Number 81, May 27, 1992 by Phillip F. Schewe and Ben Stein|
THE EXISTENCE OF VORTEX GLASS has been verified experimentally by scientists at IBM (C. Dekker et al., Physical Review Letters, 1 June 1992). One of the problems of using high-temperature superconducting materials in magnets is the fact that if the current flowing through the material is too high, the magnetic flux lines (or magnetic vortices) inside the superconductor can move, causing a dissipation of energy and a loss of the superconducting state. Several years ago a theory arose which foresaw that in contrast to this "vortex liquid" state there existed a "vortex glass" state (reached by cooling the material below a critical temperature) in which the vortex lines were frozen in random positions like the atoms in a glassy substance. The IBM scientists (contact Roger Koch of IBM; 914-945-2393) conducted extensive measurements of current and voltage in yttrium-based superconducting films first to verify that a phase transition was taking place, a fact that had been demonstrated in some previous experiments. Unlike previous experiments, however, they probed the nature of the low-temperature phase and found strong quantitative agreement with the vortex-glass model. Specifically, they determined the value of the exponent which characterizes the phase transition. This exponent, which is directly proportional to the height of the potential barrier that keeps the vortices fixed in their positions, was shown to increase with a decrease in current, a key prediction of the vortex-glass model. These new results discount competing theories, including the flux-creep model, which concludes that high-temperature superconductors in magnetic fields will always have a finite value of resistance, even if immeasurably small, at temperatures above absolute zero.
THE SILICON COMPOUND SILOXENE can be made to luminesce over a range of visible light wavelengths. Light-emitting silicon would greatly promote the growth of opto-electronic technology. Unlike luminescent porous silicon---discovered two years ago---siloxene does not need to be etched in acid and it can emit blue light. Martin S. Brandt of the Max Planck Institute for Solid State Science in Stuttgart, Germany reported at the recent meeting of the Materials Research Society in San Francisco that some of siloxene's properties have been known for a long time but not fully appreciated until now. He and his colleagues grew a calcium-silicon compound (CaSi2) on silicon and then replaced the calcium with hydroxyl groups, creating siloxene (Si6O3H6), which can be tailored by attaching various chemical side groups to produce different shades of light. (Science News, 16 May 1992.)
GEMINGA IS A GAMMA-RAY PULSAR . Data from two orbiting observatories---the x-ray satellite Rosat and the Gamma Ray Observatory---show that Geminga, an object in the Gemini constellation, bears a resemblance to Vela and the Crab, two pulsars that emit gamma rays. Unlike them, however, Geminga seems to shine mostly at gamma wavelengths, a fact which had baffled scientists ever since Geminga was discovered twenty years ago. The new satellite observations show that gamma and (more weakly) x-ray broadcasts from Geminga are modulated (in pulsar style) with a period of 237 msec. The rate at which the neutron star at the heart of Geminga is slowing down leads to an estimate of 300,000 years for the age of the pulsar. (Nature, 21 May 1992.)