PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

PHYSICS NEWS UPDATE
A digest of physics news items prepared by Phillip F. Schewe, AIP Public
Information
Number 164   February 10, 1994

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.)