Number 164, February 10, 1994 by Phillip F. Schewe and Ben Stein
THE DISCOVERY OF ELEMENT 106 HAS BEEN CONFIRMED. First discovered at
the LBL HILAC machine in 1974, element 106 has gone unnamed because of
priority disputes with Russian scientists. In 1992, the International Union
of Pure and Applied Physics (IUPAP) finally gave credit for discovery to
the LBL group, but a separate protocol in 1976 suggested that the naming
of the element should await a confirmation experiment. Such a measurement
has only now been made. Scientists using the LBL 88-inch Cyclotron bombarded
Cf-249 atoms with O- 18 ions, forming element 106 (atomic weight 263),
which decayed with a lifetime of 0.9 sec. into Rf-259 plus an alpha particle.
(UPCOMING ARTICLE: K.E. Gregorich et al., Physical Review Letters.)
OPTICAL LATTICES are three-dimensional ensembles of tens of millions
of atoms held together in a crystal-like array not by the customary action
of chemical bonds but with light waves from lasers. This pattern of trapped
atoms cannot properly be called a crystal because in current experiments
only about one in ten possible lattice sites are occupied by an atom. Furthermore,
the atoms in optical lattices are hundreds of times further apart than
in ordinary crystals. As a result this novel form of matter is a billion
times more diffuse than conventional crystals. Nevertheless, there is much
more order in an optical lattice than in "optical molasses,"
in which atoms are confined within an amorphous glob by laser light. To
make a true lattice, scientists at labs around the world---at NIST, the
University of Munich, and the Ecole Normale Superieure (ENS) in Paris---must
first cool the atoms to low temperatures (2.5 microkelvins in the case
of the ENS experiment with cesium atoms) so that the relatively weak electric
fields of the laser beams can trap the atoms. Optical lattices may be useful
in establishing a more precise form of atomic clock, if an atomic-transition
signal can be strengthened by increasing the density of atoms in the lattice,
or (in a two-dimensional form) as a means of inscribing 10-nm-wide features
on integrated circuits. (New Scientist, 29 Jan.)
ATOMIC DREIDLS, structures resembling the child's toy top of that name,
may exist at the intersections of edge dislocations at a Ge-Si semiconductor
interface. Like a game of musical chairs, the process of matching up planes
of atoms in a layer of germanium with planes in a silicon layer (whose
lattice constant is slightly different) will result in the termination
of certain planes at the interface. The termination lines are called edge
dislocations and they form a checkerboard pattern at the interface. New
simulations show that 18-atom dreidl-shaped structures (with half the atoms
on the germanium side of the interface, half on the silicon side) are to
be found at the intersections of the dislocations. The dreidls constitute
a sort of lattice with a spacing of 96 angstroms and a density of up to
10**12/sq.cm. The Oak Ridge scientists performing the simulations (contact
Ted Kaplan, 615-574-5790) believe that dreidls may be electrically active,
a property that might be significant for electronics applications. Future
x-ray scattering and photoluminesence experiments may be able to verify
the existence of these structures. (UPCOMING ARTICLE: Mark Mostoller et
al., Physical Review Letters.)
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