PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 767 February 28, 2006 by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi
ATOM WIRES. The smallest wire width in mass produced electronic devices is about 50 nm, or about 500 atoms across. The ultimate limit of thinness would be wires only one atom wide. Such wires can be made now, although not for any working electronic device, and it is useful to know their properties for future reference. Paul Snijders and Sven Rogge from the Kavli Institute of Nanoscience at the Delft University of Technology and Hanno Weitering from the University of Tennessee build the world's smallest gold necklaces by evaporating a puff of gold atoms onto a silicon substrate which has first been cleared of impurities by baking it at 1200 K. The crystalline surface was cut to form staircase corrugations. Left to themselves, the atoms then self-assemble into wires (aligned along the corrugations) of up to 150 atoms each (see figure at www.aip.org/png ). Then the researchers lower the probe of a scanning tunneling microscope (STM) over the tiny causeway of gold atoms to study the nano-electricity moving in the chain; it both images the atoms and measures the energy states of the atoms' outermost electrons. What they see is the onset of charge density waves---normally variations in the density of electrons along the wire moving in pulselike fashion. But in this case (owing to the curtailed length of the wire) a standing wave pattern is what results---as the temperature is lowered. The wave is a quantum thing; hence certain wavelengths are allowed. In other words, the charge density wave is frozen in place, allowing the STM probe to measure the wave (the electron density) at many points along the wire. Surprisingly, two or more density waves could co-exist along the wire. The charge density disturbance can also be considered as a particlelike thing, including excitations which at times possess a fractional charge. (Snijders et al., Physical Review Letters, 24 February 2006; [EMAIL PROTECTED]) FORMATION OF LARGE FLUID VORTICES: CORPORATE MERGER OR HOSTILE-TAKEOVER? Large, energetic vortex structures commonly form in irregular or turbulent two-dimensional flows. Familiar examples are Jupiter's Red Spot or hurricanes and typhoons on Earth. What is the mechanism that transfers energy from small-scale vortices to these often long-lived, large-scale circulation patterns? Many suggestions have been made, such as a merger of small vortices into larger ones. According to this scenario, the process is similar to the consolidation or merger of many small corporations into a mega-corporation. In a new paper, researchers verify by experiment and simulation a quite different mechanism based on elongation and thinning of small-scale vortices, stretched like taffy by large-scale strain. This process weakens the velocity of the small vortices and transfers their kinetic energy into the large-scales. The thinning mechanism allows the large vortices to drain the energy of the smaller ones, squeezing them dry. Thus, the process is more like a hostile takeover of many small corporations by a larger one that strips their assets and liquidates them. According to the authors, the work provides quantitative models of how a population of small-scale vortices sustains on the large-scale circulations. These results will help to model and predict formation of large-scale vortices in atmospheres and oceans (Chen et al., Physical Review Letters, 3 March 2006; contact Gregory Eyink, Johns Hopkins, [EMAIL PROTECTED]; for an example experimental image, one that shows Van Gogh-like fluid swirls, see www.aip.org/png AN ION TEMPERATURE OF 2 TO 3 BILLION KELVIN, hotter than the interior of any known star, has been achieved in New Mexico. This temperature record was set recently in a test shot at the Z Pinch device at Sandia National Lab where an immense amount of electrical charge is stored in a device called a Marx generator. Many capacitors in parallel are charged up and then suddenly switched into a series configuration, generating a voltage of 8 million volts (a process captured in a famous photograph; see www.aip.org/png ). This colossal electrical discharge constitutes a current of 20 million amps passing through a cylindrical array of wires, which implodes. The imploding material reaches the record high temperature and also emits a large amount of x-ray energy (http://www.aip.org/pnu/2004/split/702-1.html). Why the implosion process should be so hot and why it generates x rays so efficiently (10-15% of all electrical energy is turned into soft x rays) has been a mystery. Now Malcolm Haines (Imperial College) and his colleagues think they have an explanation. In the hot fireball formed after the jolt of electricity passes through, they believe, the powerful magnetic field sets in motion a myriad of tiny vortices (through instabilities in the plasma), which in turn are damped out by the viscosity of the plasma (ionized atoms). In the space of only a few nanoseconds, a great deal of magnetic energy is converted into the thermal energy of the plasma. Last but not least, the hot ions transfer much energy to the relatively cool electrons, energy which is radiated away in the form of x rays. (Haines et al., Physical Review Letters, 24 February 2006; contact Malcolm Haines at [EMAIL PROTECTED]) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. 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