bulk conditions, and the nanoconfined system has a high degree of reversibility and stability and possibly also improved thermodynamic properties. The purpose of this work was to further develop the concept of nanoconfinement by investigating a system of higher complexity. Lithium borohydride and magnesium hydride  have been studied intensively in the past due to their high theoretical hydrogen densities.
“However” explains Nielsen, “the use of lithium borohydride as a solid-state hydrogen storage material is hampered by its unfavorable high thermal stability; that is, release of hydrogen takes place at temperatures above 400°C and, importantly, uptake of hydrogen only occurs under extreme conditions. Similarly, application of the abundant and cheap metal magnesium is also impeded by unfavorable thermodynamic properties.” Jensen adds that, “fortunately, both the kinetic and thermodynamic properties of potential hydrogen storage materials can be significantly improved by combining exothermic and endothermic chemical reactions. A more favorable total enthalpy change may be obtained by the introduction of a new dehydrogenated state which may facilitate hydrogenation. This concept is referred to as reactive hydride composites (RHC), and it helps to preserve a high gravimetric hydrogen storage capacity.”

By studying the effect of nanoconfinement on the hydrogen storage properties of a lithium-borohydride/magnesium-hydride system, the team (which included scientists from the Institute of Material Research in Germany and Lund University in Sweden) found that it possesses a high degree of reversible stability and improved hydrogen desorption kinetics as compared to the bulk. Furthermore, the concept of nanoconfined chemical reactions may develop to become an important tool within the emerging area of nanotechnology for the improvement of the properties and reaction yield of a wide range of chemical reactions. This new scheme of nanoconfined chemistry may have a wide range of interesting applications in the future, for example, within the merging area of chemical storage of renewable energy.

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The technique works with multiple forms of graphene and is poised to become an important finding for the development of graphene electronics. The research is published in the Science journal. Scientists who work with nanocircuits are enthusiastic about graphene because electrons meet with less resistance when they travel along graphene compared to silicon and because today’s silicon transistors are nearly as small as allowed by the laws of physics. Graphene also has the edge due to its thickness – it’s a carbon sheet that is a single atom thick. While graphene nanoelectronics could be faster and consume less power than silicon, no one knew how to produce graphene nanostructures on such a reproducible or scalable method. That is until now.

“We’ve shown that by locally heating insulating graphene oxide, both the flakes and epitaxial varieties, with an atomic force microscope tip, we can write nanowires with dimensions down to 12 nanometers. And we can tune their electronic properties to be up to four orders of magnitude more conductive. We’ve seen no sign of tip wear or sample tearing,” said Elisa Riedo, associate professor in the School of Physics at the Georgia Institute of Technology.

On the macroscopic scale, the conductivity of graphene oxide can be changed from an insulating material to a more conductive graphene-like material using large furnaces. Now, the research team used TCNL to increase the temperature of reduced graphene oxide at the nanoscale, so they can draw graphene-like nanocircuits. They found that when it reached 130 degrees Celsius, the reduced graphene oxide began to become more conductive.

“So the beauty of this is that we’ve devised a simple, robust and reproducible technique that enables us to change an insulating sample into a conducting nanowire. These properties are the hallmark of a productive technology,” said Paul Sheehan, head of the Surface Nanoscience and Sensor Technology Section at the Naval Research Laboratory in Washington, D.C.

The research team tested two types of graphene oxide – one made from silicon carbide, the other with graphite powder. “I think there are three things about this study that make it stand out,” said William P. King, associate professor in the Mechanical Science and Engineering department at the University of Illinois at Urbana-Champaign. “First, is that the entire process happens in one step. You go from insulating graphene oxide to a functional electronic material by simply applying a nano-heater.  Second, we think that any type of graphene will behave this way. Third, the writing is an extremely fast technique. These nanostructures can be synthesized at such a high rate that the approach could be very useful for engineers who want to make nanocircuits.”

Source: original article

A surface plasmon is a collective excitation involving all conducting electrons in a metallic layer moving relatively to the static positive ions of the metal as shown in the diagram. Research on plasmonics, a relatively new branch of optics, has received an increasing level of international attention over the last decade. This interest is mainly driven by the fact that surface plasmons, travelling along the interface between a metal and a dielectric, allow confining optical energy to volumes that are significantly smaller than those accessible with conventional dielectric waveguiding structures such as optical fibers.

Apart from being of fundamental interest on its own, tightly focused optical energy can be used as a ‘nano-probe’ which provides valuable measurements in fields like solid-state physics, chemistry and the life sciences. In addition, the tight confinement of the optical field is an interesting feature as it promises optical devices with reduced dimensions. This is of particular relevance for the field of optical communications, optical computing and hybrid microelectronic/optical circuits. However, under normal circumstances, optical energy travels over very short distances in plasmonic waveguides, before it is absorbed due to Ohmic loss in the metal.

Although clever design can somewhat increase the useful length of plasmonic waveguides, it is widely accepted that the only way to completely overcome this problem is to add a mechanism that continuously amplifies the light as it travels along the plasmonic waveguide.

However, integrating such plasmonic amplification has turned out to be a challenging task. The researchers team developed a structure that provides sufficient amplification to overcome the intrinsic absorption of a plasmonic waveguide. In fact, the optical amplification is sufficient to provide a net gain of the plasmon-bound light as it travels along the waveguide. The researchers used a structure consisting of an ultra-thin gold film that was embedded in a highly fluorescent polymer, optically pumped by an ultrafast laser source. The structure was designed to channel the light generated by the fluorescent polymer to the plasmonic waveguide. As the plasmonic wave travels along the waveguide, its intensity is increased by stimulated emission of the optical energy stored in the fluorescent polymer.

“For many years the propagation loss issue in plasmonic waveguides has been a major hurdle for the development of devices that make use of surface plasmon effects,” says Klaus Meerholz. “The key to the success of our work was that we found a way to embed the plasmonic waveguides into an amplifying fluorescent polymer without affecting the properties of the waveguide too much,” explains Malte Gather.

Source: original article

Previous experiments led by Graham Fleming, a physical chemist, pointed to quantum mechanical effects as the key to the ability of green plants, through photosynthesis, to almost instantaneously transfer solar energy from molecules in light harvesting complexes to molecules in electrochemical reaction centres. Now a new collaborative team that includes Fleming have identified entanglement as a natural feature of these quantum effects. Their work is published in the Nature Physics journal. When two quantum-sized particles, for example a pair of electrons, are entangled, any change to one will be instantly reflected in the other, no matter how far apart they might be. Though physically separated, the two particles act as a single entity.

“This is the first study to show that entanglement, perhaps the most distinctive property of quantum mechanical systems, is present across an entire light harvesting complex,” says Mohan Sarovar, a post-doctoral researcher at the University of California. “While there have been prior investigations of entanglement in toy systems that were motivated by biology, this is the first instance in which entanglement has been examined and quantified in a real biological system.”

The results of this study hold implications not only for the development of artificial photosynthesis systems as a renewable non-polluting source of electrical energy, but also for the future development of quantum-based technologies in areas such as computing – a quantum computer could perform certain operations thousands of times faster than any conventional computer.

What may prove to be this study’s most significant revelation is that contrary to the popular scientific notion that entanglement is a fragile and exotic property, difficult to engineer and maintain, the Berkeley researchers have demonstrated that entanglement can exist and persist in the chaotic chemical complexity of a biological system.

“We present strong evidence for quantum entanglement in noisy non-equilibrium systems at high temperatures by determining the timescales and temperatures for which entanglement is observable in a protein structure that is central to photosynthesis in certain bacteria,” Sarovar says.

Green plants and certain bacteria are able to transfer the energy harvested from sunlight through a network of light harvesting pigment-protein complexes and into reaction centres with nearly 100-percent efficiency. Speed is the key – the transfer of the solar energy takes place so fast that little energy is wasted as heat. In 2007, Fleming and his research group reported the first direct evidence that this essentially instantaneous energy transfer was made possible by a remarkably long-lived, wavelike electronic quantum coherence.

Through photosynthesis, green plants are able to capture energy from sunlight and convert it into chemical energy. By exploiting quantum mechanical effects, the plants transfer energy from sunlight with an efficiency of nearly 100-percent. Using electronic spectroscopy measurements made on a femtosecond time-scale, Fleming and his group discovered the existence of “quantum beating” signals, coherent electronic oscillations in both donor and acceptor molecules. These oscillations are generated by the excitation energy from captured solar photons, like the waves formed when stones are tossed into a pond. The wavelike quality of the oscillations enables them to simultaneously sample all the potential energy transfer pathways in the photosynthetic system and choose the most efficient.

Source: original article

This growing interest, triggered by the numerous potential applications of graphene in future nanotechnology, motivates the creation of an interdisciplinary school on graphene. The school is aimed at PhD students, post-doctoral and young researchers in the first instance. The School is taking place in Cargèse, France from October 12 to October 22, 2010 at the Institut d’Etudes Scientifiques de Cargèse.

The school, initiated by the GdR-I «Graphene and Nanotubes», will deal with properties and characterization of monolayers and multilayers of graphene and graphene-based nano-objects, from the points of views of physicists, chemists and material scientists. Both fundamental and applied aspects will be considered. It will offer participants both background lectures, essential for the interdisciplinary approach that is proposed, and specialized courses, which will include the most recent developments in the field. Practical training on numerical simulations and experimentation such scanning tunnelling microscopy will be organized. The lecturers of the school are researchers known for their pedagogic skills and internationally recognized for their expertise in the field of graphene. The participants will be encouraged to present their own work (also in adjacent fields) during poster sessions.

Detailed information about the GDR-I and the School is available on their website.

The research team that conducted the new investigation features physicists from the US National Institutes of Standards and Technology (NIST) and the Georgia Institute of Technology (Georgia Tech). The experts say that the moire patterns can also be used on multiple grids or atom arrays, not only on two. They add that using “atomic moire interferometry” can also help scientists determine the rotational orientation of the graphene sheets used in a variety of technological applications. Their work is published in the Physical Review B journal.

Given that one of the most complex areas of graphene research today is figuring out how the material changes its properties when stacked in multiple layers, being able to obtain a map of the strains that develop within it is extremely important. The electronic and transport properties of the single-atom-thick carbon compound can now be analyzed and determined with a much higher degree of accuracy than ever before, the NIST/Georgia Tech team says. Due to its revolutionary semiconducting properties, graphene is now hailed as the material of the future, at least in the electronics industry.

For the new experiments, Georgia Tech experts developed sheets of graphene on a silicon carbide substrate. After the samples were transferred at NIST, scientists here used a custom-built scanning tunnelling microscope (STM) to look at the graphene samples. The high resolve power on this instrument allows the experts to peer deep within the sample, past the topmost layer. It was through this method that the moire patterns became visible. Distinguishing them is fairly easy because of the hexagonal arrangement of carbon atoms in graphene. Any layers that are misplaced on top of others are immediately clear.

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Despite the use of materials for the most inexpensive, easy to manufacture and flexible, large-scale commercialization of these batteries confronts two major obstacles. The electrolyte is very corrosive, causing a deficiency in sustainability. It is also very colorful, preventing light from entering and effectively limiting the photo-voltage of 0.7 volts. Moreover, platinum is an expensive material, non-transparent and rare.

Benoit Marsan and his team have been working for years to develop an electrochemical solar cell. With the help of Professor Livain Breau, also from the Department of Chemistry at University of Quebec at Montreal (UQAM), researchers have developed an electrolyte consisting of new molecules, whose concentration could be increased. The gel formed is transparent, non-corrosive and can increase the photo-voltage. Therefore, the battery is more stable and better performance. The platinum cathode has also been replaced with cobalt sulfide. This material is much less expensive than platinum. It is also more efficient, more stable and easier to produce in the laboratory.

The work conducted by Professor Benoit Marsan has solved two problems that hindered growth of the solar cell industry for 20 years: increasing the efficiency of solar cells while reducing their fabrication cost. The scientific community was excited about this work published in the Journal of American Chemical Society. Several researchers contend that the work of Professor Marsan represents a breakthrough towards the production of efficient and affordable solar cells.

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