Neutral atoms made to act like electrically charged particles

Completing the story they started by creating synthetic magnetic fields, scientists from the Joint Quantum Institute (JQI), a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland, have now made atoms act as if they were charged particles accelerated by electric fields.

Reported in the journal Nature Physics, these synthetic electric fields make each atom in a gas act, individually, as if it were a charged particle, but collectively they remain neutral, uncharged particles. This dual personality will help researchers simulate and study fundamental electrical phenomena and may lead to a deeper understanding of exotic phenomena involving charged particles such as superconductivity, the flow of electricity without resistance, or the quantum Hall effect, used by NIST to create a standard of electrical resistance.

Some aspects of electricity are difficult to study because, although oppositely charged particles are attracted to one another, similarly charged particles are repelled by one another. To get around this, NIST physicist Ian Spielman and his colleagues realized that they could make atoms, which are typically electrically neutral, act as if they are charged particles in an electric field -- extending their earlier method for making neutral atoms act like charged particles in a magnetic field.

The researchers create their synthetic electric field in an ultracold gas of several hundred thousand rubidium atoms. Using lasers, the team alters the atoms' energy-momentum relationship. This had the effect of transferring a bit of the lasers' momentum to the atoms, causing them to move. The force on each atom is physically identical -- and mathematically equivalent -- to what a charged particle would feel in an electric field.

So while the neutral atoms each experience the force of this synthetic electric field individually, they do not repel each other as would true charged particles in an ordinary electric field. This is analogous to an experienced group of dancers all following the moves of their instructor without getting in each other's way.

According to Spielman, this work may enable scientists to study the Hall effect, a phenomenon where an electromagnetic field can cause charged particles traveling through a conductor to experience a sideways force, which has of yet been unobserved in cold-atom systems. The work may also facilitate measurements of the atomic equivalents of electrical quantities such as resistance and inductance. For neutral atoms in synthetic electric fields, inductance is a measure of the energy that is stored as a result of the atoms' motion, and resistance is a measure of the dissipation, or energy loss, in the system. Measuring these quantities could provide insights into the properties of charged particles in analogous systems, including superconductors.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by National Institute of Standards and Technology (NIST).

Journal Reference:

Y-J. Lin, R. L. Compton, K. Jiménez-García, W. D. Phillips, J. V. Porto, I. B. Spielman. A synthetic electric force acting on neutral atoms. Nature Physics, 2011; DOI: 10.1038/nphys1954

Imaging the paintings under the paintings of the Old Masters

 Gaze upon Rembrandt's The Night Watch, or one of the great Dutch master's famous self-portraits. Contemplate Caravaggio's Boy with a Basket of Fruit, Supper at Emmaus, or the famed Italian artist's Seven Works of Mercy. Admire Peter Paul Rubens' Prometheus Bound, Portrait of Władysław IV, or the Flemish baroque painter's The Exchange of Princesses.

Speaking at the 241st National meeting & Exposition of the American Chemical Society, an international team of scientists have now described use of a new technique to see the paintings under the paintings of Rembrandt, Caravaggio, Rubens, and other 17th Century Old Master painters. The report by scientists in Belgium, The Netherlands and the United States was among almost two dozen studies presented as part of a symposium on chemistry and art titled "Partnerships and New Analytical Methodologies at the Interface of Chemistry and Art," presented on March 29 in Anaheim, California.

"The underpainting was the first and most important step in creating a work of art," explained lead scientist Matthias Alfeld, who is with the University of Antwerp in Belgium. "It was the sketch that guided the artist through the creative process. The Old Masters generally used to roughly indicate light, shade and contours. Observation of the underpainting would allow us to see the first execution of the artist's vision of the painting. It's a more detailed look over the shoulder of the artist at work. But the underpainting has virtually escaped all imaging efforts. So far, our methods to visualize the underpainting, except in localized cross sections, have been very limited."

Alfeld and colleagues described use of a powerful new technique called scanning macro X-ray fluorescence analysis that allows more detailed imaging of the composition of underpaintings. It is portable enough for use on-the-scene in museums and does not harm priceless artwork. The technology already has provided new insights into the nature of the paint that some Old Masters used in their underpainting.

An analysis of paintings from the workshops of Rembrandt and Caravaggio, for instance, led them to the conclusion that the Old Masters were more frugal than fussy about the paint used for the underpainting. The analysis suggested that this brown pigment mixture in underpaintings actually consisted of recycled leftovers from the artist scraping his palette clean.

"Using the new technique, we hope to disperse doubts about the authenticity of several paintings or to confirm that these paintings were not by the painter they have been attributed to," Alfeld said. "It is nice to show that the world of art can intersect with chemistry. Chemistry is such an all-encompassing science. Imagine, chemistry isn't just about molecules and reactions, but it also involves also the study of something as beautiful as great works of art."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Chemical Society.

'Spincasting' holds promise for creation of nanoparticle thin films

 Researchers from North Carolina State University have investigated the viability of a technique called "spincasting" for creating thin films of nanoparticles on an underlying substrate -- an important step in the creation of materials with a variety of uses, from optics to electronics.

Spincasting, which utilizes centrifugal force to distribute a liquid onto a solid substrate, already has a variety of uses. For example, it is used in the electronics industry to deposit organic thin films on silicon wafers to create transistors.

For this study, the researchers first dispersed magnetic nanoparticles coated with ligands into a solution. The ligands, small organic molecules that bond directly to metals, facilitate the even distribution of the nanoparticles in the solution -- and, later, on the substrate itself.

A drop of the solution was then placed on a silicon chip that had been coated with a layer of silicon nitride. The chip was then rotated at high speed, which spread the nanoparticle solution over the surface of the chip. As the solution dried, a thin layer of nanoparticles was left on the surface of the substrate.

Using this technique, the researchers were able to create an ordered layer of nanoparticles on the substrate, over an area covering a few square microns. "The results are promising, and this approach definitely merits further investigation," says Dr. Joe Tracy, an assistant professor of materials science and engineering at NC State and co-author of a paper describing the study.

Tracy explains that one benefit of spincasting is that it is a relatively quick way to deposit a layer of nanoparticles. "It also has commercial potential as a cost-effective way of creating nanoparticle thin films," Tracy says.

However, the approach still faces several hurdles. Tracy notes that modifications to the technique are needed, so that it can be used to coat a larger surface area with nanoparticles. Additional research is also needed to learn how, or whether, the technique can be modified to achieve a more even distribution of nanoparticles over that surface area.

Analysis of the nanoparticle films created using spincasting led to another development as well. The researchers adapted analytical tools to evaluate transmission electron microscopy images of the films they created. One benefit of using these graphical tools is their ability to identify and highlight defects in the crystalline structure of the layer. "These methods for image analysis allow us to gain a detailed understanding of how the nanoparticle size and shape distributions affect packing into monolayers," Tracy says.

The paper, "Formation and Grain Analysis of Spin Cast Magnetic Nanoparticle Monolayers," was published online March 24 by the journal Langmuir. The paper was co-authored by Tracy; NC State Ph.D. student Aaron Johnston-Peck; and former NC State post-doctoral research associate Dr. Junwei Wang. The research was funded by the National Science Foundation, the U.S. Department of Education, and Protochips, Inc.

NC State's Department of Materials Science and Engineering is part of the university's College of Engineering.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by North Carolina State University.

Journal Reference:

Aaron C. Johnston-Peck, Junwei Wang, Joseph B. Tracy. Formation and Grain Analysis of Spin-Cast Magnetic Nanoparticle Monolayers. Langmuir, 2011; 110324110441093 DOI: 10.1021/la200005q

Blocking carbon dioxide fixation in bacteria increases biofuel production

 Reducing the ability of certain bacteria to fix carbon dioxide can greatly increase their production of hydrogen gas that can be used as a biofuel. Researchers from the University of Washington, Seattle, report their findings in the current issue of online journal mBio®.

"Hydrogen gas is a promising transportation fuel that can be used in hydrogen fuel cells to generate an electric current with water as the only waste product," says Caroline Harwood, who conducted the study with James McKinlay. "Phototrophic bacteria, like Rhodopseudomonas palustris obtain energy from light and carbon from organic compounds during anaerobic growth. Cells can naturally produce hydrogen gas biofuel as a way of disposing of excess electrons."

Feeding these bacteria more electron rich organic compounds though, does not always produce the logically expected result of increased hydrogen production. Harwood and McKinlay analyzed metabolic functions of R. palustris grown on four different compounds to better understand what other variables might be involved.

One factor involved appears to be the Calvin cycle, a series of biochemical reactions responsible for the process known as carbon dioxide fixation. The Calvin cycle converts carbon dioxide and electrons into organic compounds. Therefore carbon dioxide-fixation and hydrogen production naturally compete for electrons.

When they tested a strain of the bacterium, which had been genetically modified to block carbon dioxide-fixation they observed an increased output of hydrogen from all four substrates.

The Calvin cycle was not the only variable affecting hydrogen production that Harwood and McKinlay identified in the paper. They also determined that the metabolic route a growth substrate took on its way to becoming a building block for making new cells also played a role.

"Our work illustrates how an understanding of bacterial metabolism and physiology can be applied to engineer microbes for the production of sustainable biofuels," says Harwood.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by American Society for Microbiology, via EurekAlert!, a service of AAAS.

Journal Reference:

J. B. McKinlay, C. S. Harwood. Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H2 Biofuel Yield in Photoheterotrophic Bacteria. mBio, 2011; 2 (2): e00323-10 DOI: 10.1128/mBio.00323-10

Biological molecules select their spin

Do the principles of quantum mechanics apply to biological systems? Until now, says Prof. Ron Naaman of the Institute's Chemical Physics Department (Faculty of Chemistry), both biologists and physicists have considered quantum systems and biological molecules to be like apples and oranges. But research he conducted together with scientists in Germany, which appeared recently in Science, definitively shows that a biological molecule -- DNA -- can discern between quantum states known as spin.

Quantum phenomena, it is generally agreed, take place in extremely tiny systems -- single atoms, for instance, or very small molecules. To investigate them, scientists must usually cool their material down to temperatures approaching absolute zero. Once such a system exceeds a certain size or temperature, its quantum properties collapse, and "every day" classical physics takes over. Naaman: "Biological molecules are quite large, and they work at temperatures that are much warmer than the temperatures at which most quantum physics experiments are conducted. One would expect that the quantum phenomenon of spin, which exists in two opposing states, would be scrambled in these molecules -- and thus irrelevant to their function."

But biological molecules have another property: they are chiral. In other words, they exist in either "left-" or "right-handed" forms that can't be superimposed on one another. Double-stranded DNA molecules are doubly chiral -- both in the arrangement of the individual strands and in the direction of the helices' twist. Naaman knew from previous studies that some chiral molecules can interact in different ways with the two different spins. Together with Prof. Zeev Vager of the Particle Physics and Astrophysics Department, research student Tal Markus, and Prof. Helmut Zacharias and his research team at the University of Münster, Germany, he set out to discover whether DNA might show some spin-selective properties.

The researchers fabricated self-assembling, single layers of DNA attached to a gold substrate. They then exposed the DNA to mixed groups of electrons with both directions of spin. Indeed, the team's results surpassed expectations: The biological molecules reacted strongly with the electrons carrying one of those spins, and hardly at all with the others. The longer the molecule, the more efficient it was at choosing electrons with the desired spin, while single strands and damaged bits of DNA did not exhibit this property. These findings imply that the ability to pick and choose electrons with a particular spin stems from the chiral nature of the DNA molecule, which somehow "sets the preference" for the spin of electrons moving through it.

In fact, says Naaman, DNA turns out to be a superb "spin filter," and the team's findings could have relevance for both biomedical research and the field of spintronics. If further studies, for instance, bear out the finding that DNA only sustains damage from spins pointing in one direction, then exposure might be reduced and medical devices designed accordingly. On the other hand, DNA and other biological molecules could become a central feature of new types of spintronic devices, which will work on particle spin rather than electric charge, as they do today.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Weizmann Institute of Science.

Journal Reference:

B. Gohler, V. Hamelbeck, T. Z. Markus, M. Kettner, G. F. Hanne, Z. Vager, R. Naaman, H. Zacharias. Spin Selectivity in Electron Transmission Through Self-Assembled Monolayers of Double-Stranded DNA. Science, 2011; 331 (6019): 894 DOI: 10.1126/science.1199339