APS News

Metallic Hydrogen, Magnetic Surgery Mark 1996 March Meeting

More than 4,500 physicists converged on the St. Louis Convention Center in Missouri, 18-22 March, for the Society's annual March Meeting. Approximately 4,200 technical papers were presented, mostly on topics in condensed matter and materials physics, as well as related fields, making it one of the largest physics meetings ever. APS units represented at the meeting included biological physics, chemical physics, condensed matter physics, fluid dynamics, high polymer physics and materials physics.

Among the technical highlights were sessions on the achievement of a metallic state of liquid hydrogen (see story below), magnetic surgery (see page 3), neural encoding of sensory information (see page 3), and stochastic resonance (see page 4). Other technical sessions reported on the latest developments in scanning probe microscopy, single-electron devices, liquid interfaces, nanoparticles, granular flows, optical communications, spatio-temporal dynamics, and cooperativity in biological macromolecules.

Nontechnical highlights included sessions on science policy in an era of political change (see page 2), the future of international science (see page 5), the current status of women and minorities in physics (see page 8), the history of radioactivity (see page 8), and physical methods of waste management (see page 9).

The meeting also featured sessions on physics journals on the Internet and educational issues, as well as numerous career-related talks and activities. For example, volunteers from the APS Forum on Industrial and Applied Physics (FIAP) served as mentors to groups of students and as individual counselors during the meeting in conjunction with the placement center and two career services workshops. Also featured was a talk by Mitchell Fromstein, CEO of Manpower, Inc., on forces driving the current job market and the potential role of a temporary employment agency for physicists in this market.

The traditional ceremonial session for the bestowal of prizes and awards was held Monday evening, followed by a reception hosted by APS President J. Robert Schrieffer (Florida State 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 1996 issue of APS News.

Technical Sessions

Chemical Sensitivity in Scanning Probe Microscopy. On Monday morning, scientists reported on some of the latest advances in using state-of-the-art microscopes to identify chemical bonds in atoms and molecules, and even to tailor the strength of the bonds. IBM's Phaedon Avouris discussed how the electric field in an STM can be used to tune the strength of a chemical bond, resulting in Stark-shifts of the excitation energies which can suppress lateral energy transfer processes to allow very localized bond breaking.

Furthermore, chemical modification of the force microscope probes and sample surfaces with self-assembled monolayers provides a reducible system for quantifying adhesive and frictional forces between distinct functional groups in liquid media, according to Harvard University's Dmitri Vezenov. This technique is called chemical force microscopy (CFM).

Single-Electron Devices. During the past year, a spectacular breakthrough in the fabrication of nanometer-scale single- electron transistors was achieved by several research groups, and these devices exhibit correlated single-electron tunneling with much higher electron addition energies at room-temperature. For example, LSI Laboratories in Japan fabricated a silicon single-electron transistor, and the University of Minnesota is producing single-hole quantum dot transistors. These ultrasmall structures will allow systematic studies of previously inaccessible features of correlated tunneling, and may eventually find their way into mainstream ULSI technology.

Liquid Interfaces. Michael Wilson of the University of California, San Francisco, described the latest ideas on the chemical mechanisms for anesthesia. For almost a century, it was believed that the site of anesthetics action was located in the oily interiors of neuronal membranes. Recently, however, numerous compounds have been discovered _ such as lipophilic compounds and water-soluble compounds like ethanol or butanol _ for which this basic hypothesis doesn't apply, leading to the conclusion that anesthetic compounds act at water-membrane, or possibly water-protein, interfaces. Using molecular dynamics computer simulations, Wilson and his colleagues have been examining how various solute molecules interact with the water-oil and water- membrane interfaces at a microscopic level of detail not yet available experimentally.

Critical Behavior of Magnetic Films. Many materials become magnetic at low temperatures, and there are existing models to determine the critical temperatures at which this transition occurs for two- and three-dimensional bulk materials. However, to date there has been no general method for calculating critical temperatures for magnetic films of arbitrary thickness, and it is difficult to describe magnetic properties in terms of atomic behavior. D.L. Lin of SUNY-Buffalo has developed a theoretical model that will help scientists understand how the critical temperature changes when film thickness increases from one single atomic layer to many layers. The theory may also be extended to investigate -various properties of magnetic films from an atomic viewpoint, particularly by looking at the cooperative behavior of magnetic atoms in ultrathin films. Thin magnetic films are very useful in electronic devices, especially for information storage.

All-Optical Communications Networks. Transistors and other electronic devices could someday be replaced by optical switches in the telecommunications industry, according to speakers at a Friday morning session, who addressed efforts to produce truly transparent optical communication networks, in which the optical signals are never converted to electrical signals and back again. Such transparent, multiwavelength networks could prove to be high capacity, flexible, reconfigurable and cost-effective, as well as compatible with existing systems and easily upgraded _ all qualities needed to meet the demand for broadband multimedia services.

Dynamics of Granular Flows. Paul Umbanhowar of the University of Texas at Austin described the discovery of a new type of pattern that occurs in the flow of granular materials, such as corn, pebbles and wheat grain. Such knowledge is important for the mixing, processing and manufacturing of such materials in industrial and agricultural settings. The effects of air between the grains and the interaction between the grains and the walls were minimized by using a wide, shallow container. When the grain sample is vibrated, numerous patterns form: stripes, hexagons, and baseball stitches.

Metallic Nanocrystals. Uzi Landman of Georgia Tech described the first formulation of large-scale crystals, measuring about one micron on a side, out of metallic "nanocrystals" measuring just billionths of a meter on a side. Materials structures of nanometer-scale dimensions often exhibit properties and behavior different from those in the bulk, and Landman is currently investigating the energetics, crystalline structure and morphology, electronic and transport properties, stability, and mechanical response characteristics of several forms of these gold nanostructures. In addition to the crystals, other forms include nanowires formed as junctions between materials bodies, and dots supported on a substrate.

Self-Assembly of Nanostructures. Samuel Stupp of the University of Illinois reported on the discovery of molecules that assemble themselves into 1-micron-thick films of "nanomushrooms" of about 100 molecules each that have potentially useful properties. For example, the film can double the frequency of light by converting infrared radiation to green light. It also sticks tenaciously to glass, while its top surface remains non-sticky, so that it resembles a piece of tape. Furthermore, the sticky surface ends up on the top instead of the bottom of the film by drying the precursor solution on a non-sticky surface like teflon. The discovery is helping scientists to understand the principles that rule self-assembly of nanostructrures, such as the proteins produced in nature.

Constant Materials. According to Rick Moore of the Georgia Institute of Technology, percolating materials - special materials composed of conducting and non-conducting elements - surprisingly conduct better as bulk materials rather than thin layers. These materials have potential applications as antennas and as devices to protect against electromagnetic interference. He found that the structures underwent a transition from two-dimensional to three-dimensional systems as thickness increased.

Ultrafast Scanned Probe Microscopes. Geffrey Nunes of Dartmouth College reported on the status of his efforts to make a "movie" of the charge cloud at single defect sites in semiconductors by building up a time sequence with picosecond resolution of STM measurements. He described two approaches to achieve such resolution. One exploits the intrinsic nonlinearity of the current-voltage characteristic in the STM to mix a fast signal with a short electrical "probe" pulse, similar to ultrafast optical cross-correlation techniques. The other performs boxcar measurements of time-dependent STM signals by varying the tunneling impedance, using a magnetorestrictive tip to control the tunneling distance on nanosecond time scales.

Quantum Biology. Advances in theory and computational power have allowed scientists to make models of biological systems at the atomic and molecular scales, where the quantum energy levels of electrons and other quantum-mechanical effects come into play. Duke University's Weitao Yang reported on the development of a linear scaling electronic structure method suitable for biological macromolecules. The method has been used to calculate the electrostatic potential of proteins and nucleic acids, and studies of thermodynamic properties are underway. At SUNY-Buffalo, a hybrid quantum mechanical and molecular mechanical method has been developed for the simulation of molecules in solution, and scientists at Columbia University have developed applicable novel algorithms and electron correlation methods.

Frontiers in Research. The APS Forum on Education sponsored a session featuring topics on the frontiers of physics research aimed at educators and general audiences. Speakers explored physicists' latest insights into the mysteries of how ice and water drops form; new knowledge on the pretty and intricate patterns formed by groups of living cells; the miniaturization of single magnets to nanometer scales and potential applications; and recent studies allowing physicists to stretch a single DNA molecule to test its mechanical strength and other properties.

Nontechnical Sessions

Physics Journals on the Internet. The advent of electronic publishing is having a profound affect on scientific journals. On Tuesday morning, representatives from the American Institute of Physics (AIP) and the American Astronomical Society (AAS) described their efforts to produce on-line versions of their journals. AIP's Applied Physics Letters became available online in January 1995, and the APS journal Physical Review Letters became available online soon thereafter. Other journals are coming online monthly.

Human Rights in China. The APS Committee on the International Freedom of Scientists (CIFS) sponsored a special session in honor of Henry Wu, a former political prisoner in China who captured world headlines last year when the Chinese government first sentenced him to a harsh prison sentence, but then expelled him from China shortly before Beijing hosted the United Nations' Fourth World Conference on Women.

Wu, who emigrated to the U.S. in 1985 and has a degree in geology, spoke informally at the reception, addressing the past and present condition of intellectuals in China. Also making informal comments were Chinese astrophysicist Fang Lizhi (University of Arizona) and physicist Betty Tsang (Michigan State University), both past chairs of CIFS.

Biological Effects of EMFs. The debate continues as to whether the low-frequency electric and magnetic fields from power lines and household appliances affect the cells of living organisms. Some physicists say that ongoing laboratory studies show possible evidence for subtle, though not necessarily damaging, effect of the fields on biological systems. Speakers at a Thursday morning session reviewed recent results from such studies, including work on the ELF charge-to-mass signature, and on the effects of ELF magnetic fields on immune cells.

Special thanks to Philip F. Schewe and Benjamin Stein of the American Institute of Physics' Public Information Office for significant contributions to the coverage of technical sessions in this issue.


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Editor: Barrett H. Ripin