Highlights from Kansas City March Meeting
Approximately 4,400 physicists converged on the Kansas City Convention Center in Kansas City, Missouri, 17-21 March, for the Society's annual March Meeting. Over 4,500 technical papers were presented, mostly on topics in condensed matter and materials physics, as well as related fields. APS units represented at the meeting included the Divisions of Biological Physics, Chemical Physics, Condensed Matter Physics, Fluid Dynamics, High Polymer Physics, and Materials Physics. Recently established Topical Groups on Statistical and Nonlinear Physics and Magnetisim and its Applications as well as the Forum on Industrial and Applied Physics were also represented.
Among the technical highlights were numerous sessions on carbon nanotubes, as well as the measurement of atto-Newton forces using magnetic resonance microscopy, the latest on switchable mirrors, the observance of self-organized criticality in mammalian brain cells, the development of x-ray microprobes, and splitting DNA using optical tweezers. There was also a special session featuring lectures by this year's Nobel Prize winners.
Nontechnical highlights included a session on the challenges of handling nuclear waste, a look at entrepreneurial physics, and a humorous series of lectures exploring the "lighter" side of science, the third such session organized by the APS Topical Group on Instrument and Measurement Science since 1994. The meeting also featured celebrations of the 100th anniversary of the discovery of the electron and the 50th anniversary of the transistor.
Prizes and Awards. The traditional ceremonial session for the bestowal of prizes and awards was held Monday evening, followed by a reception hosted by APS President D. Allan Bromley (Yale University). Ten APS prizes and awards were presented, and the recipients gave lectures on their respective award-winning topics at various sessions throughout the week. Citations and brief biographical summaries of the recipients appeared in the March 1997 issue of APS News.
Room-Temperature Mid-Infrared Lasers. At two Thursday morning sessions, researchers described various new designs for mid-infrared lasers, devices that deliver light with a wavelength of 2-6 microns. Since this light is transparent in the atmosphere, mid-infrared lasers would be ideal for detecting pollutants, and serving as a part of a collision-avoidance system in automobiles. Although it has traditionally been difficult to produce laser light in this part of the spectrum, researchers in the past year have successfully produced room-temperature versions. In addition, they are striving to improve the notorious inefficiency of these devices. Steven Pei of the University of Houston reported details on a brand-new mid-infrared laser design based on antimony.
Chocolate and Shock Absorbers. An electrorheological (ER) fluid is one whose stiffness can be controlled with electric fields. Years of research in this area have yet to result in practical applications, partly through a lack of sufficient control over the ER process. A notable exception is the no-fat ER chocolate bar described to the left on page 4. But this may change soon. At a Friday morning session, Clark Radcliffe of Michigan State University announced the development of a feedback control mechanism using laser light to sense the amount of "chaining" among molecules in the fluid. One byproduct of the precision control of the ER process itself is a 30-fold increase in the ER response time. This should encourage the advent of ER-based components for hydraulic valves, clutches, and shock absorbers. Moreover, one MSU researcher has indeed been studying how ER can be used in the manufacture of low-fat chocolate.
Compact Discs and Blue Lasers. On Monday morning, Shuji Nakamura of Nichia Chemical Industries in Japan announced the development of a room-temperature, continuous-operation, blue-light laser with a much longer lifetime than previous devices. Light-emitting diodes, already in use in display systems, are efficient producers of light (red and blue) and will soon be used in automobile brake lights and traffic lights. For optical storage applications, however, one needs diode lasers. For one thing, the digital versatile discs (DVDs) appearing on the market now would benefit further from the use of blue-light lasers in place of red-light lasers. The data capacity of the new DVD models, now about 4.7 gigabytes (GB)), could be increased to 15 GB with the use of blue lasers. But because of the larger quantum energy gap that must be bridged, blue light is harder to produce than red light in semiconductor lasers. Last year Nichia developed a pulsed blue diode laser, based on gallium-nitride materials, which are thought to possess better commercial prospects than GaAs-based lasers.
Two Types of Superconductivity at the Same Time? Putting a direct current (dc) through a metallic sample, physicists can observe the onset of superconductivity as the resistance drops to zero at the critical temperature. In alternating-current (ac) studies, one looks for peaks in a plot of the impedance (analogous to resistance) versus temperature. Performing ac microwave measurements of YBCO superconductors, Srinivas Sridhar of Northeastern University has observed two peaks, one at a temperature just below 93 K (the recognized critical temperature) and another at 65 K, which he interprets as a hint for the onset of a further kind of electron pairing. Signs of this second type of superconductivity may have been present in previous experiments, Sridhar said, but only now, with the use of new high-purity crystal samples, is direct evidence available.
Single Magnetic Atoms. Single magnetic atoms have been found to disrupt superconductivity on an atomic scale, according to researchers at IBM Almaden. As part of their microscopic study of magnetism, Ali Yazdani and his IBM colleagues deposit single manganese (Mn) and gadolinium (Gd) atoms, each of which exerts magnetic forces, onto a niobium metal, which is a superconductor at low temperatures. By measuring the tunneling current that flows from the surface to the probe of a scanning tunneling microscope, the researchers detected the loss of superconductivity in the vicinity of the isolated magnetic atoms. This represents the first time a local loss of the superconducting state at the atomic scale has been detected. The researchers theorize that the atoms break up nearby electron pairs which constitute supercurrents.
Novel Scanning Probes. When electrons trapped at the interface between two semiconductors are subjected to high magnetic fields and are held at low temperature, they can form into strange configurations, some of which appear to have fractional charges. Physicists naturally want to map this "quantum Hall effect" with as much precision as possible. At a Monday morning session, Ray Ashoori of MIT announced the development of a scanning probe which, sitting outside the semiconductor structure in which the 2-dimensional electron structures are forming, can nevertheless render images of the structures with 40-nm spatial resolution. Answering some questions, the images pose new challenges, in the form of unexplained lateral oscillations of the electron clumps. Ashoori's probe may be useful in electronic chip design and testing since it can measure the local density of electrons even hundreds of angstroms inside a semiconductor.
New Liquid Crystal Phase Discovered With DNA. While many scientists study DNA to unravel the mysteries of life, others take advantage of the fact that DNA is an easily replicated and manipulated version of a polymer-a long, chainlike molecule-to discover new intricacies in materials physics. Helmet Strey and his colleagues at the National Institute of Health have prepared a dense array of DNA molecules in solution to form a liquid crystal, a state of matter in which molecules are arranged to a degree of order that lies between that of crystals (highly ordered) and liquids (less highly ordered). Using x rays to observe their samples, they have discovered a new configuration, or phase, of liquid crystal never before observed in any material. Known as the line hexatic phase, it was originally predicted by John Toner of the University of Oregon in the 1980s. The researchers hope that using DNA will allow them to find and to investigate more types of liquid crystalline arrangements.
Waste is a Terrible Thing to Mind. As the Cold War recedes into history and as weapons containing radioactive warheads are dismantled, a vast amount of chemical and fissionable material must be inventoried and disposed of. The cleanup at U.S. Department of Energy labs alone - 120 sites in 36 states - will require many years and billions of dollars. Speakers at a Tuesday afternoon session revealed the extent to which this tremendous social/technical undertaking has turned an important corner, just in the past year. For example, an emphasis on paper studies and litigation has shifted to actual cleanup activities at specific sites. Chaired by David L. Bodde of the University of Missouri at Kansas City, the session speakers included Alvin L. Alm, DOE Assistant Secretary for Environmental Management; Frank L. Parker of Vanderbilt University; Thomas Winston of the Ohio Environmental Protection Agency; and Charles Powers of the Environmental and Occupational Health Sciences Institute.
Separating Microscopic Particles by Exploiting Brownian Motion. At a Tuesday afternoon session, Martin Bier of the University of Chicago described efforts to build a machine that separates different-sized colloids (tiny, slightly charged particles in fluids) by exposing them to fluctuating electric fields and taking advantage of their Brownian motion, the random movements that they experience when they collide with fluid molecules. Bier's team has demonstrated that it is possible to design a device for separating different-sized colloids because particles of unlike sizes experience different levels of friction in a fluid and thereby undergo differing amounts of Brownian motion. One would be able to add particles for separation continuously to such a device, unlike centrifuges, which must be started and stopped every time new particles are added.
10 Megagauss Fields. For a variety of condensed matter experiments (such as studies of the quantum Hall effect), physicists need strong magnetic fields. The highest steady fields attainable are about 40 Tesla (T), or 40,000 gauss. Operating in a pulsed mode, magnets at Los Alamos have reached 70 T, but only for periods of 20 to 50 msec. With nested pulsed magnets, fields in excess of 100 T will be reached in the near future. By resorting to explosive compression of the magnetic flux in a magnet, even higher fields can be achieved. James Brooks of Florida State University reported on the attempt to carry out condensed matter studies in extremely high magnetic fields, albeit for a period of only a microsecond or so. By using 20 kg of explosives (the skills of weapons engineers came in handy at this stage), a solenoid magnet, made in Russia and consisting of a mesh of thin copper wires was imploded, resulting in fields estimated to have been as high as 1000 T, or 10 megagauss.
SQUIDs for Nondestructive Evaluation of Aircraft. Superconducting Quantum Interference Devices (SQUIDs), the most sensitive magnetic field sensors known to date, are now of sufficient sensitivity and reliability to find practical use in biomagnetism, geomagnetism and nondestructive evaluation of aircraft. The latter is the focus of a recent German research collaboration, led by Rohmann GmbH, which has been in the nondestructive testing business for 20 years. According to Hans- Joachim Krause (Institute of Thin Film and Ion Technology, Juelich), SQUIDs are superior to conventional eddy current testing with coils because their high sensitivity at low frequencies allow for larger penetration depths.
Recently, aircraft wheel testing was successfully demonstrated with a prototype SQUID system in the Lufthansa maintenance facility at Frankfurt's airport. One challenge is to operate the SQUID sensor integrated into a handheld system during movement in the strong ambient fields commonly found in aircraft maintenance facilities. Also, the SQUID must be equipped with mobile cooling with a lightweight nitrogen cryostat. In the near future, an automated wheel testing unit will be development. For fuselage testing, the SQUID will be integrated with an industrial scanning system. The project continues until the summer of 1998, after which Rohmann intends to introduce the new SQUID device into the marketplace.
Hydrogen and Deuterium in Silicon Device Processing. Metal oxide semiconductor (MOS) transistors are the basic building blocks of silicon integrated circuits. Their stability and lifetime depend on the degree of perfection of the interface between the semiconductor, silicon and the oxide. The high quality is achieved by introducing hydrogen atoms into the process to passivate any defects, such as dangling bonds. However, recent surface science experiments using a scanning tunneling microscope have shown that incorporating deuterium instead of hydrogen leads to significant improvement in the lifetime of MOS transistors, because it is even harder to desorb from silicon surfaces. In addition to its impact on current generations of devices, the suppression of defects becomes even more essential for future devices, in which oxide thickness and devise dimensions will shrink still further and the effects of any remaining defects will be magnified.
Advanced Memories and CMR. Information technology is a rapidly evolving industry with a continually changing landscape, but one constant factor is the ongoing need for more information storage for a broad range of applications encompassing small portable digital cameras to huge data warehouses used by global companies. According to James Brug of Hewlett-Packard Laboratories, who spoke at a Wednesday morning session, storing information magnetically has long been the dominant technology, but this becomes more difficult as areal densities increase. However, new understanding of the interaction of electrons with the magnetization in thin films and superlattices is enabling the possiblity of storage architectures other than rotating disks. Some alternatives being considered include new magneto-transport effects, such as colossal magnetoresistance and spin tunneling, which might be utilized in advanced memory applications.
Entrepreneurial Physics. On Tuesday morning, the Forum on Industrial and Applied Physics held a session on entrepreneurial physics organized by Ray O'Neal. W.K. Warburton of X-Ray Instrumentation Associates, a small company that develops specialized electronics to support the detectors used at synchrotron radiation facilities, reviewed the issues a small company must deal with in order to survive in the business world. His company is currently extending its expertise to the development of spectrometry instruments for specialized medical markets. Virgil Elings, president of Digital Instruments, also reviewed the history and development of what is now the world's largest manufacturer of scanning probe microscopes, and offered suggestions for entrepreneurial-minded physicists interested in starting a technology business.
Civic Science. Changing the increasingly negative perception of science by the general public and legislature was the focus of a Tuesday morning session on civic science. Speakers included Robert Greenler of the University of Wisconsin, Milwaukee, founder of a highly popular series of science programs for the public called "The Science Bag," now in its 24th year. Since its inception, the program has drawn a cumulative audience of well over 100,000. In addition, Carl M. Bender described how the physics department of Washington University in St. Louis has established a physics and society course as part of a continuing effort to raise the level of science literacy among bright students who do not intend to take further courses in science or mathematics. The course considers such issues as the availability of energy, nuclear weapons, the greenhouse effect, the ozone hole, risk analysis, the scientific method, and claims of the paranormal.
Centenary of the Electron. Wednesday afternoon's program featured a special symposium commemorating the centennial of the discovery of the electron in 1897. According to William Evenson of Brigham Young University, who chaired the session, the electron's discovery culminated an era of cathode ray physics, during which it was not clear whether cathode rays were particles of electricity or waves. Nor was it suspected that there might be particles of subatomic size until J.J. Thomson's classic experiments on the mass-to-charge ratio for cathode rays. The discovery of the electron led not only into investigations of the subatomic world, but ultimately to nuclear physics and quantum physics as well.
Furthermore, it was the particle that showed the existence of intrinsic spin, and also led the way in the development of our understanding of the properties of large collections of subatomic particles. More recently, the study of systems of strongly interacting electrons has led to the discovery of unexpected new phenomena, such as high temperature superconductivity, intermediate valences, and the fractional quantum Hall effect. Although these are not unique to electron systems, their discovery in those systems "has changed the way we understand quantum mechanics and enriched our understanding of how interactions can change physical systems," said Evenson.
Additional highlights and session summaries from the Kansas City meeting are available for viewing or down loading in the online version of APS News located under the APS News button on the APS home page.
Special thanks to Philip F. Schewe and Benjamin Stein of the American Institute of Physics' Public Information Office for contributing to the coverage of technical sessions in this issue.
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