PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 800 November 9, 2006 by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi www.aip.org/pnu
COHERENT EXCITON MATTER has been reported by a UC San Diego group. They say their cold swarm of excitons acts like a Bose-Einstein condensate (BEC) but might also be some new kind of quantum condensate in which many particles act as if they were a single entity. Excitons are artificial tiny paired objects adrift in a semiconductor and consist of an electron excited from its home orbital plus a vacancy left behind. The negatively charged electron and the positively charged hole are bound to each other and will usually, after a nanosecond or so, recombine, an event which knocks the electron back into its home band and releases a tiny parcel of light. This is how light emitting diodes (LEDs) produce their illumination. In the UCSD experiment, the excitons live much longer than usual--long enough to study as if there were a species of atom--since the pair partners are held somewhat apart in nano-size structures (quantum wells) in the body of the semiconductor sample. The excitons can move about in the plane of the quantum well semiconductor structure and, in a 20-micron-wide ring, constitute a sort of gas which can be cooled down to ultralow temperatures. When chilled below 5 K, this gas began to show signs that it had spontaneously condensed into a kind of quantum coherent state. Just as atomic BECs (in which atoms are cooled to the point where their matter waves overlap and become, in effect, a single coherent quantum system) are imaged by releasing the atoms from their confining magnetic fields, so in this case the exciton condensate reveals itself by telltale photons. The photons, coming from the annihilation of electrons with their hole partners, leave the ring area, enter an optical device and, in the form of waves (which are proxies for the original excitons), produce a high-contrast interference pattern, suggesting the coherent nature of the exciton gas. Furthermore, the contrast of the interference fringes provides the coherence length--about 2 microns (images at http://ucsdnews.ucsd.edu/graphics/images/2006/exciton06_bg.jpg ). San Diego researcher Leonid Butov ([EMAIL PROTECTED]) believes that controversy has surrounded previous reports of exciton condensates and believes the new results are particularly clear in displaying an interference pattern and in demonstrating quantum coherence. Like others who study coherent matter, Butov predicts that practical quantum devices will follow from this line of research. (Yang et al., Physical Review Letters, 3 November 2006; UCSD press release at http://ucsdnews.ucsd.edu/newsrel/science/exciton.asp) MEASURING ABSOLUTE MAGNETIC MOMENTS. As long ago as Lord Kelvin's time, a century ago, scientists recognized that a change in magnetic environment could alter a material's electrical resistance. Back then this was a 5 percent effect. By the 1990's, however, if the material consisted of a sandwich of alternating magnetic and nonmagnetic films the effect could amount to as much as 60 percent. This "giant magnetoresistance" is now the operative mechanism behind much of hard-drive and tape data storage. By forcing the tiny induced electric currents to jump across insulating layers as it passes from one magnetic layer after another, the change in resistance can be as high as 400 percent. This translates into data storage densities as high as 300 billion bits per square inch. Fabio da Silva of the University of Colorado at Denver and Health Science Center ([EMAIL PROTECTED]) and his colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado now employ the magnetoresistance effect not to store data but to study the very process by which the tiny domains in the storage medium are magnetized in the first place. Using anisotropic magnetoresistance (AMR), in which the resistance of the sample changes when the magnetic material is remagnetized in a direction different from that of the flowing current, the researchers could measure the fundamental magnetism of the tiny (4-micron) domain itself. They hope these results can serve as a reference for calibrating conventional magnetometers, devices that measure magnetic fields in objects ranging from consumer electronics devices to geophysical specimens. These results are being reported next week at the AVS (vacuum science) symposium in San Francisco. (Wednesday, November 15, 2006, 8:40am; Abstract at http://www.avssymposium.org/paper.asp?abstractID=199 ) *********** 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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