NIST Tech Beat - February 15, 2007

President George W. Bush’s fiscal year (FY) 2008 budget proposal, submitted to the Congress on February 5, requests $640.7 million is for the National Institute of Standards and Technology (NIST).

The proposed NIST budget includes $594.4 million for NIST’s core research and facilities programs, an 11 percent increase over the President’s FY 2007 request and a 21 percent increase over the proposed FY 2007 continuing resolution passed by the Congress. The President’s request will implement key components of the American Competitiveness Initiative (ACI), launched in 2006, which is designed to enhance our nation’s capacity to innovate.

The budget requests includes funding for five major new research initiatives in:

Continued funding also is provided for 12 research initiatives called for in the FY 2007 budget in the areas of nanotechnology, neutron research facilities, the hydrogen economy, manufacturing, quantum information science, structural safety, synchrotron research, international standards, measurement science, bioimaging, cyber security and biometrics.

A new cryogenic refrigerator has been demonstrated at the National Institute of Standards and Technology (NIST) that operates at twice the usual frequency, achieving a long-sought combination of small size, rapid cooling, low temperatures and high efficiency. The cryocooler could be used to chill instruments for space and military applications, and is a significant step toward even smaller, higher-frequency versions for integrated circuits and microelectromechanical (MEM) systems.

The new cryocooler, described in the current issue of Applied Physics Letters,* is a “pulse tube” design that uses oscillating helium gas to transport heat, achieving very cold temperatures (-223 degrees C or -370 degrees F) in a matter of minutes without any cold moving parts. With cold components about 70 by 10 millimeters in size, the device operates at 120 cycles per second (hertz), compared to the usual 60 Hz, which enables use of a much smaller oscillator to generate gas flow, as well as faster cool-down. Because changing the size of one component can negatively affect others, the researchers used a NIST-developed computer model to find the optimal combination of frequency, pressure and component geometry.

The new cryocooler is as efficient as the low-frequency version because it uses a higher average pressure and a finer screen mesh in the regenerator—a stainless steel tube packed with screening that provides a large surface area for transfer of heat between the gas and the steel. This is a key part of the cooling process. The helium gas is pre-cooled by the screen in the regenerator before entering the pulse tube, where the gas is expanded and chilled. The cold gas reverses its direction and carries heat away from the object to be cooled before it enters the regenerator again and picks up stored heat from the screen. Then it is compressed again for a new cycle. Compared to a prototype NIST mini-cryocooler flown on a space shuttle in 2001, the new version is about the same size but gets much colder.

Pulse tube cryocoolers are more durable than conventional (Stirling) cryocoolers typically used in applications where small size is essential. These applications include cooling infrared sensors in space-based instruments used to measure temperature and composition of the atmosphere and oceans for studies of global warming and weather forecasting, and cooling night-vision sensors for tanks, helicopters, and airplanes. With continued work, the NIST researchers hope to increase operating frequencies to 1,000 Hz, which could enable development of chip-scale cryocoolers. Many difficult technical challenges need to be overcome to attain frequencies that high while maintaining high efficiency, such as the design of regenerators with pores just 10 micrometers in diameter.

*S. Vanapalli, M. Lewis, Z. Gan, and R. Radebaugh. 120 Hz pulse tube cryocooler for fast cooldown to 50 K. Applied Physics Letters. 90, 072504 (2007)

Physicists at JILA are using ultrashort pulses of laser light to reveal precisely why some electrons, like ballet dancers, hold their spin positions better than others—work that may help improve spintronic devices, which exploit the magnetism or “spin” of electrons in addition to or instead of their charge. One thing spinning electrons like, it turns out, is some disorder.

JILA is a joint venture of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.

Electrons act like tiny bar magnets whose poles can point up or down. So-called “spintronic” circuits that sense changes in electron spin already are used in very high-density data storage devices, and other spin-based devices are under study. Greater exploitation of spintronics will require spins to be stable—in this case meaning that electrons can maintain their spin states for perhaps tens of nanoseconds while also traveling microscale distances through electronic circuits or between devices.

Scientists have suspected for some time that electrons best maintain the same spin direction at a “magic” electron density. New JILA measurements, described in Nature Physics,* suggest where the magic originates, revealing that electrons actually hold their spins for the longest time—three nanoseconds—when confined around defects, or disordered areas, in semiconductors. They lose their spin alignment in just a few hundred picoseconds when flowing through perfect areas of the crystal. This finding explains the role of density: at very low density, electrons are strongly confined to different local environments, whereas at extremely high density, electrons start hitting each other and lose spin control very fast. The magic point of maximum spin memory occurs at the cross-over between these two conditions.

The JILA research is the first to characterize the so-called electronic disorder in semiconductors and connect it to the spin dynamics. Disorder may arise because, when thin films are being made, imperfections consisting of even one extra layer of a few atoms create islands where electrons act as if they were trapped in stationary molecules. The new findings present a design challenge for spintronic devices, because the conditions that best preserve memory are not conducive to optimum transport properties.

The JILA team confined electrons in “quantum wells,” and used a visible laser beam of varying intensity to systematically vary electron density in the wells. For the measurements, infrared laser pulses were applied in pairs. The first pulse excites some electrons and gives them a spin, creating a temporary magnet. The polarization of light from the second pulse, reflected off the quantum wells, is rotated by the electrons. By measuring the magnitude of that rotation, the researchers infer how many electrons have the same spin. Then an external magnetic field is applied and the electrons rotate around the field, flipping their spins up and down as they go, and causing the reflected light’s polarization to oscillate. Based on the oscillation patterns, scientists can infer electron disorder and calculate spin retention times.

The research was supported in part by the National Science Foundation. The quantum wells were provided by the University of Manchester, United Kingdom.

* Z. Chen, S.G. Carter, R. Bratschitsch, P. Dawson and S.T. Cundiff. Effects of disorder on electron spin dynamics in a semiconductor quantum well. Nature Physics. Posted online Feb. 11, 2007.

Years of comparisons among the world’s best atomic clocks—based on different atoms—have established the most precise limits ever achieved in the laboratory for detecting possible changes in so-called “constants” of nature. The comparisons at the National Institute of Standards and Technology (NIST) may help scientists test the latest theories in physics and develop a more complete understanding of the history of the universe.

Some astronomical and geological studies suggest there might have been very small changes in the values of fundamental constants over billions of years, although the results have been inconsistent and controversial. If fundamental constants are changing, the present-day rates of change are too small to be measured using conventional methods. However, a new comparison of NIST’s cesium fountain and mercury ion clocks, scheduled to appear in this week’s issue of Physical Review Letters,* has narrowed the range in which one of them—the “fine-structure constant”— possibly could be changing by a factor of 20. Widely used in physical theory and experiments, the fine-structure constant, represents the strength of the interaction between electrons and photons.

Astronomers and geologists have attempted to detect changes in natural constants by examining phenomena dating back billions of years. The NIST experiments attained the same level of precision by comparing the relative drifts in the “ticks” of an experimental mercury ion clock, which operates at optical frequencies, and NIST-F1, the national standard cesium clock, which operates at lower microwave frequencies. These data can be plugged into equations to obtain upper limits for possible rates of change of the fine structure constant in recent times.

A second study, based on seven years of comparisons of cesium and hydrogen clocks at NIST and in Europe,** achieved record limits on Local Position Invariance, the principle that two clocks based on natural frequencies of different atoms should undergo proportional frequency shifts when subjected to the same changes in gravitational field. The new experiments lowered the upper limit for a possible violation of LPI, by more than 20 times.

Changes in physical constants such as the fine structure constant or the gravitational constant would violate Albert Einstein’s original theory of general relativity. Such violations are predicted in recent theories aimed at unifying gravitation and quantum mechanics. NIST scientists now plan an all-optical-frequency comparison of the mercury ion clock with an aluminum ion atomic clock, which could increase measurement precision further, offering a more stringent test of the theoretically predicted changes. Conducting such tests with many different types of atomic clocks offers the best chance of eliminating extraneous factors to clearly identify which, if any, of the fundamental “constants” are changing over time.

Partial support for staff and equipment was provided by Los Alamos National Laboratory.

* T.M. Fortier, N. Ashby, J.C. Bergquist, M.J. Delaney, S.A. Diddams, T.P. Heavner, L. Hollberg, W.M. Itano, S.R. Jefferts, K. Kim, F. Levi, L. Lorini, W.H. Oskay, T.E. Parker, J. Shirley and J.E. Stalnaker. Precision atomic spectroscopy for improved limits on variation of the fine structure constant and local position invariance. Physical Review Letters. Feb. 16, 2007.

** N. Ashby, T. P. Heavner, S. R. Jefferts, T. E. Parker, A. G. Radnaev and Y. O. Dudin. Testing local position invariance with four cesium-fountain primary frequency standards and four NIST hydrogen masers. Physical Review Letters. Feb. 16, 2007.

A new report from the National Institute of Standards and Technology (NIST), An Assessment of the United States Measurement System: Addressing Measurement Barriers to Accelerate Innovation, details results of the agency’s first-ever assessment of the capacity of the nation’s measurement infrastructure—a large, diverse collection of private and public-sector organizations—to sustain U.S. innovation at a world-leading pace.

Innovation is vital to the long-term health of the U.S. economy: the better we do in conceiving, developing and applying new technology, the brighter our nation’s future. Innovation has helped the United States sustain the world’s most productive workforce, raise our standard of living and open new avenues of opportunity that inspire a continuing quest to discover, invent and be first to market with new products and services.

In all, more than 1,000 people from industry, academia and government contributed to the wide-ranging NIST assessment of the state of the nation’s measurement system and its impact on innovation. The result is a snapshot appraisal that was formed by surveying measurement needs across 11 industrial sectors and technology areas. These ranged from materials to software and from building and construction to nanotechnology. Altogether, more than 700 measurement-related barriers to innovation were identified and evaluated.

Measurement challenges distilled in the report were identified in 15 specially convened workshops, reviews of more than 160 technology “roadmaps” produced by private and public sector organizations, and interviews. Examples include the need for versatile, high-accuracy methods to measure the three-dimensional geometry of manufactured products and the need for tools for measuring the properties of nanodevices and materials.

For its part, the report says, NIST will use this assessment to focus its own work in support of U.S. innovation and competitiveness. The report’s results and findings, along with input gathered in follow-up activities, will inform NIST’s strategic planning decisions. NIST also plans to work with other organizations in both the private and public sectors to raise awareness of the important role that advances in measurement science and technology play in boosting innovation.

An Assessment of the United States Measurement System: Addressing Measurement Barriers to Accelerate Innovation is available on the NIST Web site at: http://usms.nist.gov/.

Just a little mechanical strain can cause a large drop in the maximum current carried by high-temperature superconductors, according to novel measurements carried out by the National Institute of Standards and Technology (NIST). The effect, which is reversible, adds a new dimension to designing superconducting systems—particularly for electric power applications—and it also provides a new tool that will help scientists probe the fundamental mechanism behind why these materials carry current with no resistance.

The measurements, reported in Applied Physics Letters,* revealed a 40 percent reduction in critical current, the point at which superconductivity breaks down, at just 1 percent compressive strain. This effect can be readily accommodated in the engineering design of practical applications, NIST project leader Jack Ekin says, but knowing about it ahead of time will be important to the success of many large-scale devices. The effect was measured in three types of yttrium-barium-copper-oxide (YBCO), a brittle ceramic considered the best prospect for making low-cost, high-current, superconducting wires. The researchers developed a “four point” bend technique that enables studies of superconducting properties over a wide range of uniform strain at high current levels. The superconductor is soldered on top of a flexible metal beam, which is then bent up or down at both ends while the critical current is measured.

The discovery is the first major reversible strain effect found in practical high-temperature superconductors, which generally have been tested under smaller tensile strains only, or at strains so high they caused the material to break down permanently. The newly discovered effect is totally reversible and symmetric for both compressive and tensile (pushing and pulling) strains, suggesting it is intrinsic to the fundamental mechanism of superconductivity in YBCO.

The NIST team is now pursuing the possibility of using the effect as a new tool for probing the elusive mechanism underlying high-temperature superconductivity. The next step is to investigate how magnetic fields affect the strain effect, and several collaborations are under way with universities and other research organizations to study the interplay of the effect with other factors affecting high-temperature superconductivity. The research described in the new paper was supported in part by the U.S. Department of Energy.

* D.C. van der Laan and J.W. Ekin. Large intrinsic effect of axial strain on the critical current of high-temperature superconductors for electric power applications. Applied Physics Letters, 90, 052506, 2006. Posted online Jan. 31.

Air pollution sources are everywhere in the home, from the bacon and eggs frying in the kitchen, to the woodburning stove in the family room, the newly painted hallway, and even the carpet in the living room. To help estimate the seriousness of these and other indoor pollutant sources as well as to devise ways to reduce possible health impacts, the National Institute of Standards and Technology (NIST) has developed searchable databases of relevant product emission studies.

NIST researchers also have created a software tool called ContamLink that can transfer selected information from the databases into CONTAM, an indoor air quality modeling software program that predicts airflows and contaminant concentrations in multizone building systems. Together—the electronic databases, ContamLink, and the CONTAM program—should significantly accelerate our understanding of indoor air pollution.

The new databases allow investigators to access immediately information that previously was available in scientific literature, but required significant time to locate. The databases include emission rates for consumer products, cooking and combustion appliances (such as gas stoves); and data on contaminant transport mechanisms, including particle deposition, contaminant sorption and different ventilation systems filters. Two of the five databases are from the U.S. Environmental Protection Agency (EPA) and the National Research Council of Canada. Researchers can download the databases, and with ContamLink, selectively obtain relevant information for inclusion in CONTAM or other indoor air quality models. Database entries are not intended to be all-inclusive, but rather representative of the literature. Researchers and other practitioners are encouraged to expand the databases with their own data using the data entry format provided.

The U.S. Department of Housing and Urban Development’s (HUD) Healthy Homes Initiative supported NIST’s development of the databases and software tool. Information on both are available in NISTIR 7364, Database Tools for Modeling Emissions and Control of Air Pollutants from Consumer Products, Cooking and Combustion available on the CONTAM Web site: www.bfrl.nist.gov/IAQanalysis/software/.

Physicists at JILA have demonstrated that the warmer a surface is, the stronger its subtle ability to attract nearby atoms, a finding that could affect the design of devices that rely on small-scale interactions, such as atom chips, nanomachines, and microelectromechanical systems (MEMS).

The research highlights an underappreciated aspect of the elusive Casimir-Polder force, one of the stranger effects of quantum mechanics. The force arises from the ever-present random fluctuation of microscopic electric fields in empty space. The fluctuations get stronger near a surface, and an isolated neutral atom nearby will feel them as a subtle pull—a flimsy, invisible rubber band between bulk objects and atoms that may be a source of friction, for example, in tiny devices. The JILA group previously made the most precise measurement ever of Casimir-Polder, measuring forces hundreds of times weaker than ever before and at greater distances (more than 5 micrometers). JILA is a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.

Now, as reported in last week’s Physical Review Letters*, the JILA team has made the first measurement of the temperature dependence of this force. By using a combination of temperatures at opposite extremes—making a glass surface very hot while keeping the environment neutral and using ultracold atoms as a measurement tool—the new research underscores the power of surfaces to influence the Casimir-Polder force. That is, electric fields within the glass mostly reflect inside the surface but also leak out a little bit to greatly strengthen the fluctuations in neighboring space. As a result, says group leader and NIST Fellow Eric Cornell, “warm glass is stickier than cold glass.”

The experiments demonstrate the practical use of a Bose-Einstein Condensate (BEC), a form of matter first created at JILA a decade ago. In a BEC, thousands of ultracold atoms coalesce into a “superatom” in a single quantum state. Cornell, who shared the 2001 Nobel Prize in Physics for this development, says the purity and sensitivity of a BEC makes it uniquely useful as a tool for measuring very slight forces and changes.

To measure the Casimir-Polder force, a BEC of about 250,000 rubidium atoms in a magnetic trap was placed a few micrometers from a glass plate. As the BEC was brought closer to the surface, the “wiggling” of the condensate was observed over time. Based on the changes in the oscillation frequency, the researchers calculated the force. In the latest experiment, measurements were made as a laser beam was used to heat the glass plate from room temperature (about 37 degrees C or 98 degrees F) to very hot (about 330 degrees C or 630 degrees F), while the surrounding environment was kept near room temperature. The strength of the force was shown to be nearly three times larger when the glass temperature doubled. The researchers also were able to separate the forces emanating from the surface versus the environment.

The research is supported in part by the National Science Foundation. The JILA group collaborated with theorists from the University of Trento, Italy.

* J.M. Obrecht, R.J. Wild, M. Antezza, L.P. Pitaevskii, S. Stringari and E.A. Cornell. Measurement of the temperature dependence of the Casimir-Polder force. Physical Review Letters. Vol. 98, No. 6, Feb. 9, 2007.

Physicists at the National Institute of Standards and Technology (NIST) have taken the first ever two-dimensional pictures of a “frequency comb,” providing extra information that enhances the comb’s usefulness in optical atomic clocks, secure high-bandwidth communications, real-time chemical analysis, remote sensing, and the ultimate in precision control of atoms and molecules.

The work, described in the Feb. 8 issue of Nature,* demonstrates a novel method for separating and identifying thousands of individual colors—or frequencies—of visible light while simultaneously measuring intensity and imaging the results in real time. The pictures transform frequency combs, long imagined as one-dimensional, like hair combs in which individual teeth represent specific frequencies, into two-dimensional brushes, in which many rows of bristles represent frequencies.

“This is really the first time we’ve seen individual elements of the stabilized comb, without interacting it with atoms or probing it with another laser, and it turns out to look more like a brush than a comb,” says lead author Scott Diddams. “We now can see all the bristles at once with high precision.”

Frequency combs are a measurement tool designed and used at NIST and other laboratories for frequency metrology and optical atomic clocks. By providing a second dimension to the typical output of a frequency comb, the new technique efficiently packs more data into a given area without sacrificing precision. All light waves are displayed simultaneously, with a comb resolution as narrow as any other yet demonstrated. In the latest experiments reported in Nature, the researchers made a comb using an ultrafast laser that emits a continuous train of about a billion pulses of light per second, each lasting just a few millionths of a billionth of a second and containing millions of different colors.

The new technique will enable scientists to measure and manipulate optical frequencies in a massively parallel manner, and could make it possible to reliably pack more communications channels with greater security into the same spectrum. The technique also may be useful in optical signal processing, boosting the power of surveillance, remote sensing, trace gas detection and high-speed computing systems, and it could enable the “ultimate” in precision control of atoms and molecules, according to Diddams.

The authors of the Nature paper include a NIST guest researcher from the Council for Scientific and Industrial Research – National Metrology Laboratory and the University of the Witwatersrand, South Africa. The research was supported in part by the Defense Advanced Research Projects Agency.

*S.A. Diddams, L. Hollberg, and V. Mbele. Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb. Nature. Feb 8, 2007

William A. Jeffrey, director of the National Institute of Standards and Technology (NIST), has named three distinguished industry and business experts to serve on the Visiting Committee on Advanced Technology (VCAT), the agency’s primary private-sector policy advisory group. The new VCAT members—who will serve three-year terms until Jan. 31, 2010—bring the body’s number to 12.

Starting their service on the VCAT are Vinton G. Cerf, a vice president at Google; Elsa Reichmanis, Bell Labs Fellow and director of Materials for Communications Research at Alcatel-Lucent; and William Happer Jr., professor of physics at Princeton University.

Cerf received the U.S. National Medal of Technology in 1997 for his work as the co-designer (with Robert Kahn) of the TCP/IP protocols and basic architecture of the Internet. In 2005 Cerf and Kahn received the highest civilian honor bestowed in the U.S., the Presidential Medal of Freedom. It recognized that their work on the software code used to transmit data across the Internet has put them “at the forefront of a digital revolution that has transformed global commerce, communication, and entertainment”.

Reichmanis, president in 2003 of the American Chemical Society, currently co-chairs the National Research Council’s Board on Chemical Sciences and Technology. Among her awards are the 2001 Perkin Medal, 1999 American Chemical Society Award in Applied Polymer Science, 1998 Photopolymer Science and Technology Award; 1996 ASM Engineering Materials Achievement Award, and the 1993 Society of Women Engineers Achievement Award.

Happer, a specialist in laser spectroscopy, optical pumping, radio frequency spectroscopy and magnetic resonance, is a member of JASON, a group of nationally known scientists who advise government agencies on defense, energy and other technical issues. He was the director of the U.S. Department of Energy’s Office of Energy from July 1991 to May 1993.

The VCAT was established by Congress in 1988 to review and make recommendations on NIST’s policies, organization, budget and programs. The current VCAT members are: David Spong (VCAT chair), Retired President Integrated Defense Systems, The Boeing Company, Thomas M. Baer, Executive Director, Stanford Photonics Research Center, Stanford University; John F. Cassidy, Retired Senior Vice President, Science & Technology, United Technologies Corporation; Paul A. Fleury, Dean of Engineering & Frederick W. Beinecke Professor of Engineering and of Applied Physics, Yale University; Gary D. Floss, Director, Quality Assurance & Continual Improvement, Marvin Windows and Doors; Lou Ann Heimbrook, VP, Global Site Services, Merck & Co. Inc.,; James W. Serum, (VCAT vice chair), President, SciTek Ventures; W. Wyatt Starnes, Chairman & CEO, SignaCert, Inc.; and Robert T. Williams, VP Track-Type Tractors Division, Caterpillar Inc.

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Editor: Michael Baum

Date created: February 15, 2007
Date updated: February 16, 2007
Contact: inquiries@nist.gov