Friday, February 25, 2011

Same rules apply to some experimental systems regardless of scale

New experiments show that common scientific rules can apply to significantly different phenomena operating on vastly different scales.

The results raise the possibility of making discoveries pertaining to phenomena that would be too large or impractical to recreate in the laboratory, said Cheng Chin, associate professor in physics and the James Franck Institute at the University of Chicago. Chin and associates Chen-Lung Hung, Xibo Zhang and Nathan Gemelke will publish their results in the Feb. 10, 2011 issue of the journal Nature.

Chin aspires to simulate the impossibly hot conditions that followed the big bang, during the earliest moments of the universe, by using an ultracold vacuum chamber in his laboratory. "It's fascinating to think about all these connections," he said.

The UChicago experiments demonstrate the validity of two widely discussed topics in the physics community today: scale invariance and universality.

Theoretical physicist Lev Pitaevskii had predicted that scale invariance would apply to a two-dimensional, cold-atom gas in 1997. Scale invariance means that the properties of a given phenomenon will remain the same, no matter how much its size is expanded or contracted. This contrasts sharply the three-dimensional world of everyday life, where dynamics change dramatically.

In the biological world, for example, scale invariance does not apply to complex organisms like humans, but exists in simple biological structures like nautilus shells, ferns and even broccoli. In physics, special cases also exist that exhibit scale invariance. Fractal structures have been observed in nature, which manifest similar structures whether magnified 10, 1,000 or a million times.

"There are only a few systems in nature that can display this kind of scale invariance, and we have shown that our two-dimensional system belongs to this very special class," Chin explained. "Once you identify these special cases and see how they are all linked together, then you can bring all these physical phenomena under the same umbrella," Chin said. "Now they can be fully described using the same language."

Exotic transformation

The universality concept applies to matter that undergoes smooth phase transitions. In the physics of everyday life, a phase transition occurs when water freezes to ice on a cold winter day. The phase transition in the UChicago experiment is more exotic: In the experiment, cesium atoms transform from a gas to a superfluid, a form of matter that exists only at temperatures of hundreds of degrees below zero.

Theoretical physicists in the early 1970s predicted that weakly interacting two-dimensional gases would exhibit similar behaviors under a variety of conditions as they neared the critical point of phase transition. Their prediction has remained unverified until now.

In their experiment, the UChicago researchers super-cooled thousands of cesium atoms to 10 nano-Kelvin, billionths of a degree above absolute zero (-459.67 degrees Fahrenheit), then loaded them into a pancake-like laser trap. The trap simulated a two-dimensional system by restricting the atoms' motion vertically but allowed a significant degree of horizontal freedom.

Chin's team was able to control the properties of this cold-atom gas system to make it non-interacting, weakly interacting or strongly interacting and then compared the results.

"At the same time, we can prepare the two-dimensional system at different sizes and also at different temperatures," Chin said. They could adjust the size parameters from 10 to 100 microns (a human hair is approximately 50 microns in diameter), and the temperature parameters from 10 to 100 nano-Kelvin.

Their experiment showed that no matter how they changed these three parameters, just one general description could characterize the resulting dynamics.

"There's a strong reason to believe that this kind of scale invariance can be extrapolated and on a more fundamental level can be mapped to other types of two-dimensional systems," Chin said. "The bigger question is whether our observation can shed light on other complex phenomena in nature. So our next step will be to explore going beyond two-dimensional systems."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Chicago.

Journal Reference:

Chen-Lung Hung, Xibo Zhang, Nathan Gemelke, Cheng Chin. Observation of scale invariance and universality in two-dimensional Bose gases. Nature, 2011; DOI: 10.1038/nature09722

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World's first programmable nanoprocessor: Nanowire tiles can perform arithmetic and logical functions

 Engineers and scientists collaborating at Harvard University and the MITRE Corporation have developed and demonstrated the world's first programmable nanoprocessor.

The groundbreaking prototype computer system, described in a paper appearing in the journal Nature, represents a significant step forward in the complexity of computer circuits that can be assembled from synthesized nanometer-scale components.

It also represents an advance because these ultra-tiny nanocircuits can be programmed electronically to perform a number of basic arithmetic and logical functions.

"This work represents a quantum jump forward in the complexity and function of circuits built from the bottom up, and thus demonstrates that this bottom-up paradigm, which is distinct from the way commercial circuits are built today, can yield nanoprocessors and other integrated systems of the future," says principal investigator Charles M. Lieber, who holds a joint appointment at Harvard's Department of Chemistry and Chemical Biology and School of Engineering and Applied Sciences.

The work was enabled by advances in the design and synthesis of nanowire building blocks. These nanowire components now demonstrate the reproducibility needed to build functional electronic circuits, and also do so at a size and material complexity difficult to achieve by traditional top-down approaches.

Moreover, the tiled architecture is fully scalable, allowing the assembly of much larger and ever more functional nanoprocessors.

"For the past 10 to 15 years, researchers working with nanowires, carbon nanotubes, and other nanostructures have struggled to build all but the most basic circuits, in large part due to variations in properties of individual nanostructures," says Lieber, the Mark Hyman Professor of Chemistry. "We have shown that this limitation can now be overcome and are excited about prospects of exploiting the bottom-up paradigm of biology in building future electronics."

An additional feature of the advance is that the circuits in the nanoprocessor operate using very little power, even allowing for their miniscule size, because their component nanowires contain transistor switches that are "nonvolatile."

This means that unlike transistors in conventional microcomputer circuits, once the nanowire transistors are programmed, they do not require any additional expenditure of electrical power for maintaining memory.

"Because of their very small size and very low power requirements, these new nanoprocessor circuits are building blocks that can control and enable an entirely new class of much smaller, lighter weight electronic sensors and consumer electronics," says co-author Shamik Das, the lead engineer in MITRE's Nanosystems Group.

"This new nanoprocessor represents a major milestone toward realizing the vision of a nanocomputer that was first articulated more than 50 years ago by physicist Richard Feynman," says James Ellenbogen, a chief scientist at MITRE.

Co-authors on the paper included four members of Lieber's lab at Harvard: Hao Yan (Ph.D. '10), SungWoo Nam (Ph.D. '10), Yongjie Hu (Ph.D. '10), and doctoral candidate Hwan Sung Choe, as well as collaborators at MITRE.

The research team at MITRE comprised Das, Ellenbogen, and nanotechnology laboratory director Jim Klemic. The MITRE Corporation is a not-for-profit company that provides systems engineering, research and development, and information technology support to the government. MITRE's principal locations are in Bedford, Mass., and McLean, Va.

The research was supported by a Department of Defense National Security Science and Engineering Faculty Fellowship, the National Nanotechnology Initiative, and the MITRE Innovation Program.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Harvard University, via EurekAlert!, a service of AAAS.

Journal Reference:

Hao Yan, Hwan Sung Choe, SungWoo Nam, Yongjie Hu, Shamik Das, James F. Klemic, James C. Ellenbogen, Charles M. Lieber. Programmable nanowire circuits for nanoprocessors. Nature, 2011; 470 (7333): 240 DOI: 10.1038/nature09749

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Scientists customize a magnet's performance by strategically replacing key atoms

Scientists have given us a plethora of new materials -- all created by combining individual elements under varying temperatures and other conditions. But to tweak an intermetallic compound even more, in order to give it the attributes you desire, you have to go deeper and re-arrange individual atoms.

It's a process similar to what bioengineers employ when they add and delete genes to create synthetic organisms, and it was the focus of a group of researchers at the U.S. Department of Energy's Ames Laboratory, when they replaced key atoms in a gadolinium-germanium magnetic compound with lutetium and lanthanum atoms.

The group was led by Vitalij Pecharsky, Ames Lab senior scientist and Distinguished Professor of Materials Science and Engineering at Iowa State University, and included his Lab colleagues, Karl Gschneidner Jr., Ames Lab senior metallurgist and Distinguished Professor of MS&E at ISU, and Gordon Miller, Ames Lab senior scientist and ISU professor of chemistry, along with assistant scientists Yaroslav Mudryk and Durga Paudyal. Also participating was Sumohan Misra, research associate at the DOE's SLAC National Accelerator in Menlo Park, Calif., formerly a Ph.D. student of Miller's.

Creating materials by design is no easy task, especially in the case of the complex gadolinium-germanium -- Gd5Ge4 -- compound. Making things even more difficult, the compound's structure is highly symmetrical, which is common in intermetallics, so predicting which atoms are key to changing the material's characteristics would be difficult if not impossible unless some methodology was available to help in the selection process.

The Gd5Ge4 compound's uniformity results from the fact that like nearly all metallic solids' atoms are arranged in a highly symmetrical crystal structure called a lattice. The more complex the material, the more intricate its lattice. And while the individual elements making up the lattice influence its characteristics, in some cases the location of specific atoms within the lattice can also have a profound influence on such things as its melting point, mechanical strength or -- in the case of magnets -- ferromagnetic properties.

"Individuality doesn't happen often among the atoms of metallic crystals," Pecharsky explained, "But atoms still are able to 'cooperate' with one another in areas such as magnetic ordering and superconductivity."

By discovering these cooperative relationships, scientists can determine what will happen if they replace one or more of the atoms with those of another element, which is precisely what the team accomplished.

"We revealed that a single site occupied by the Gd atoms is much more active than all of the other Gd sites when it comes to bringing the ferromagnetic order in a complex crystal structure of gadolinium germanide," Pecharsky said.

Pecharsky, Gschneidner and other researchers at the Ames Lab have spent years working with gadolinium alloys, because of the magnetic compound's use in the green, energy-saving field of magnetic refrigeration. However, that was not the main reason the Ames Lab researchers chose Gd5Ge4 for their work.

As it turns out, "the metal exhibits an impressive combination of intriguing and potentially important properties, the researchers explained in their paper, "Controlling Magnetism of a Complex Metallic System Using Atomic Individualism," published in the August 10, 2010 Physical Review Letters. "The extraordinary responsiveness to relatively weak external stimuli makes Gd5Ge4 and related compounds a phenomenal playground for condensed matter science."

Besides being unusually responsive, Gd5Ge4 was an ideal alloy for the work, because any changes in its magnetic properties resulting from the group's manipulations could be easily measured.

In 2008, Pecharsky and members of the same research team had already discovered that adding silicon to the alloy resulted in a magnetostructural transition that occurred without the application of a magnetic field. Chemical pressure alone was able to enhance the material's ferromagnetism.

That earlier finding led the team to experiment with other additions to the alloy. To ferret out precisely which atoms in the lattice were the best candidates for manipulation, the researchers called upon density functional theory, which is a means of studying the electronic structure of solids developed by Nobel Prize winning physicist Walter Kohn.

Kohn's methodology enabled the group to model the effects substituting small amounts of gadolinium atoms within the Gd5Ge4 solid with the elements lutetium and lanthanum. With the modeled results in hand, the group's next step was to create the actual alloys in the lab, in order to test the accuracy of their computer-based predictions.

In fact, the complex fabrication process confirmed the modeling results. The researchers found if they replaced just a few gadolinium atoms with lutetium, the result would be a severe loss in the alloy's ferromagnetism. By contrast, substituting an equal number of lanthanum atoms had no significant effect; though substituting greater amounts of lanthanum might have a more pronounced impact on the resulting alloy's ferromagnetism, the researchers speculated.

Going forward, the lessons learned in this experiment could have important far-reaching implications, as materials scientists search for new exotic substances to be used in this and future generations of high-tech products. "Knowing how to identify key atomic positions is similar to understanding the roles specific genes play in an organism's DNA sequence," Pecharsky said. "And that knowledge could ultimately lead to materials by design."

This research was funded by the DOE Office of Science.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Ames Laboratory.

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X-rays show why van Gogh paintings lose their shine

 Scientists have identified a complex chemical reaction responsible for the degradation of two paintings by Vincent van Gogh and other artists of the late 19th century. This discovery is a first step to understanding how to stop the bright yellow colours of van Gogh's most famous paintings from being covered by a brown shade, and fading over time. In the meantime, the results suggest shielding affected paintings as much as possible from UV and sunlight.

The results are published in the 15 February 2011 issue of Analytical Chemistry.

The work was carried out by an international team of scientists from four countries led by Koen Janssens of Antwerp University (Belgium), with Letizia Monico, an Italian chemist preparing a Ph.D. at Perugia University (Italy), taking the centre stage in the experiments. As an Erasmus student, she worked for one year in Janssens' research group in Antwerp, and is also the lead author of the two papers. Scientists from the CNR Institute of Molecular Science and Technologies (Perugia, Italy), the CNRS C2RMF (Paris, France), TU Delft (Netherlands) and the van Gogh Museum (Amsterdam, Netherlands) were also part of the team.

Uncovering the secrets of the chemical reaction needed deployment of an impressive arsenal of analytical tools, with synchrotron X-rays at the ESRF in Grenoble (France) providing the final answers. "For every Italian, conservation of masterpieces has always mattered. I am pleased that science has now added a piece to a puzzle that is a big headache for so many museums" says Letizia Monico from University of Perugia.

The experiment reads like a crime scene investigation. The scientists employed an X-ray beam of microscopic dimensions to reveal a complex chemical reaction taking place in the incredibly thin layer where the paint meets the varnish. Sunlight can penetrate only a few micrometers into the paint, but over this short distance, it will trigger a hitherto unknown chemical reaction turning chrome yellow into brown pigments, altering the original composition.

Van Gogh's decision to use novel bright colours in his paintings is a major milestone in the history of art. He deliberately chose colours that conveyed mood and emotion, rather than employing them realistically. At the time, this was completely unheard of and, without major innovations in pigment manufacturing made in the 19th century, would also have been impossible.

It was the vibrancy of new industrial pigments such as chrome yellow which allowed van Gogh to achieve the intensity of, for example, his series of Sunflowers paintings. He started to paint in these bright colours after leaving his native Holland for France where he became friends with artists who shared his new ideas about the use of colours. For one of them, Paul Gauguin, he started painting yellow sunflower motifs as a decoration for his bedroom.

The fact that yellow chrome paint darkens under sunlight has been known since the early 19th Century. However, not all period paintings are affected, nor does it always happen at the same speed. As chrome yellow is toxic, artists quickly switched to new alternatives in the 1950s. However, Vincent van Gogh did not have this choice, and to preserve his work and that of many comtemporaries, interest in the darkening of chrome yellow is now rising again.

To solve a chemical puzzle nearly 200 years old, the team around Janssens used a two-step approach: first, they collected samples from three left-over historic paint tubes. After these samples had been artificially aged for 500 hours using an UV-lamp, only one sample, from a paint tube belonging to the Flemish Fauvist Rik Wouters (1882-1913), showed significant darkening. Within 3 weeks, its surface of originally bright yellow had become chocolate brown. This sample was taken as the best candidate for having undergone the fatal chemical reaction, and sophisticated X-ray analysis identified the darkening of the top layer as linked to a reduction of the chromium in the chrome yellow from Cr(VI) to Cr(III). The scientists also reproduced Wouters' chrome yellow paint and found that the darkening effect could be provoked by UV light.

In the second step, the scientists used the same methods to examine samples from affected areas of two van Gogh paintings, View of Arles with Irises (1888) and Bank of the Seine (1887), both on display in the Van Gogh Museum in Amsterdam.

"This type of cutting edge research is crucial to advance our understanding of how paintings age and should be conserved for future generations," says Ella Hendriks of the van Gogh Museum Amsterdam.

Because the affected areas in these multicoloured samples were even more difficult to locate than in the artificially aged ones, an impressive array of analytical tools had to be deployed which required the samples travelling to laboratories across Europe. The results indicate that the reduction reaction from Cr(VI) to Cr(III) is likely to also have taken place in the two van Gogh paintings.

The microscopic X-ray beam also showed that Cr(III) was especially prominent in the presence of chemical compounds which contained barium and sulphur. Based on this observation, the scientists speculate that van Gogh's technique of blending white and yellow paint might be the cause of the darkening of his yellow paint.

"Our next experiments are already in the pipeline. Obviously, we want to understand which conditions favour the reduction of chromium, and whether there is any hope to revert pigments to the original state in paintings where it is already taking place.," summarises Koen Janssens from University of Antwerp.

The techniques used by the scientists included X-ray diffraction along with various spectroscopies employing infrared radiation, electrons and X-rays at the universities of Antwerp and Perugia, and at two synchrotrons (ESRF and DESY).

"I am not aware of a similarly big effort ever having been made for the chemistry of an oil painting," says Joris Dik, Professor at Delft Technical University.

In the decisive step, two techniques were combined using a single X-ray beam at the ESRF: X-Ray fluorescence (XRF) and X-Ray absorption near-edge spectroscopy (XANES). For the XRF, the microscopic beam size (0.9 x 0.25 µm2) made possible to separate the study of degraded and unaffected areas, and the XANES technique proved the speciation of chromium, i.e. the reduction from Cr(VI) to Cr(III).

"Our X-ray beam is one hundred times thinner than a human hair, and it reveals subtle chemical processes over equally minuscule areas. Making this possible has opened the door to a whole new world of discovery for art historians and conservators," says Marine Cotte, an ESRF scientist also working at CNRS/Musée du Louvre.

The reduction of chromium that had been observed in the artificially aged sample from the atelier of Rik Wouters was finally confirmed in both microsamples from the van Gogh paintings.

The study was completed with a nanoscopic investigation of the discoloured paint using electron energy loss spectroscopy at the University of Antwerp, which confirmed the results and showed that the newly formed Cr(III) compounds were formed as a nanometer-thin coating of the pigment particles that constitute the paint.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by European Synchrotron Radiation Facility.

Journal Reference:

Letizia Monico, Geert Van der Snickt, Koen Janssens, Wout De Nolf, Costanza Miliani, Joris Dik, Marie Radepont, Ella Hendriks, Muriel Geldof, Marine Cotte. Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 2. Original Paint Layer Samples. Analytical Chemistry, 2011; 83 (4): 1224 DOI: 10.1021/ac1025122

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.