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In This Issue...
JILA Finds Flaw in Model Describing DNA Elasticity
DNA, the biomolecule that provides the blueprint for life, has a lesser-known identity as a stretchy polymer. JILA scientists have found a flaw in the most common model for DNA elasticity, a discovery that will improve the accuracy of single-molecule research and perhaps pave the way for DNA to become an official standard for measuring picoscale forces, a notoriously difficult challenge. JILA is a joint venture of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.
The JILA experiments, described in a new paper,* reveal that a classic model for measuring the elasticity of double-stranded DNA leads to errors when the molecules are short. For instance, measurements are off by up to 18 percent for molecules 632 nanometers long, and by 10 percent for molecules about twice that length. (By contrast, the DNA in a single human cell, if linked together and stretched out, would be about 2 meters long.)
The old elasticity model assumes that polymers are infinitely long, whereas the most popular length for high precision single-molecule studies is 600 nm to 2 microns, NIST/JILA biophysicist Tom Perkins says. Accordingly, several university collaborators developed a new theory, the finite worm-like chain (FWLC) model, which improves accuracy by incorporating three previously neglected effects, including length.
The work builds on JILA expertise in measuring positions of microscopic objects. A DNA molecule (green in the animation) is linked at one end to a moveable stage and at the other end to a polystyrene bead trapped by an infrared laser. While moving the stage to extend the DNA molecule, scientists measure changes in bead position using custom electronics and a second laser. By calculating the force exerted on the bead, based in part on the intensity of the laser, and comparing it to the position of the bead in the optical trap, which acts like a spring, scientists can measure DNA elasticity.
The JILA work is part of a NIST project studying possible use of DNA as a picoforce standard, because enzymes build DNA with atomic precision. DNA already is used informally to calibrate atomic force microscopes. An official standard could, for the first time, enable picoscale measurements that are traceable to internationally accepted units. DNA elasticity could provide a force standard from 0.1 -10 pico-Newtons (pN), where 1 pN is the approximate weight of 100 E. coli bacteria cells, and roughly 6 pN is the force exerted by 1 milliwatt of light reflected off a mirror.
The JILA group collaborated with theorists from the universities of Colorado and Pennsylvania. The work was supported by the Alfred P. Sloan Foundation, a Burroughs Wellcome Fund Career Award in the Biomedical Sciences, the Butcher Foundation, a W.M. Keck Grant in the RNA Sciences, NIST, and the National Science Foundation.
* Y. Seol, J. Li, P.C. Nelson, T.T. Perkins and M.D. Betterton. Elasticity of short DNA molecules: theory and experiment for contour lengths of 0.6–7 µm. Biophysical Journal. Published on-line in BioFAST, Aug. 31, 2007.
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‘Radio Wave Cooling’ Offers New Twist on Laser Cooling
Visible and ultraviolet laser light has been used for years to cool trapped atoms—and more recently larger objects—by reducing the extent of their thermal motion. Now, applying a different form of radiation for a similar purpose, physicists at the National Institute of Standards and Technology (NIST) have used radio waves to dampen the motion of a miniature mechanical oscillator containing more than a quadrillion atoms, a cooling technique that may open a new window into the quantum world using smaller and simpler equipment.
Described in a forthcoming issue of Physical Review Letters,* this demonstration of radio-frequency (RF) cooling of a relatively large object may offer a new tool for exploring the elusive boundary where the familiar rules of the everyday, macroscale world give way to the bizarre quantum behavior seen in the smallest particles of matter and light. There may be technology applications as well: the RF circuit could be made small enough to be incorporated on a chip with tiny oscillators, a focus of intensive research for use in sensors to detect, for example, molecular forces.
The NIST experiments used an RF circuit to cool a 200 x 14 x 1,500 micrometer silicon cantilever—a tiny diving board affixed at one end to a chip and similar to the tuning forks used in quartz crystal watches—vibrating at 7,000 cycles per second, its natural “resonant” frequency. Scientists cooled it from room temperature (about 23 degrees C, or 73 degrees F) to -228 C (-379 F). Other research groups have used optical techniques to chill micro-cantilevers to lower temperatures, but the RF technique may be more practical in some cases, because the equipment is smaller and easier to fabricate and integrate into cryogenic systems. By extending the RF method to higher frequencies at cryogenic temperatures, scientists hope eventually to cool a cantilever to its “ground state” near absolute zero (-273 C or -460 F) , where it would be essentially motionless and quantum behavior should emerge.
Laser cooling is akin to using the kinetic energy of millions of ping-pong balls (particles of light) striking a rolling bowling ball (such as an atom) to slow it down. The RF cooling technique, lead author Kenton Brown says, is more like pushing a child on a swing slightly out of synch with its back-and-forth motion to reduce its arc. In the NIST experiments, the cantilever’s mechanical motion is reduced by the force created between two electrically charged plates, one of which is the cantilever, which store energy like electrical capacitors. In the absence of any movement, the force would be stable, but in this case, it is modulated by the cantilever vibrations. The stored energy takes some time to change in response to the cantilever’s movement, and this delay pushes the cantilever slightly out of synch, damping its motion.
* K.R. Brown, J. Britton, R.J. Epstein, J. Chiaverini, D. Leibfried, and D.J. Wineland. 2007. Passive cooling of a micromechanical oscillator with a resonant electric circuit. Physical Review Letters. [Forthcoming].
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NIST Firebrand Device Could Save U.S. and Japanese Homes
Crackling embers and glowing firebrands might make for a romantic evening in front of the fireplace, but for homeowners in high fire-risk areas, windborne fire material is the stuff of nightmares. To learn how to mitigate such threats, National Institute of Standards and Technology (NIST) researchers have built a firebrand generator that can be used to study the way firebrands ignite structures. The unique device allows for the generation of controlled and repeatable firebrands that can be adjusted to be representative of typical firebrands produced from burning vegetation.
With Japanese colleagues, the NIST researchers are running experiments with the machine in the Fire Research Wind Tunnel Facility (FRWTR) at the Building Research Institute (BRI) in Tsukuba, Japan. The research should help design homes to be more resistant to firebrand ignition in the path of such fires in Wildland-Urban Interface (WUI) zones, areas where structures exist amidst undeveloped land. It should also lead to new building codes and standards to protect homes in the United States and Japan.
Usually fires in WUI zones are caused by “spot” fires that propagate away from the main fire line due to airborne chunks of burning wood and vegetation. In Japan, fires often occur in the aftermath of earthquakes, when displaced traditional ceramic roofing tiles expose the bare wood roof underpinning. As the structures burn, firebrands are produced, and disperse by high winds, resulting in large-scale urban fires. In the United States, the principal threat is forest fires that spread to homes. In California an estimated 41 percent of homes lie in WUI zones, with 3.2 million homes at significant risk from wildfires, according to a report from the Government Accountability Office. Destruction from a single WUI fire event can be severe—in 2003, WUI fires near San Diego, Calif., displaced nearly 100,000 people, destroyed more than 3000 homes and cost over $2 billion in insured losses.*
NIST and BRI fire engineers recently used the NIST firebrand apparatus to observe, for the first time, the mechanism of firebrand penetration through building vents fitted with screens. They generated a controlled firebrand shower onto a structure erected in the Fire Research Wind Tunnel Facility. A gable vent on the front face of the structure and three different-sized steel screens installed behind the gable vent were unable to block firebrands from penetrating the openings. The scientists saw the firebrands burn until the glowing embers fit through the screen openings and ignite fires inside the structure. The results demonstrated the need to design building vents both in Japan and the United States that can resist firebrands.
* Reported at the Second Fire Behavior and Fuels Conference in Destin, Fla., March 26-30, 2007.
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NIST Team Develops Novel Method for Nanostructured Polymer Thin Films
All researchers at the National Institute of Standards and Technology (NIST) wanted was a simple, quick method for making thin films of block copolymers or BCPs (chemically distinct polymers linked together) in order to have decent samples for taking measurements important to the microelectronics industry. What they got for their efforts, as detailed in the Sept. 12, 2007, Nano Letters,* was an unexpected bonus: a unique annealing process that may make practical the use of BCP thin films for patterning nanoscale features in next-generation microchips and data storage devices.
BCP thin films have been highly desired by semiconductor manufacturers as patterns for laying down very fine features on microchips, such as arrays of tightly spaced, nanoscale lines. Annealing certain BCP films—a controlled heating process—causes one of the two polymer components to segregate into regular patterns of nanocylinder lines separated by distances as small as five nanometers or equally regular arrays of nanoscale dots. Chemically removing the other polymer leaves the pattern behind as a template for building structures on the microchip.
In traditional oven annealing the quality of the films is still insufficient even after days of annealing. A process called hot zone annealing—where the thin film moves at an extremely slow speed through a heated region that temporarily raises its temperature to a point just above that at which the cylinders become disordered—has previously been used for creating highly ordered BCP thin films with a minimum of defects but little orientation control. For some polymer combinations, the order-disorder transition temperature is so high that it is virtually impossible for manufacturers to heat them sufficiently without degradation occurring.
To eliminate the time and temperature restraints without losing the order yielded by hot zone annealing, the NIST researchers developed a “cold zone” annealing system where the polymers are completely processed well below their order-disorder transition temperature. Properly controlled, the lower-temperature processing not only works with BCPs for which hot-zone annealing is impractical, but, as the NIST experiments showed, also repeatedly produces a highly ordered thin film in a matter of minutes. NIST researchers also discovered that the alignment of the cylinders was controlled by the “cold zone” annealing conditions. Because it is simple, yields consistent product quality and has virtually no limitations on sample dimensions, the NIST method is being evaluated by microelectronic companies to fabricate highly ordered sub 30 nm features.
The next step, the NIST researchers say, is to better understand the fundamental processes that make the cold zone annealing system work so well and refine the measurements needed to evaluate its performance.
* B.C. Berry, A.W. Bosse, J.F. Douglas, R.L. Jones and A. Karim.Orientational order in block copolymer films zone annealed below the order-disorder transition temperature. Nano Letters, Vol. 7, No. 9, pp. 2789-2794, (Sept. 12, 2007).
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Probing a Rare Material Spin State at NIST
A team of international physicists that includes researchers from the National Institute of Standards and Technology (NIST) has found experimental evidence of a highly sought-after type of arrangement of atomic magnetic moments, or spins, in a series of materials. Their work, one of the very few studies of this particular spin state, which has been postulated as a possible underlying mechanism for high-temperature superconductivity, may eventually serve as a test of current and future theoretical models of exotic spin states.
At the NIST Center for Neutron Research (NCNR) and the Hahn-Meitner Institute in Berlin, Germany, the scientists used intense beams of neutrons to probe a series of antiferromagnets, materials in which each spin—an intrinsic property of an atom that produces a tiny magnetic field called a magnetic “moment”—cancels another, giving the material a net magnetic field of zero. The results, described in the Aug. 26 online edition of Nature Materials,* revealed evidence of a rare and pporly understood “quantum paramagnetic” spin state, in which neighboring spins pair up to form “entangled spin singlets” that have an ordered pattern and that allow the material to weakly respond to an outside magnetic field—i.e., become paramagnetic.
The antiferromagnets used in this work are composed mainly of zinc and copper, and are distinguished by their proportions of each, with the number of copper ions determined by the number of zinc ions. At the atomic level, the material is formed of many repeating layers. The atoms of each layer are arranged into a structure known as a “kagome lattice,” a pattern of triangles laid point-to-point whose basic unit resembles a six-point star.
Physicists have been studying antiferromagnets with kagome structures over the last 20 years because they suspected these materials harbored interesting spin structures. But good model systems, like the zinc/copper compounds used by this group, had not been identified.
At the NCNR, the researchers determined how varying concentrations of zinc and copper and varying temperatures affected fluctuations in the way the spins are arranged in these materials. Using a neutron spectrometer at the Hahn-Meitner Institute, they also investigated the effect of external magnetic fields of varying strengths. The group uncovered several magnetic phases in addition to the quantum paramagnetic state and were able to construct a complete phase diagram as a function of the zinc concentration and temperature. They are planning further experimental and theoretical studies to learn more about the kagome system.
This work was led by S.-H. Lee at the University of Virginia. The other participating institutions are the University of Fukui in Japan, and the Hahn-Meitner Institute and the Technical University of Berlin, both in Germany.
* S.-H. Lee, H. Kikuchi, Y. Qiu, B. Lake, Q. Huang, K. Habicht and K. Kiefer. Quantum-spin-liquid states in the two-dimensional kagome antiferromagnets ZnxCu4-x(OD)6Cl2. Nature Materials advance online publication DOI:10.1038/nmat1986
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New Guide Available for Fractography of Ceramics and Glasses
The National Institute of Standards and Technology (NIST) recently issued a guide to Fractography of Ceramics and Glasses (NIST Special Publication SP 960-16), which aims to increase the use of fractography by scientists and engineers for the analysis of broken brittle materials.
Fractography is a powerful but underutilized tool for the analysis of fractured glasses and ceramics. It applies to fractures created under controlled conditions in the laboratory as well as material failures in real-life situations. It can identify the cause and origin of a failure, determine whether a material contains atypical flaws or if a part was simply overloaded or misused, or why one part broke but others did not. Fractography also can yield quantitative information about the loading conditions at fracture.
The guide emphasizes practical approaches for fracture analysis and problem solving. It also stresses fractography’s value in other applications, such as routine mechanical testing and materials processing. As fractography is largely about pattern recognition, the guide is illustrated with 725 pictures and drawings to aid fractographers, including students, in identifying patterns. Additionally, it includes an extensive bibliography and a list of published case studies.
The guide may be downloaded in Adobe Acrobat (.pdf) format at www.nist.gov/public_affairs/practiceguides/SP960-16.pdf. Single print copies may be obtained free-of-charge by contacting George D. Quinn, (301) 975-5765, firstname.lastname@example.org. Requests for multiple copies will be reviewed on a case by case basis.
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"Guide to Secure Web Services" Provides Blueprint to Safer Web 2.0
Many Web-based services, from shopping to online word processing, allow computer programs to talk to each other and exchange user data across several Web sites without human intervention. Many of the attractive features of this “Web 2.0,” including greater access to information and one-stop transactions that process information from several websites, are at odds with traditional ways of maintaining computer security.
A new NIST publication, called “Guide to Secure Web Services” (NIST Special Publication 800-95), provides details on how to make Web 2.0 more secure while maintaining its flexible and convenient features.
“The security challenges presented by the web services approach are formidable and unavoidable,” according to the publication. “Difficult and unsolved problems exist,” it continues, citing examples such as maintaining confidentiality and integrity in data that is transmitted via intermediary Web sites. Firewalls, which often protect single computers or networks from certain types of attack, are often inadequate to safeguard Web services data traveling between Web sites.
The publication recommends several steps to make Web services more secure. One recommended measure for content providers is to replicate their data and services at backup sites. This would improve the availability of their services in the event of “denial of service” (DoS) attacks intended to shut down a target Web site. Another recommendation is better and more uniform logging of visitors and actions on Web sites. The publication also outlines several existing security techniques for making web services more secure, such as adding encryption to data transmitted through XML (eXtensible Markup Language), a protocol that allows the sharing and manipulation of data across different computer platforms.
The publication is free of charge and available at http://csrc.nist.gov/publications/nistpubs/800-95/SP800-95.pdf.
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Voting Standards Guidance Sent to Election Commission
Recommendations for a new set of requirements intended to make future voting systems more “secure, reliable, and easier for all voters to use” have been submitted in a 598-page report by an advisory panel to the Election Assistance Commission (EAC).
The recommendations are a “complete rewrite” of similar guidelines issued in 2005 by the Technical Guidelines Development Committee (TGDC), an advisory panel established along with the EAC by the Help America Vote Act of 2002. The TGDC is chaired by the director of the National Institute of Standards and Technology (NIST) with technical support provided by NIST staff.
The EAC is expected to conduct a series of public reviews of the TGDC recommendations, consider comments made, and then issue a final version, most likely in 2009. Voting systems will then be required to meet the final guidelines to receive federal certification.
Key new recommendations made by the TGDC to the EAC include guidelines that
The TGDC’s recommendations are called a “Voluntary Voting System Guideline (VVSG),” because individual states and U.S. territories determine their own election laws and thus are not obligated to use voting systems that have received federal certification.
To download a copy of the full report see: http://vote.nist.gov/vvsg-report.htm.
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NIST, SRC-NRI Partner to Advance Next-Generation Computer Technology
The National Institute of Standards and Technology (NIST) and Semiconductor Research Corporation (SRC), a university-research consortium for semiconductors and related technologies, today announced a public-private partnership to support research and innovation in nanoelectronics, with a goal of developing a radical, yet practical, successor to the basic electronic building blocks in today’s computers. Over the next year, NIST will contribute $2.76 million to the effort, which, when combined with funds from industry, will fund nearly $4 million for the first year of a planned five-year program.
The partnership will fund a variety of high-priority research projects identified by the Nanoelectronics Research Initiative (NRI), one of three research program entities of SRC that coordinates work in nanoelectronics among major universities across the country. Through the initiative, researchers will strive to replace the world’s most commonly used electronic component, known as the Complementary Metal-Oxide Semiconductor Field-Effect Transistor (CMOS FET), which has driven the world’s computers for more than 30 years but may hit its technological limits in the next decade. NIST chose NRI as a partner to accelerate research in electronics that goes beyond CMOS though an open competition launched in May, 2007. The competition was part of a NIST-wide effort to explore new models of public-private partnerships for R&D investment to accelerate and promote innovation.
For more details, see NIST, SRC-NRI Enter Partnership to Drive Search for Next-Generation Computer Technology.
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Tech Transfer Event to Showcase NIST Microfluidics Technologies
The National Institute of Standards and Technology (NIST), in cooperation with the MIT Enterprise Forum™ and TEDCO, will host a technology transfer workshop on Oct. 9, 2007, to showcase several of the agency’s microfluidics technologies that have potential for commercial development. The evening meeting will be held at the NRECA Conference Center in Ballston, Va.
Microfluidics is a key technology underlying the development of highly miniaturized “labs on a chip” that have shown promise for enabling rapid, low-cost biochemical analysis and diagnostics. NIST, which has pursued an active research program in microfluidics for several years, will offer presentations on several of its technologies, including:
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