Number 36, May 31, 1991 by Phillip F. Schewe and Ben Stein|
A SUPERCONDUCTING FORM OF GALLIUM-ARSENIDE has been discovered by a group of materials' scientists at the University of California at Berkeley (Eicke R. Weber, 415-642-0446). They grew arsenide-rich layers of GaAs by molecular beam epitaxy carried out at the relatively low temperature of 200 C; they then discovered a superconducting phase, with a transition temperature of 10 K, inside a semi-insulating phase of the material. The Berkeley scientists believe that other superconductive semiconductor compounds may be discovered with even higher transition temperatures, and that "It is now possible, for the first time, to think of combining on the same chip insulating, semiconducting, and superconducting devices, based on epitaxially grown layers. (Upcoming article in Physical Review Letters.)
SILICON HAS BEEN MADE TO EMIT LIGHT in tests conducted in Britain and France. In semiconductors using elements from columns III and V of the Periodic Table, such as GaAs, the energy released in the recombination of electrons and holes can take the form of a photon; this process is harnessed in opto-electronic devices. In silicon, by contrast, the recombination energy usually appears as heat. At last month's Materials Research Society meeting in Anaheim, Leigh Canham from the Royal Signals and Radar Establishment (UK) reported that his group had succeeded in getting light (of all colors) out of tiny silicon quantum wires. R. Romestain of the University of Grenoble reported getting light (infrared up into the visible) from silicon quantum dots. (Science, 17 May 1991.)
THE MAGELLAN SPACECRAFT , having mapped 84% of Venus' surface, has finished its nominal mission. Its orbit will now be shifted slightly in order to do some additional mapping from a different perspective. Stephen Saunders of JPL said that because of repeated lava flows, the surface of Venus has an average age of only 400 million years. (The New York Times, 30 May 1991.)
GAMMA-RAY BURSTS ARE GALACTIC IN ORIGIN . This is the conclusion of a French-Soviet team of astronomers who have carried out a statistical survey of burst directions recorded by a number of spacecraft-based detectors. Gamma-ray bursts are brief flashes (usually only a few seconds long) of gamma photons from sources which are never heard from again. Because the bursts are so ephemeral and because their distribution appears to be isotropic across the sky, it has been difficult to determine whether they come primarily from within the Milky Way or from extra-galactic sources. According to Charles Dermer of Rice University, the Gamma Ray Observatory is presently recording about one new burst per day. (Nature, 23 May 1991.)
THE MOON HAS A COMET-LIKE COMA AND TAIL . Michael Mendillo of Boston University, reporting at this week's meeting of American Geophysical Union in Baltimore, showed that the moon has a tail, consisting of sodium gas, extending at least 15,000 miles away from the lunar surface. The sodium, Mendillo believes, is released from lunar rocks by meteorite impacts and is later dissipated into space where it is formed into a tail by the force of solar radiation.