How Seawater Could Corrode Nuclear Fuel

Japan used seawater to cool nuclear fuel at the stricken Fukushima-Daiichi nuclear plant after the tsunami in March 2011 -- and that was probably the best action to take at the time, says Professor Alexandra Navrotsky of the University of California, Davis.

But Navrotsky and others have since discovered a new way in which seawater can corrode nuclear fuel, forming uranium compounds that could potentially travel long distances, either in solution or as very small particles. The research team published its work Jan. 23 in the Proceedings of the National Academy of Sciences.

"This is a phenomenon that has not been considered before," said Alexandra Navrotsky, distinguished professor of ceramic, earth and environmental materials chemistry. "We don't know how much this will increase the rate of corrosion, but it is something that will have to be considered in future."

Japan used seawater to avoid a much more serious accident at the Fukushima-Daiichi plant, and Navrotsky said, to her knowledge, there is no evidence of long-distance uranium contamination from the plant.

Uranium in nuclear fuel rods is in a chemical form that is "pretty insoluble" in water, Navrotsky said, unless the uranium is oxidized to uranium-VI -- a process that can be facilitated when radiation converts water into peroxide, a powerful oxidizing agent.

Peter Burns, professor of civil engineering and geological sciences at the University of Notre Dame and a co-author of the new paper, had previously made spherical uranium peroxide clusters, rather like carbon "buckyballs," that can dissolve or exist as solids.

In the new paper, the researchers show that in the presence of alkali metal ions such as sodium -- for example, in seawater -- these clusters are stable enough to persist in solution or as small particles even when the oxidizing agent is removed.

In other words, these clusters could form on the surface of a fuel rod exposed to seawater and then be transported away, surviving in the environment for months or years before reverting to more common forms of uranium, without peroxide, and settling to the bottom of the ocean. There is no data yet on how fast these uranium peroxide clusters will break down in the environment, Navrotsky said.

Navrotsky and Burns worked with the following co-authors: postdoctoral researcher Christopher Armstrong and project scientist Tatiana Shvareva, UC Davis; May Nyman, Sandia National Laboratory, Albuquerque, N.M.; and Ginger Sigmon, University of Notre Dame. The U.S. Department of Energy supported the project.

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Journal Reference:

1. C. R. Armstrong, M. Nyman, T. Shvareva, G. E. Sigmon, P. C. Burns, A. Navrotsky. Uranyl peroxide enhanced nuclear fuel corrosion in seawater. Proceedings of the National Academy of Sciences, 2012; DOI:10.1073/pnas.1119758109

Astronomers discover complex organic matter exists throughout the universe

ScienceDaily (Oct. 26, 2011) — Astronomers report in the journal Nature that organic compounds of unexpected complexity exist throughout the Universe. The results suggest that complex organic compounds are not the sole domain of life but can be made naturally by stars.

Prof. Sun Kwok and Dr. Yong Zhang of The University of Hong Kong show that an organic substance commonly found throughout the Universe contains a mixture of aromatic (ring-like) and aliphatic (chain-like) components. The compounds are so complex that their chemical structures resemble those of coal and petroleum. Since coal and oil are remnants of ancient life, this type of organic matter was thought to arise only from living organisms. The team's discovery suggests that complex organic compounds can be synthesized in space even when no life forms are present.

The researchers investigated an unsolved phenomenon: a set of infrared emissions detected in stars, interstellar space, and galaxies. These spectral signatures are known as "Unidentified Infrared Emission features." For over two decades, the most commonly accepted theory on the origin of these signatures has been that they come from simple organic molecules made of carbon and hydrogen atoms, called polycyclic aromatic hydrocarbon (PAH) molecules. From observations taken by the Infrared Space Observatory and the Spitzer Space Telescope, Kwok and Zhang showed that the astronomical spectra have features that cannot be explained by PAH molecules. Instead, the team proposes that the substances generating these infrared emissions have chemical structures that are much more complex. By analyzing spectra of star dust formed in exploding stars called novae, they show that stars are making these complex organic compounds on extremely short time scales of weeks.

Not only are stars producing this complex organic matter, they are also ejecting it into the general interstellar space, the region between stars. The work supports an earlier idea proposed by Kwok that old stars are molecular factories capable of manufacturing organic compounds. "Our work has shown that stars have no problem making complex organic compounds under near-vacuum conditions," says Kwok. "Theoretically, this is impossible, but observationally we can see it happening."

Most interestingly, this organic star dust is similar in structure to complex organic compounds found in meteorites. Since meteorites are remnants of the early Solar System, the findings raise the possibility that stars enriched the early Solar System with organic compounds. The early Earth was subjected to severe bombardments by comets and asteroids, which potentially could have carried organic star dust. Whether these delivered organic compounds played any role in the development of life on Earth remains an open question.

Prof. Sun Kwok is the Dean of Science and Chair Professor of Physics of the University of Hong Kong. He serves as Vice President of Division VI (interstellar matter) of the International Astronomical Union, and is the incoming Vice President of Commission 51 (bioastronomy) of the International Astronomical Union. He has published many books, including the recent book "Organic Matter in the Universe" (Wiley, 2011). Dr. Yong Zhang is a Research Assistant Professor at the University of Hong Kong. This work was supported by the Research Grants Council of Hong Kong.

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Sun Kwok, Yong Zhang. Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features. Nature, 2011; DOI: 10.1038/nature10542

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Scientists discover way to determine when water was present on Mars and Earth

ScienceDaily (Oct. 20, 2011) — The discovery of the mineral jarosite in rocks analyzed by the Mars Rover, Opportunity, on the Martian surface had special meaning for a team of Syracuse University scientists who study the mineral here on Earth. Jarosite can only form in the presence of water. Its presence on Mars means that water had to exist at some point in the past. The trick is in figuring out if jarosite can be used as a proxy for determining when, and under what conditions, water was present on the planet.

The SU scientists have done just that. In a recent study published in an October issue issue of Earth and Planetary Science Letters, Suzanne Baldwin, professor of Earth Sciences in SU's College of Arts and Sciences; and Joseph Kula, research associate and corresponding author for the study, established the "diffusion parameters" for argon in jarosite. In simpler terms, they discovered a way to use the noble gas argon, which accumulates in jarosite over time, to determine the age of the mineral and the surface conditions under which it formed.

The new study is the first in a series of experiments designed to provide a roadmap of sorts for scientists who may someday study Martian samples brought back to Earth. "Our experiments indicate that over billion-year timescales and at surface temperatures of 20 degrees Celsius (68 degrees Fahrenheit) or colder, jarosite will preserve the amount of argon that has accumulated since the crystal formed," Kula says, "which simply means that jarosite is a good marker for measuring the amount of time that has passed since water was present on Mars."

Moreover, since the development of life requires water, knowing when and for how long water was present on the Martian surface has implications for the search for potential habitats harboring life, the scientists say. "Jarosite requires water for its formation, but dry conditions for its preservation," Baldwin says. "We'd like to know when water formed on the surface of Mars and how long it was there. Studying jarosite may help answer some of these questions."

Jarosite is a byproduct of the weathering of rocks exposed at the surface of a planet (such as Earth and Mars). The mineral forms when the right mixture of oxygen, iron, sulfur, potassium and water is present. Once formed, the crystals begin to accumulate argon, which is produced when certain potassium isotopes in the crystals decay. Potassium decay is a radioactive process that occurs at a known rate. By measuring the isotopes of argon trapped within the crystals, scientists can determine the age of the crystals.

However, because argon is a gas, it can potentially escape rapidly from the crystals under hot conditions or slowly over long durations at cold conditions. In order to determine the reliability of the "argon clock" in jarosite, the scientists had to determine the temperature limits to which the crystals could be subjected and still retain the argon. Using a combination of experiments and computer modeling, the team found that argon remains trapped inside the crystals for long periods of time over a range of planetary surface temperatures.

"Our results suggest that 4 billion-year-old jarosite will preserve its argon and, along with it, a record of the climate conditions that existed at the time it formed," Baldwin says. The scientists are in the process of conducting further studies on jarosite that formed less than 50 million years ago in the Big Horn Basin in Wyoming, which they hope will reveal when the minerals formed and how fast environmental conditions changed from water-saturated to dry. The results can be used as a context for interpreting findings on other planets.

Baldwin and Kula are members of the NASA-funded New York Center for Astrobiology at Rensselaer Polytechnic Institute in Troy, N.Y. The center is one of 10 such centers nationally that are part of the NASA Astrobiology Institute, located at NASA's Ames Research Center at Moffett Field, Calif. Their jarosite research is funded by NASA.

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Joseph Kula, Suzanne L. Baldwin. Jarosite, argon diffusion, and dating aqueous mineralization on Earth and Mars. Earth and Planetary Science Letters, 2011; 310 (3-4): 314 DOI: 10.1016/j.epsl.2011.08.006

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Researchers discover material with graphene-like properties

ScienceDaily (Oct. 14, 2011) — After the Nobel Prize in Physics was awarded to two scientists in 2010 who had studied the material graphene, this substance has received a lot of attention. Together with colleagues from Korea, Dr. Frederik Wolff-Fabris from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now developed and analyzed a material which possesses physical properties similar to graphene. Its structure also resembles iron pnictides, i.e. high temperature superconductors, and it definitely has a promising future: Due to the position of the individual components in the Periodic Table of Elements, some of the atoms can simply be replaced by foreign atoms.

This creates new materials which can be superconductive, magnetic, or behave like topological insulators.

Earlier this year, Dr. Jun Sung Kim came from South Korea to use HZDR's Dresden High Magnetic Field Laboratory to analyze a number of material samples in high magnetic fields. For the first time ever, he and his colleague from Dresden, Dr. Frederik Wolff-Fabris, studied the metal SrMnBi2 and observed something amazing: The material consisting of the three elements strontium, manganese, and bismuth behaves physically similar to the "magical material" graphene.

Due to its composition and the position of its elements in the Periodic Table, SrMnBi2 permits simple and uncomplicated doping with foreign atoms. Inserting small amounts of foreign atoms alters the physical properties of a material. This might result in the creation of new magnets or superconductors.

SrMnBi2 is currently also in the focus of other research groups; but only the use of ultra-high magnetic fields, such as those generated in the Dresden High Magnetic Field Laboratory, permitted these precise results and, thus, a publication in the scientific journal Physical Review Letters. Later this year, Dr. Jun Sung Kim will return to Dresden to conduct additional experiments on SrMnBi2 with Dr. Wolff-Fabris.

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Joonbum Park, G. Lee, F. Wolff-Fabris, Y. Koh, M. Eom, Y. Kim, M. Farhan, Y. Jo, C. Kim, J. Shim, J. Kim. Anisotropic Dirac Fermions in a Bi Square Net of SrMnBi2. Physical Review Letters, 2011; 107 (12) DOI: 10.1103/PhysRevLett.107.126402

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Venus weather not boring after all, scientists discover

ScienceDaily (Sep. 27, 2011) — At first glance, a weather forecaster for Venus would have either a really easy or a really boring job, depending on your point of view. The climate on Venus is widely known to be unpleasant -- at the surface, the planet roasts at more than 800 degrees Fahrenheit under a suffocating blanket of sulfuric acid clouds and a crushing atmosphere more than 90 times the pressure of Earth's. Intrepid future explorers should abandon any hope for better days, however, because it won't change much.

"Any variability in the weather on Venus is noteworthy, because the planet has so many features to keep atmospheric conditions the same," says Dr. Tim Livengood, a researcher with the National Center for Earth and Space Science Education, Capitol Heights, Md., and now with the University of Maryland, College Park, Md.

"Earth has seasons because its rotation axis is tilted by about 23 degrees, which changes the intensity of sunlight and the length of the day in each hemisphere throughout the year. However, Venus has been tilted so much, it's almost completely upside down, leaving it with a net tilt of less than three degrees from the sun, so the seasonal effect is negligible," explains Livengood, who is stationed at NASA's Goddard Space Flight Center in Greenbelt, Md. "Also, its orbit is even more circular than Earth's, which prevents it from getting significantly hotter or cooler by moving closer to or further away from the sun. And while you might expect things to cool down at night -- especially since Venus rotates so slowly that its night lasts almost two Earth months -- the thick atmosphere and sulfuric acid clouds act like a blanket while winds move heat around, keeping temperatures pretty even. Finally, almost all the planet's water has escaped to space, so you don't get any storms or precipitation like on Earth where water evaporates and condenses as clouds."

However, higher up, the weather gets more interesting, according to a new study of old data by NASA and international scientists. The team detected strange things going on in data from telescopic observations of Venus in infrared light at about 68 miles (110 kilometers) above the planet's surface, in cold, clear air above the acid clouds, in two layers called the mesosphere and the thermosphere.

"Although the air over the polar regions in these upper atmospheric layers on Venus was colder than the air over the equator in most measurements, occasionally it appeared to be warmer," said Dr. Theodor Kostiuk of NASA Goddard. "In Earth's atmosphere, a circulation pattern called a 'Hadley cell' occurs when warm air rises over the equator and flows toward the poles, where it cools and sinks. Since the atmosphere is denser closer to the surface, the descending air gets compressed and warms the upper atmosphere over Earth's poles. We saw the opposite on Venus. In addition, although the surface temperature is fairly even, we've seen substantial changes -- up to 54 degrees Fahrenheit (about 30 K change) -- within a few Earth days in the mesosphere -- thermosphere layers over low latitudes on Venus. The poles appeared to be more stable, but we still saw changes up to 27 degrees Fahrenheit (about 15 K change)."

Kostiuk and Livengood are co-authors of a paper about these observations that appeared July 23 in the online edition of the journal Icarus.

"The mesosphere and thermosphere of Venus are dynamically active," said lead author Dr. Guido Sonnabend of the University of Cologne, Germany. "Wind patterns resulting from solar heating and east to west zonal winds compete, possibly resulting in altered local temperatures and their variability over time."

This upper atmospheric variability could have many possible causes, according to the team. Turbulence from global air currents at different altitudes flowing at more than 200 miles per hour in opposite directions could exchange hot air from below with cold air from above to force changes in the upper atmosphere. Also, giant vortexes swirl around each pole. They, too, could generate turbulence and change the pressure, causing the temperature to vary.

Since the atmospheric layers the team observed are above the cloud blanket, they may be affected by changes in sunlight intensity as day transitions to night, or as latitude increases toward the poles. These layers are high enough that they could even be affected by solar activity (the solar cycle), such as solar explosions called flares and eruptions of solar material called coronal mass ejections.

Changes were seen over periods spanning days, to weeks, to a decade. Temperatures measured in 1990-91 are warmer than in 2009. Measurements obtained in 2007 using Goddard's Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) observed warmer temperature in the equatorial region than in 2009. Having seen that the atmosphere can change, a lot more observations are needed to determine how so many phenomena can affect Venus' upper atmosphere over different intervals, according to the team.

"In addition to all these changes, we saw warmer temperatures than those predicted for this altitude by the leading accepted model, the Venus International Reference Atmosphere model," said Kostiuk. "This tells us that we have lots of work to do updating our upper atmospheric circulation model for Venus."

Although Venus is often referred to as Earth's twin, since they are almost the same size, it ended up with a climate very different from Earth. A deeper understanding of Venus' atmosphere will let researchers compare it to the evolution of Earth's atmosphere, giving insight as to why Earth now teems with life while Venus suffered a hellish fate.

The team measured temperature and wind speeds in Venus' upper atmosphere by observing an infrared glow emitted by carbon dioxide (CO2) molecules when they were energized by light from the sun. Infrared light is invisible to the human eye and is perceived by us as heat, but it can be detected by special instruments. In the research, it appeared as a line on a graph from a spectrometer, an instrument that separates light into its component colors, each of which corresponds to a specific frequency. The width of the line revealed the temperature, while shifts in its frequency gave the wind speed.

The researchers compared observations from 1990 and 1991 using Goddard's Infrared Heterodyne Spectrometer instrument at NASA's Infrared Telescope Facility on Mauna Kea, Hawaii, to observations from 2009 using the Cologne Tunable Heterodyne Infrared Spectrometer instrument at the National Optical Astronomy Observatory's McMath Telescope at Kitt Peak, Ariz.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by NASA/Goddard Space Flight Center.

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Guido Sonnabend, Peter Krötz, Frank Schmülling, Theodor Kostiuk, Jeff Goldstein, Manuela Sornig, Dušan Stupar, Timothy Livengood, Tilak Hewagama, Kelly Fast, Arnaud Mahieux. Thermospheric/mesospheric temperatures on Venus: Results from ground-based high-resolution spectroscopy of CO2 in 1990/1991 and comparison to results from 2009 and between other techniques. Icarus, 2011; DOI: 10.1016/j.icarus.2011.07.015

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Scientists discover dielectron charging of water nano-droplet

ScienceDaily (June 28, 2011) — Scientists have discovered fundamental steps of charging of nano-sized water droplets and unveiled the long-sought-after mechanism of hydrogen emission from irradiated water. Working together at the Georgia Institute of Technology and Tel Aviv University, scientists have discovered when the number of water molecules in a cluster exceeds 83, two excess electrons may attach to it -- forming dielectrons -- making it a doubly negatively charged nano droplet. Furthermore, the scientists found experimental and theoretical evidence that in droplets composed of 105 molecules or more, the excess dielectrons participate in a water-splitting process resulting in the liberation of molecular hydrogen and formation of two solvated hydroxide anions.

The results appear in the June 30 issue of the Journal of Physical Chemistry A.

It has been known since the early 1980s that while single electrons may attach to small water clusters containing as few as two molecules, only much larger clusters may attach more than single electrons. Size-selected, multiple-electron, negatively-charged water clusters have not been observed -- until now.

Understanding the nature of excess electrons in water has captured the attention of scientists for more than half a century, and the hydrated electrons are known to appear as important reagents in charge-induced aqueous reactions and molecular biological processes. Moreover, since the discovery in the early 1960s that the exposure of water to ionizing radiation causes the emission of gaseous molecular hydrogen, scientists have been puzzled by the mechanism underlying this process. After all, the bonds in the water molecules that hold the hydrogen atoms to the oxygen atoms are very strong. The dielectron hydrogen-evolution (DEHE) reaction, which produces hydrogen gas and hydroxide anions, may play a role in radiation-induced reactions with oxidized DNA that have been shown to underlie mutagenesis, cancer and other diseases.

"The attachment of multiple electrons to water droplets is controlled by a fine balancing act between the forces that bind the electrons to the polar water molecules and the strong repulsion between the negatively charged electrons," said Uzi Landman, Regents' and Institute Professor of Physics, F.E. Callaway Chair and director of the Center for Computational Materials Science (CCMS) at Georgia Tech.

"Additionally, the binding of an electron to the cluster disturbs the equilibrium arrangements between the hydrogen-bonded water molecules and this too has to be counterbalanced by the attractive binding forces. To calculate the pattern and strength of single and two-electron charging of nano-size water droplets, we developed and employed first-principles quantum mechanical molecular dynamics simulations that go well beyond any ones that have been used in this field," he added.

Investigations on controlled size-selected clusters allow explorations of intrinsic properties of finite-sized material aggregates, as well as probing of the size-dependent evolution of materials properties from the molecular nano-scale to the condensed phase regime.

In the 1980s Landman, together with senior research scientists in the CCMS Robert Barnett, the late Charles Cleveland and Joshua Jortner, professor of chemistry at Tel Aviv University, discovered that there are two ways that single excess electrons can attach to water clusters -- one in which they bind to the surface of the water droplet, and the other where they localize in a cavity in the interior of the droplet, as in the case of bulk water. Subsequently, Landman, Barnett and graduate student Harri-Pekka Kaukonen reported in 1992 on theoretical investigations concerning the attachment of two excess electrons to water clusters. They predicted that such double charging would occur only for sufficiently large nano-droplets. They also commented on the possible hydrogen evolution reaction. No other work on dielectron charging of water droplets has followed since.

That is until recently, when Landman, now one of the world leaders in the area of cluster and nano science, and Barnett teamed up with Ori Chesnovsky, professor of chemistry, and research associate Rina Giniger at Tel Aviv University, in a joint project aimed at understanding the process of dielectron charging of water clusters and the mechanism of the ensuing reaction -- which has not been observed previously in experiments on water droplets. Using large-scale, state-of-the-art first-principles dynamic simulations, developed at the CCMS, with all valence and excess electrons treated quantum mechanically and equipped with a newly constructed high-resolution time-of-flight mass spectrometer, the researchers unveiled the intricate physical processes that govern the fundamental dielectron charging processes of microscopic water droplets and the detailed mechanism of the water-splitting reaction induced by double charging.

The mass spectrometric measurements, performed at Tel Aviv, revealed that singly charged clusters were formed in the size range of six to more than a couple of hundred water molecules. However, for clusters containing more than a critical size of 83 molecules, doubly charged clusters with two attached excess electrons were detected for the first time. Most significantly, for clusters with 105 or more water molecules, the mass spectra provided direct evidence for the loss of a single hydrogen molecule from the doubly charged clusters.

The theoretical analysis demonstrated two dominant attachment modes of dielectrons to water clusters. The first is a surface mode (SS'), where the two repelling electrons reside in antipodal sites on the surface of the cluster. The second is another attachment mode with both electrons occupying a wave function localized in a hydration cavity in the interior of the cluster -- the so-called II binding mode. While both dielectron attachment modes may be found for clusters with 105 molecules and larger ones, only the SS' mode is stable for doubly charged smaller clusters.

"Moreover, starting from the II, internal cavity attachment mode in a cluster composed of 105 water molecules, our quantum dynamical simulations showed that the concerted approach of two protons from two neighboring water molecules located on the first shell of the internal hydration cavity, leads, in association with the cavity-localized excess dielectron, to the formation of a hydrogen molecule. The two remnant hydroxide anions diffuse away via a sequence of proton shuttle processes, ultimately solvating near the surface region of the cluster, while the hydrogen molecule evaporates," said Landman.

"What's more, in addition to uncovering the microscopic reaction pathway, the mechanism which we discovered requires initial proximity of the two reacting water molecules and the excess dielectron. This can happen only for the II internal cavity attachment mode. Consequently, the theory predicts, in agreement with the experiments, that the reaction would be impeded in clusters with less than 105 molecules where the II mode is energetically highly improbable. Now, that's a nice consistency check on the theory," he added.

As for future plans, Landman remarked, "While I believe that our work sets methodological and conceptual benchmarks for studies in this area, there is a lot left to be done. For example, while our calculated values for the excess single electron detachment energies are found to be in quantitative agreement with photoelectron measurements in a broad range of water cluster sizes -- containing from 15 to 105 molecules -- providing a consistent interpretation of these measurements, we would like to obtain experimental data on excess dielectron detachment energies to compare with our predicted values," he said.

"Additionally, we would like to know more about the effects of preparation conditions on the properties of multiply charged water clusters. We also need to understand the temperature dependence of the dielectron attachment modes, the influence of metal impurities, and possibly get data from time-resolved measurements. The understanding that we gained in this experiment about charge-induced water splitting may guide our research into artificial photosynthetic systems, as well as the mechanisms of certain bio-molecular processes and perhaps some atmospheric phenomena."

"You know," he added. "We started working on excess electrons in water clusters quite early, in the 1980s -- close to 25 years ago. If we are to make future progress in this area, it will have to happen faster than that."

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

Journal Reference:

Robert N. Barnett, Rina Giniger, Ori Cheshnovsky, Uzi Landman. Dielectron Attachment and Hydrogen Evolution Reaction in Water Clusters. The Journal of Physical Chemistry A, 2011; 110603091014098 DOI: 10.1021/jp201560n

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Researchers discover superatoms with magnetic shells

ScienceDaily (June 9, 2011) — A team of Virginia Commonwealth University scientists has discovered a new class of 'superatoms' -- a stable cluster of atoms that can mimic different elements of the periodic table -- with unusual magnetic characteristics.

The superatom contains magnetized magnesium atoms, an element traditionally considered as non-magnetic. The metallic character of magnesium along with infused magnetism may one day be used to create molecular electronic devices for the next generation of faster processors, larger memory storage and quantum computers.

In a study published online in the Early Edition of the Proceedings of the National Academy of Sciences, the team reports that the newly discovered cluster consisting of one iron and eight magnesium atoms acts like a tiny magnet that derives its magnetic strength from the iron and magnesium atoms. The combined unit matches the magnetic strength of a single iron atom while preferentially allowing electrons of specific spin orientation to be distributed throughout the cluster.

Through an elaborate series of theoretical studies, Shiv N. Khanna, Ph.D., a Commonwealth professor in the VCU Department of Physics, and his team examined the electronic and magnetic properties of clusters having one iron atom surrounded by multiple magnesium atoms. The team included instructor J. Ulises Reveles and Victor M. Medel, a post-doctoral associate, both from VCU; A. W. Castleman Jr., Ph.D., the Evan Pugh Professor of Chemistry and Physics, and Eberly Distinguished chair in Science in the Department of Chemistry at Penn State University; and Prasenjit Sen and Vikas Chauhan from the Harish-Chandra Research Institute in Allahabad, India.

"Our research opens a new way of infusing magnetic character in otherwise non-magnetic elements through controlled association with a single magnetic atom. An important objective was to discover what combination of atoms would lead to a species that is stable as we put multiple units together," said Khanna.

"The combination of magnetic and conducting attributes was also desirable. Magnesium is a good conductor of electricity and, hence, the superatom combines the benefit of magnetic character along with ease of conduction through its outer skin," he said.

The team found that when the cluster had eight magnesium atoms it acquired extra stability due to filled electronic shells that were far separated from the unfilled shells. An atom is in a stable configuration when its outermost shell is full and far separated from unfilled shells, as found in inert gas atoms. Khanna said that such phenomena commonly occur with paired electrons which are non-magnetic, but in this study the magnetic electronic shell showed stability.

According to Khanna, the new cluster had a magnetic moment of four Bohr magnetons, which is almost twice that of an iron atom in solid iron magnets. A magnetic moment is a measure of the magnetic strength of the cluster. Although the periodic table has more than one hundred elements, there are only nine elements that exhibit magnetic character in solid form.

"A combination such as the one we have created here can lead to significant developments in the area of "molecular electronics" where such devices allow the flow of electrons with particular spin orientation desired for applications such as quantum computers. These molecular devices are also expected to help make denser integrated devices, higher data processing, and other benefits," said Reveles.

Khanna and his team are conducting preliminary studies on the assemblies of the new superatoms and have made some promising observations that may have applications in spintronics. Spintronics is a process using electron spin to synthesize new devices for memory and data processing.

This research was supported by the U.S. Department of Energy.

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

Journal Reference:

Victor M. Medel, Jose Ulises Reveles, Shiv N. Khanna, Vikas Chauhan, Prasenjit Sen, A. Welford Castleman. Hund's rule in superatoms with transition metal impurities. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1100129108

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Electronic life on the edge: Scientists discover the edge states of graphene nanoribbons

ScienceDaily (May 9, 2011) — As far back as the 1990s, long before anyone had actually isolated graphene -- a honeycomb lattice of carbon just one atom thick -- theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the "edge states" of well-ordered nanoribbons.

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn't have.

"Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge," says Michael Crommie of Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Division, who led the research. "We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope."

The team's research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research in Nature Physics, available May 8, 2011 in advanced online publication.

The well-tempered nanoribbon

"Making flakes and sheets of graphene has become commonplace," Crommie says, "but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity" -- typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of "unzipping" carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or "chiral vector," of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as "armchair" edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab's MSD was the research team's theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature -- not the artifact of a folded ribbon or a flattened nanotube -- the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

"Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there's no band gap," Crommie says. "Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up."

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

"In the quantum world, electrons can be described as waves in addition to being particles," Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie, "The optimist says, 'Wow, look at all the ways we can control these states -- this might allow a whole new technology!' The pessimist says, 'Uh-oh, look at all the things that can disturb a nanoribbon's behavior -- how are we ever going to achieve reproducibility on the atomic scale?'"

Crommie himself declares that "meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon's edge."

Although getting there won't be simple -- "The edges have to be controlled," Crommie emphasizes -- "what we've shown is that it's possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries."

"Spatially resolving edge states of chiral graphene nanoribbons," by Chenggang Tao, Liying Jiao, Oleg V. Yazyev, Yen-Chia Chen, Juanjuan Feng, Xiaowei Zhang, Rodrigo B. Capaz, James M. Tour, Alex Zettle, Steven G. Louie, Hongjie Dai, and Michael F. Crommie, appears in Nature Physics and is available online at http://www.nature.com/nphys/index.html. Funded at Berkeley Lab by DOE's Helios Energy Research Center, this collaborative project was supported jointly by the Office of Naval Research, the National Science Foundation, and the U.S. Department of Energy's Office of Science.

Story Source:

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

Journal Reference:

Chenggang Tao, Liying Jiao, Oleg V. Yazyev, Yen-Chia Chen, Juanjuan Feng, Xiaowei Zhang, Rodrigo B. Capaz, James M. Tour, Alex Zettl, Steven G. Louie, Hongjie Dai, Michael F. Crommie. Spatially resolving edge states of chiral graphene nanoribbons. Nature Physics, 2011; DOI: 10.1038/nphys1991

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.

View the original article here

Electronic life on the edge: Scientists discover the edge states of graphene nanoribbons

ScienceDaily (May 9, 2011) — As far back as the 1990s, long before anyone had actually isolated graphene -- a honeycomb lattice of carbon just one atom thick -- theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the "edge states" of well-ordered nanoribbons.

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn't have.

"Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge," says Michael Crommie of Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Division, who led the research. "We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope."

The team's research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research in Nature Physics, available May 8, 2011 in advanced online publication.

The well-tempered nanoribbon

"Making flakes and sheets of graphene has become commonplace," Crommie says, "but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity" -- typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of "unzipping" carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or "chiral vector," of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as "armchair" edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab's MSD was the research team's theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature -- not the artifact of a folded ribbon or a flattened nanotube -- the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

"Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there's no band gap," Crommie says. "Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up."

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

"In the quantum world, electrons can be described as waves in addition to being particles," Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie, "The optimist says, 'Wow, look at all the ways we can control these states -- this might allow a whole new technology!' The pessimist says, 'Uh-oh, look at all the things that can disturb a nanoribbon's behavior -- how are we ever going to achieve reproducibility on the atomic scale?'"

Crommie himself declares that "meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon's edge."

Although getting there won't be simple -- "The edges have to be controlled," Crommie emphasizes -- "what we've shown is that it's possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries."

"Spatially resolving edge states of chiral graphene nanoribbons," by Chenggang Tao, Liying Jiao, Oleg V. Yazyev, Yen-Chia Chen, Juanjuan Feng, Xiaowei Zhang, Rodrigo B. Capaz, James M. Tour, Alex Zettle, Steven G. Louie, Hongjie Dai, and Michael F. Crommie, appears in Nature Physics and is available online at http://www.nature.com/nphys/index.html. Funded at Berkeley Lab by DOE's Helios Energy Research Center, this collaborative project was supported jointly by the Office of Naval Research, the National Science Foundation, and the U.S. Department of Energy's Office of Science.

Story Source:

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

Journal Reference:

Chenggang Tao, Liying Jiao, Oleg V. Yazyev, Yen-Chia Chen, Juanjuan Feng, Xiaowei Zhang, Rodrigo B. Capaz, James M. Tour, Alex Zettl, Steven G. Louie, Hongjie Dai, Michael F. Crommie. Spatially resolving edge states of chiral graphene nanoribbons. Nature Physics, 2011; DOI: 10.1038/nphys1991

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.

View the original article here

Toward faster transistors: Physicists discover physical phenomenon that could boost computers' clock speed

ScienceDaily (May 15, 2011) — In the 1980s and '90s, competition in the computer industry was all about "clock speed" -- how many megahertz, and ultimately gigahertz, a chip could boast. But clock speeds stalled out almost 10 years ago: Chips that run faster also run hotter, and with existing technology, there seems to be no way to increase clock speed without causing chips to overheat.

In this week's issue of the journal Science, MIT researchers and their colleagues at the University of Augsburg in Germany report the discovery of a new physical phenomenon that could yield transistors with greatly enhanced capacitance -- a measure of the voltage required to move a charge. And that, in turn, could lead to the revival of clock speed as the measure of a computer's power.

In today's computer chips, transistors are made from semiconductors, such as silicon. Each transistor includes an electrode called the gate; applying a voltage to the gate causes electrons to accumulate underneath it. The electrons constitute a channel through which an electrical current can pass, turning the semiconductor into a conductor.

Capacitance measures how much charge accumulates below the gate for a given voltage. The power that a chip consumes, and the heat it gives off, are roughly proportional to the square of the gate's operating voltage. So lowering the voltage could drastically reduce the heat, creating new room to crank up the clock.

MIT Professor of Physics Raymond Ashoori and Lu Li, a postdoc and Pappalardo Fellow in his lab -- together with Christoph Richter, Stefan Paetel, Thilo Kopp and Jochen Mannhart of the University of Augsburg -- investigated the unusual physical system that results when lanthanum aluminate is grown on top of strontium titanate. Lanthanum aluminate consists of alternating layers of lanthanum oxide and aluminum oxide. The lanthanum-based layers have a slight positive charge; the aluminum-based layers, a slight negative charge. The result is a series of electric fields that all add up in the same direction, creating an electric potential between the top and bottom of the material.

Ordinarily, both lanthanum aluminate and strontium titanate are excellent insulators, meaning that they don't conduct electrical current. But physicists had speculated that if the lanthanum aluminate gets thick enough, its electrical potential would increase to the point that some electrons would have to move from the top of the material to the bottom, to prevent what's called a "polarization catastrophe." The result is a conductive channel at the juncture with the strontium titanate -- much like the one that forms when a transistor is switched on. So Ashoori and his collaborators decided to measure the capacitance between that channel and a gate electrode on top of the lanthanum aluminate.

They were amazed by what they found: Although their results were somewhat limited by their experimental apparatus, it may be that an infinitesimal change in voltage will cause a large amount of charge to enter the channel between the two materials. "The channel may suck in charge -- shoomp! Like a vacuum," Ashoori says. "And it operates at room temperature, which is the thing that really stunned us."

Indeed, the material's capacitance is so high that the researchers don't believe it can be explained by existing physics. "We've seen the same kind of thing in semiconductors," Ashoori says, "but that was a very pure sample, and the effect was very small. This is a super-dirty sample and a super-big effect." It's still not clear, Ashoori says, just why the effect is so big: "It could be a new quantum-mechanical effect or some unknown physics of the material."

There is one drawback to the system that the researchers investigated: While a lot of charge will move into the channel between materials with a slight change in voltage, it moves slowly -- much too slowly for the type of high-frequency switching that takes place in computer chips. That could be because the samples of the material are, as Ashoori says, "super dirty"; purer samples might exhibit less electrical resistance. But it's also possible that, if researchers can understand the physical phenomena underlying the material's remarkable capacitance, they may be able to reproduce them in more practical materials.

Triscone cautions that wholesale changes to the way computer chips are manufactured will inevitably face resistance. "So much money has been injected into the semiconductor industry for decades that to do something new, you need a really disruptive technology," he says.

"It's not going to revolutionize electronics tomorrow," Ashoori agrees. "But this mechanism exists, and once we know it exists, if we can understand what it is, we can try to engineer it."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology. The original article was written by Larry Hardesty, MIT News Office.

Journal Reference:

L. Li, C. Richter, S. Paetel, T. Kopp, J. Mannhart, R. C. Ashoori. Very Large Capacitance Enhancement in a Two-Dimensional Electron System. Science, 2011; 332 (6031): 825 DOI: 10.1126/science.1204168

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.

View the original article here

Toward faster transistors: Physicists discover physical phenomenon that could boost computers' clock speed

ScienceDaily (May 15, 2011) — In the 1980s and '90s, competition in the computer industry was all about "clock speed" -- how many megahertz, and ultimately gigahertz, a chip could boast. But clock speeds stalled out almost 10 years ago: Chips that run faster also run hotter, and with existing technology, there seems to be no way to increase clock speed without causing chips to overheat.

In this week's issue of the journal Science, MIT researchers and their colleagues at the University of Augsburg in Germany report the discovery of a new physical phenomenon that could yield transistors with greatly enhanced capacitance -- a measure of the voltage required to move a charge. And that, in turn, could lead to the revival of clock speed as the measure of a computer's power.

In today's computer chips, transistors are made from semiconductors, such as silicon. Each transistor includes an electrode called the gate; applying a voltage to the gate causes electrons to accumulate underneath it. The electrons constitute a channel through which an electrical current can pass, turning the semiconductor into a conductor.

Capacitance measures how much charge accumulates below the gate for a given voltage. The power that a chip consumes, and the heat it gives off, are roughly proportional to the square of the gate's operating voltage. So lowering the voltage could drastically reduce the heat, creating new room to crank up the clock.

MIT Professor of Physics Raymond Ashoori and Lu Li, a postdoc and Pappalardo Fellow in his lab -- together with Christoph Richter, Stefan Paetel, Thilo Kopp and Jochen Mannhart of the University of Augsburg -- investigated the unusual physical system that results when lanthanum aluminate is grown on top of strontium titanate. Lanthanum aluminate consists of alternating layers of lanthanum oxide and aluminum oxide. The lanthanum-based layers have a slight positive charge; the aluminum-based layers, a slight negative charge. The result is a series of electric fields that all add up in the same direction, creating an electric potential between the top and bottom of the material.

Ordinarily, both lanthanum aluminate and strontium titanate are excellent insulators, meaning that they don't conduct electrical current. But physicists had speculated that if the lanthanum aluminate gets thick enough, its electrical potential would increase to the point that some electrons would have to move from the top of the material to the bottom, to prevent what's called a "polarization catastrophe." The result is a conductive channel at the juncture with the strontium titanate -- much like the one that forms when a transistor is switched on. So Ashoori and his collaborators decided to measure the capacitance between that channel and a gate electrode on top of the lanthanum aluminate.

They were amazed by what they found: Although their results were somewhat limited by their experimental apparatus, it may be that an infinitesimal change in voltage will cause a large amount of charge to enter the channel between the two materials. "The channel may suck in charge -- shoomp! Like a vacuum," Ashoori says. "And it operates at room temperature, which is the thing that really stunned us."

Indeed, the material's capacitance is so high that the researchers don't believe it can be explained by existing physics. "We've seen the same kind of thing in semiconductors," Ashoori says, "but that was a very pure sample, and the effect was very small. This is a super-dirty sample and a super-big effect." It's still not clear, Ashoori says, just why the effect is so big: "It could be a new quantum-mechanical effect or some unknown physics of the material."

There is one drawback to the system that the researchers investigated: While a lot of charge will move into the channel between materials with a slight change in voltage, it moves slowly -- much too slowly for the type of high-frequency switching that takes place in computer chips. That could be because the samples of the material are, as Ashoori says, "super dirty"; purer samples might exhibit less electrical resistance. But it's also possible that, if researchers can understand the physical phenomena underlying the material's remarkable capacitance, they may be able to reproduce them in more practical materials.

Triscone cautions that wholesale changes to the way computer chips are manufactured will inevitably face resistance. "So much money has been injected into the semiconductor industry for decades that to do something new, you need a really disruptive technology," he says.

"It's not going to revolutionize electronics tomorrow," Ashoori agrees. "But this mechanism exists, and once we know it exists, if we can understand what it is, we can try to engineer it."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology. The original article was written by Larry Hardesty, MIT News Office.

Journal Reference:

L. Li, C. Richter, S. Paetel, T. Kopp, J. Mannhart, R. C. Ashoori. Very Large Capacitance Enhancement in a Two-Dimensional Electron System. Science, 2011; 332 (6031): 825 DOI: 10.1126/science.1204168

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.

View the original article here

Exercise protects the heart via nitric oxide, researchers discover

ScienceDaily (May 4, 2011) — Exercise both reduces the risk of a heart attack and protects the heart from injury if a heart attack does occur. For years, doctors have been trying to dissect how this second benefit of exercise works, with the aim of finding ways to protect the heart after a heart attack.

Researchers at Emory University School of Medicine have identified the ability of the heart to produce and store nitric oxide as an important way exercise protects the heart from injury.

Nitric oxide, a short-lived gas generated within the body, turns on chemical pathways that relax blood vessels to increase blood flow and activate survival pathways. Both the chemical nitrite and nitrosothiols, where nitric oxide is attached to proteins via sulfur, appear to act as convertible reservoirs for nitric oxide in situations where the body needs it, such as a lack of blood flow or oxygen.

The Emory team's results, published online in the journal Circulation Research, strengthen the case for nitrite and nitrosothiols as possible protectants from the damage of a heart attack.

The first author is John Calvert, PhD, assistant professor of surgery at Emory University School of Medicine. The senior author is David Lefer, PhD, professor of surgery at Emory University School of Medicine and director of the Cardiothoracic Research Laboratory at Emory University Hospital Midtown. Collaborators included scientists at University of Colorado, Boulder, and Johns Hopkins University.

"Our study provides new evidence that nitric oxide generated during physical exercise is actually stored in the bloodstream and heart in the form of nitrite and nitrosothiols. These more stable nitric oxide intermediates appear to be critical for the cardioprotection against a subsequent heart attack," Lefer says.

Timing is key -- the benefits of exercise don't last In experiments with mice, the researchers showed that four weeks of being able to run on a wheel protected them from having a coronary artery was blocked; the amount of heart muscle damaged by the blockage was less after the exercise period. Importantly, the mice are still protected a week after the wheel is taken away.

The researchers found that voluntary exercise boosted levels of an enzyme that produces nitric oxide (eNOS, endothelial nitric oxide synthase). Moreover, the levels of eNOS in heart tissue, and nitrite and nitrosothiols in the blood as well as heart tissue, stayed high for a week after exercise ceased, unlike other heart enzymes stimulated by exercise. The protective effects of exercise did not extend beyond four weeks after the exercise period was over, when nitrite and nitrosothiols in the heart returned to baseline.

In mice that lack the eNOS enzyme, exercise did not protect the heart from a coronary blockage, although these mice appeared to lack the ability to exercise as much as normal mice.

Another molecule that appears to be important for the benefits of exercise is the beta-3-adrenergic receptor, which allows cells to respond to the hormones epinephrine and norepinephrine. All of the beneficial effects of voluntary exercise are lost in mice that are deficient in this receptor. One of the effects of stimulating the receptor appears to be activating eNOS. Additional animal studies are currently underway in Lefer's lab to determine the potential benefit of beta-3-adrenergic receptor activating drugs following a heart attack.

The research was supported by the American Diabetes Association, the National Institutes of Health and the Carlyle Fraser Heart Center of Emory University Hospital Midtown.

Story Source:

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

Journal Reference:

J. W. Calvert, M. Elston, J. Pablo Aragon, C. K. Nicholson, B. F. Moody, R. L. Hood, A. Sindler, S. Gundewar, D. R. Seals, L. A. Barouch, D. J. Lefer. Exercise Protects Against Myocardial Ischemia-Reperfusion Injury via Stimulation of ß3-Adrenergic Receptors and Increased Nitric Oxide Signaling: Role of Nitrite and Nitrosothiols. Circulation Research, 2011; DOI: 10.1161/CIRCRESAHA.111.241117

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

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment. Views expressed here do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Beetle bling: Researchers discover optical secrets of 'metallic' beetles

ScienceDaily (Apr. 25, 2011) — Costa Rica was once regarded as the poorest of all the colonies of the Spanish Empire, sadly deficient in the silver and gold so coveted by conquistadors. As it turns out, all of the glittering gold and silver those explorers could have ever wanted was there all along, in the country's tropical rainforests -- but in the form of two gloriously lustrous species of beetle.

Today, the brilliant gold- (Chrysina aurigans) and silver-colored (Chrysina limbata) beetles have given optics researchers new insights into the way biology can recreate the appearance of some of nature's most precious metals, which in turn may allow researchers to produce new materials based on the natural properties found in the beetles' coloring.

A team of researchers at the University of Costa Rica has found that the beetles' metallic appearance is created by the unique structural arrangements of many dozens of layers of exo-skeletal chitin in the elytron, a hardened forewing that protects the delicate hindwings that are folded underneath. A paper about the discovery appears in the first issue of the Optical Society's (OSA) newest open access journal, Optical Materials Express, which launched this month.

The beetles were captured in the University of Costa Rica's Alberto Brenes Mesén Biological Reserve, a tropical rainforest environment. "The metallic appearance of these beetles may allow them to be unnoticed, something that helps them against potential predators," says physicist and study leader William E. Vargas. The surface of their elytra "reflects light in a way that they look as bright spots seen from any direction," he explains. "In a tropical rainforest, there are many drops of water suspended from the leaves of trees at ground level, along with wet leaves, and these drops and wet leaves redirect light by refraction and reflection respectively, in different directions. Thus, metallic beetles manage to blend with the environment."

To interpret the cause of this metallic look, Vargas and his team assumed that a sequence of layers of chitin appears through the cuticle, with successive layers having slightly different refractive indices.. In these beetles, the cuticle, which is just 10 millionths of a meter deep, has some 70 separate layers of chitin -- a nitrogen-containing complex sugar that creates the hard outer skeletons of insects, crabs, shrimps, and lobsters. The chitin layers become progressively thinner with depth, forming a so-called "chirped" structure.

"Because the layers have different refractive indices," Vargas says, "light propagates through them at different speeds. The light is refracted through -- and reflected by -- each interface giving, in particular, phase differences in the emerging reflected rays. For several wavelengths in the visible range, there are many reflected rays whose phase differences allow for constructive interference. This leads to the metallic appearance of the beetles."

This is similar to the way in which a prism breaks white light into the colors of the rainbow by refraction, but in the case of these beetles, different wavelengths, or colors of light are reflected back more strongly by different layers of chitin. This creates the initial palette of colors that enable the beetles to produce their distinctive hues. The mystery the researchers still needed to understand in more detail, however, was how the beetles could so perfectly create the structure causing the brilliant metallic tones of silver and gold.

Using a device they specially designed to measure the reflection of light when it strikes the curved surface of the beetles' elytra, Vargas and his colleagues found that as light strikes the interface between each successive layer (the first interface being the boundary between the outside air and the top chitin layer), some of its energy is reflected and some is transmitted down to the next interface.

"This happens through the complete sequence of interfaces," Vargas says.

Because a portion of the light is reflected, it combines with light of the exact same wavelength as it passes back through layer upon layer of chitin, becoming brighter and more intense. Ocean waves can exhibit the same behavior, combining to produce rare but powerful rogue waves. In the case of the beetles, this "perfect storm" of light amplification produces not only the same colors but also the striking sheen and glimmer that we normally associate with fine jewelry.

In the two beetle species, interference patterns are produced by slightly different wavelengths of light, thus producing either silver or gold colors. "For the golden-like beetle, the constructive interference is found for wavelengths larger than 515 nm, the red part of the visible wavelength range," Vargas says, "while for the silver-like beetle it happens for wavelengths larger than 400 nm -- that is, for the entire visible wavelength range."

"The detailed understanding of the mechanism used by the beetles to produce this metallic appearance opens the possibility to replicate the structure used to achieve it," Vargas says, "and thus produce materials that, for example, might look like gold or silver but are actually synthesized from organic media."

This potentially could lead to new products or consumer electronics that can perfectly mimic the appearance of precious metals. Other products could be developed for architectural applications that require coatings with a metallic appearance. Vargas notes that in the solar industry, for example, chirped multilayer reflectors could be used as back layers supporting the active or light-absorbing medium, to improve the absorption of the back-reflected light.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Optical Society of America.

Journal Reference:

Cristian Campos-Fernández, Daniel E. Azofeifa, Marcela Hernández-Jiménez, Adams Ruiz-Ruiz, William E. Vargas. Visible light reflection spectra from cuticle layered materials. Optical Materials Express, 2011; 1 (1): 85 DOI: 10.1364/OME.1.000085

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