APS News

Quantum Computing, MEMs, Spintronics Mark 1998 L.A. March Meeting

FranMore than 5000 physicists converged on the Los Angeles Convention Center in Los Angeles, California, 16-20 March, for the Society's annual March Meeting. Approximately 4500 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 subunits 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 papers on quantum computing (see page 4), an emerging applied physics field based on spin-dependent phenomena, dubbed "spintronics", and microelectomechanical systems (MEMs). Nontechnical highlights included a special lecture by Ernest Moniz, Under Secretary of the Department of Energy (DOE), providing an overview of the current national funding priorities and impact on DOE research programs, as well as talks on physics, stockpile stewardship, new technologies for energy conservation, and preparing physics graduate students for careers outside the academic Ivory Tower.

The traditional ceremonial session for the bestowal of prizes and awards was held Monday evening, followed by a reception hosted by APS President Andrew Sessler (Lawrence Berkeley Laboratory). Eleven APS prizes and awards were presented, and the winners 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 1998 issue of APS News.

cent Vortex Crystals. Many fluids in nature move principally in two dimensions; one example is the Great Red Spot of Jupiter, the hurricane-like tempest on the giant planet. The mathematical equations describing ideal, frictionless versions of these two-dimensional fluids are identical to those governing the movement of charged particles trapped in a strong magnetic field. Exploiting this connection, Fred Driscoll and his colleagues at the University of California, San Diego have recently built an apparatus that for the first time allows a full spatial imaging of the flow of electrons trapped in a magnetic field. The density of electric charge in such flows is directly analogous to the density of vortices, the whirlpool-like eddies that can exist in a fluid. This approach has enabled them to study phenomena such as the formation of "vortex crystals," repeating patterns of vortices that can stay frozen in place in the fluid.

Eberhardt Bodenschatz of Cornell University reported on another new method that enables physicists to track in detail for the first time the accelerations of a particle moving through flows with atmospheric-level turbulence. He described how a light-sensitive diode measured the movements of a particle jiggling through a fluid at up to 200 times the acceleration of gravity. For a preview of upcoming experiments, the group has installed a "silicon-strip detector" used in high-energy physics to make up to 100,000 measurements per second of multiple particles in the fluid, the better to study how particles that are initially close together move apart in a very turbulent flow such as a volcanic eruption.

Weird Behavior in Quantum Dots. Interesting things happen when particles are confined in a tiny box. Researchers at MIT, led by Raymond Ashoori, make themselves such a box, a quantum dot, out of semiconductors: a layer of gallium arsenide between layers of aluminum gallium arsenide. On top of this sandwich sits a metal gate electrode which feeds electrons into the dot and controls the arrival or departure of electrons one at a time. Building up from just one electron, the MIT physicists collect a puddle of electrons and observe how the arrival of each newcomer must overcome (with the help of an increasing voltage) the mutual repulsion ("Coulomb blockade") of those already in place.

For small dots (0.2 microns across) a graph of charge-vs-voltage would look like a staircase. Such an effect is at the heart of single-electron transistors (SET), which act as sensitive detectors of electrical charge (just as superconducting quantum interference devices - SQUIDS - are sensitive detectors of magnetic flux). For larger dots (1 micron across) the MIT scientists were astonished to observe an unexpected and mysterious pairing: for each stepwise voltage increase not one but two electrons were able to join the puddle. The pairing has not yet been explained but might have something to do with the electrons' spins. For medium-sized dots (0.5 microns) the physics gets even weirder: the pairing occurs only for every fourth or fifth electron. The goal now is to understand how small puddles coalesce into larger puddles and how the pairing comes about.

Magnetic Refrigeration. In a new type of refrigerator, physicists have exploited the fact that the rare earth element gadolinium (Gd) has a large "magnetocaloric effect": applying a magnetic field to a chunk of gadolinium will heat it up, while removing the magnetic field will cool it down and enable it to absorb heat from its surroundings. Not only is this process more efficient than conventional refrigeration (in which the compressing and expanding fluid refrigerants lose energy to processes like turbulence), it is more environmentally friendly: the solid Gd material can't leak out, and the only working fluid is water, which carries heat to the Gd refrigerant from the objects to be cooled. Carl Zimm of Astronautics Corporation in Wisconsin described a Gd-based magnetic refrigerator, built in collaboration with the Ames DOE Laboratory, that has efficiency rivaling that of a conventional unit and has been operating for more than a year. Other physicists at the same session discussed similar approaches. This potentially more efficient design can be used in supermarkets, which could conceivably lower the cost of groceries by using less energy- hungry refrigerators.

Ultrafast Laser Pulses. The interaction of matter with ultrafast and ultra-intense laser pulses is a current frontier of science. New discoveries often result from the ability to explore a new regime. Here one is exploring both extremely short time scales (below one ten-trillionth of a second) and extremely high intensities (above a trillion watts per square centimeter), according to Roland Allen of Texas A&M University, who spoke at a Wednesday morning session. The usual approximations of theoretical physics and chemistry break down under these conditions, and both electrons and atoms exhibit new kinds of behavior. In fullerenes, an ultrashort but less intense pulse can be used to "pluck" the molecule and start it vibrating.

It has already been observed that porovskites can undergo structural changes when exposed to electromagnetic radiation. In semiconductors like Si and GaAs, an ultrashort laser pulse can cause electrons to be quickly promoted from bonding states (in the valence band) to antibonding states (in the conduction band). The lattice is then destabilized, and the band gap collapses to yield a metal-like state. In current experiments, the material undergoes a nonthermal "melting", similar to the thermal melting observed in earlier experiments on longer time scales. Allen's simulations revealed that lattice destabilization occurs at about the same threshold intensity for Si and GaAs, in agreement with the experiments. The reason is that Si has tighter chemical bonding, being a smaller atom, but also has a higher population of excited electrons because of its narrower band gap.

Liquefied Particle Physics. One of the most complex processes known to physicists is the process of turbulence, in which a fluid exhibits irregular flow patterns that vary randomly in space and time. A Cornell group is applying particle physics detection technology to fluid mechanics, by installing a silicon-strip detector also used in Cornell's CLEO accelerator. The detector can make up to 100,000 measurements per second of a particle-and can track perhaps up to 6 particles at a time, promising insights into such questions as how two particles that are initially close together in an extremely turbulent fluid fly apart. Such studies may aid understanding of problems such as the transport of pollutants in the atmosphere.

Quick as a Flash: Femtosecond Lasers. New discoveries often happen at the frontiers of physics. Examples include the discovery of the top quark at Fermilab (the highest obtained particle collision energies) and Bose-Einstein condensates of gases inside magneto-optic traps (the lowest obtained temperatures). A relatively new frontier area is the domain of short-time (100 femtosecond), high-intensity (terrawatts/cm2) bursts of laser light. Pulses generated by femtosecond lasers can almost instantly destabilize a semiconductor, turning it into something like a metal, at least temporarily. Papers presented at a Tuesday session considered what these pulses do in a variety of materials, including C-60 molecules (buckyballs), and human-vision (retinal) and photosynthesis complexes.

Polymer LECs. Alan Heeger of UC-Santa Barbara described a new type of light-emitting device based on a semiconducting polymer. In this design, known as a light-emitting electrochemical cell (LEC), researchers create a blend of a light-emitting polymer material and a solid electrolyte, a substance that transports ions. According to Heeger, polymer LECs have important advantages over polymer LEDs. For example, the way in which electrical charge is injected into the semiconducting polymer is the same for all LECs independent of the color of the emitted light, and the performance of LECs is insensitive to the thickness of the material.

Wavelet Analysis of Heartbeat Patterns. Scientists from Boston University's College of Engineering have developed the first objective diagnostic tool to determine whether a patient suffers from congestive heart failure. A simple mathematical analysis of the pattern of a person's heartbeat detects the problem with 100 percent accuracy. Using a standard data set, the scientists, led by Malvin C. Teich, obtained clinically significant results using this multiresolution wavelet analysis technique to determine how much the time duration of a collection of heartbeats fluctuated and determine whether or not a patient suffers from congestive heart failure.

Traditionally, congestive heart failure is diagnosed by a physician through visual observation, using indicators such as swelling in the patient's ankles and through stethoscopic observation of heart and lung sounds. Teich's technique uses objective criteria to make the diagnosis. "It is perfectly reasonable that using this information we could develop a simple, portable device that an individual would wear to monitor their heartbeat pattern and warn them of a developing problem," Teich said. "Now that we know that this technique can predict congestive heart failure, we will apply it to other cardiac disorders that are more difficult to diagnose."

The Noisy World of the Cell. For the physicist, noise is more than just annoying sounds - it is any type of random and unpredictable variability, especially at the molecular level. There are many types of noise that occur in biological cells. One of the most interesting is ion channel noise. Ion channels are tiny pores that can open or close and allow important substances, such as sodium, potassium, and chlorine in and out of the cell, and are the machinery behind many important cellular processes such as the firing of nerves. But how do random, noisy ion channels produce precise, organized activity? Answers to this fascinating question are just beginning to be revealed, according to Paul C. Gailey of Oak Ridge National Laboratory.

One surprise is that cells may be able to actually use noise to their benefit. For example, ion channel noise can provide just enough of a tickle to make certain nerves fire spontaneously. While too much noise would make for erratic activity, too little can result in no firing at all. The amount of noise in the cell depends in part on the number of ion channels in its membrane. Cells may exploit this fact and control the number of channels to produce optimum noise levels. Some studies indicate that noise can enhance the energy production of cells, amplify signals, and assist in transporting substances in and out of the cell. Others suggest that cells may utilize feedback processes to help suppress ion channel noise. "With characteristic creativity, nature appears to be using noise just as it would any other available resource, said Gailey.

Laser Medicine. If you have ever been the beneficiary of laser surgery, thank a physicist. Rangaswamy Srinivasan of UVTech Associates in New York showed at IBM in the early 1980s that ultrashort laser pulses could vaporize specific regions of biological material without damaging the surrounding material, helping to lead to the use of lasers in medicine. Honored at the APS Biological Physics Prize session, he described the physical explanations of how laser beams can safely remove tissue. The session also featured Michael Berns of UC-Irvine, who discussed how "laser scissors" and "optical tweezers" can now cut and paste chromosomes and sequence genes. Alexander Oraevsky of the University of Texas discussed how aiming laser light at skin lesions can induce sound waves that provide information on whether the lesion is benign or cancerous.

The Mysteries of Water. Essential for life on Earth and the beautiful geological features on the surface of our planet, water has many unusual properties which are not completely understood. For example, heating water shrinks it, unlike most other liquids. Three sessions at the meeting dealt with the physics and chemistry of water. Describing the latest quantum-mechanics based simulations of water, David Clary of University College in London was part of the team that recently discovered that water only begins to act like the liquid with which we are familiar when at least six molecules of H20 are clustered together. Gene Stanley of Boston University discussed an hypothesis that predicts a previously unknown, low-temperature "critical point" for water in which two liquid phases-a higher-density liquid and a lower-density liquid-can coexist.

Making Nanopattern Surfaces. Researchers are pursuing an important new frontier in nanotechnology: to make nanometer-scale patterns of two or more chemically and physically distinct materials. Such nanopattern surfaces could be used as chemical and biological sensors or the "masks" that etch tiny circuit patterns in computer chips. Martin Moeller of Ulm University in Germany described a new technique for creating such surfaces. The technique involves embedding a metal or semiconducting nanoparticle inside a polymer whose interior acts as a "nanocompartment." Subsequently, these nanoparticle-containing polymers are deposited onto polymer films. Afterwards, the polymer shells can be removed with plasma beams similar to those used to etch patterns in computer chips. This leaves metal or semiconducting nanoparticles (1-20 nm in size, depending upon the amount of space in the nanocompartment) which can be separated by 10-200 nm (depending on the overall size of the polymer which embeds the nanoparticle).

Making Smooth Silicon Surfaces With Chemistry. The silicon-silicon dioxide interface in ultrasmall silicon-based transistors must be smooth on the atomic level or else their performance is degraded. If the boundary is too rough, electrons moving through the semiconducting silicon layer can scatter from the insulating SiO2 boundary, increasing electrical resistance to undesirable levels. Melissa Hines of Cornell showed that an ammonium fluoride solution could etch away surface roughness on Si(111) and produce surfaces of near-atomic smoothness over a large area. Hines hopes to find similar chemical methods for the Si(100) surfaces used in integrated circuits.

Marcus Weldon of Lucent Technologies presented studies of how H2O reacts with silicon at elevated temperatures during the beginning stages of forming a silicon dioxide layer. Marrying infrared spectroscopy and quantum chemistry calculations, Weldon and colleagues discovered for the first time a silicon "epoxide," a triangular arrangement of silicon-oxygen-silicon that apparently dominates the surface at the intermediate stages of these reactions. Controlling the quality of SiO2 layers is increasingly important in state-of-the-art silicon devices; one recently fabricated SiO2 layer has a thickness of just three SiO2 molecular units in an ultrasmall silicon transistor announced by Lucent last year and envisioned for mass production by 2010. Built by Greg Timp and colleagues at Lucent, this 60-nm transistor is four times smaller, five times faster, and needs 60 to 160 times less power than present transistors.

Saving Energy in the 21st Century. The research of Mark Levine, director of the Environmental Energy Technologies Division at Lawrence Berkeley Laboratory, has led to enormous energy savings for a number of billion-dollar technologies, such as lighting and windows. He reported on new developments in duct sealants (energy waste in ducts can be as high as 30%); ultraviolet water purification; new incandescent lights; and a redesigned torchiere lamp. Still in the future is an energy-efficient fume hood to provide a safe working environment in laboratories and factories. Levine maintains that while immediate energy savings can be realized by switching to alternative fuels and requiring permits for carbon emissions, new technologies - as well as new materials, processes, and manufacturing methods - will be essential in the long run.

Beyond the Ivory Tower. A joint session of the APS Committee on the Status of Women in Physics and the Forum on Industrial and Applied Physics featured four prominent female physicists who have established successful careers outside of academia, who offered advice to physicists seeking nontraditional employment opportunities. Barbara Wilson (Jet Propulsion Laboratory) described her experiences working in a federally funded R&D center, outlining possible employment areas. Mary Young (Hughes Research Laboratories) focused on some of the key differences between an academic and industrial environment. Both she and Bell Labs' Cherry Murray cited numerous strategies to prepare graduate students for jobs in industry. And Barbara Jones of IBM's Almaden Research Center identified exciting research opportunities in the world of data storage.

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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