Number 279, July 15, 1996 by Phillip F. Schewe and Ben Stein
GENERATING "SOUND WAVES" IN BOSE-EINSTEIN CONDENSATES : A NIST-University
of Colorado team (contact Deborah Jin, 303-492-7784) and an MIT group (contact
Marc-Oliver Mewes, 617-253-4178) have independently produced collective
oscillations in BECs, collections of atoms that become so cold and so dense
that the atoms essentially overlap and fall into a single quantum state.
Aiming to study the fluid properties of this new state of matter, both
teams wiggle BECs by adding a small oscillating magnetic field to the usual
magnetic fields that trap the BEC. The researchers then turn off the perturbing
field and let the BEC assume its natural oscillations. The resulting disturbances
propagate through the BEC like sound waves traveling through air. The Colorado
group observed one mode in which the BEC sloshes back and forth like a
squeezed water balloon and another similar to a spiraling football. The
MIT group found a pair of modes that resemble a sort of breathing motion
of the condensate. Both teams found these excitations to be analogous to
those in superfluid helium, and indeed the observed oscillation frequencies
agree with those that emerge from theories that predict superfluidity for
BECs. However, the researchers emphasize that more comprehensive studies
of the fluid properties and excitation modes of BECs are needed to establish
superfluidity. (D.S. Jin et al., Physical Review Letters, 15 July 1996;
M.-O. Mewes et al, Physical Review Letters, 5 August 1996.)
ELECTRICAL AND MECHANICAL PROPERTIES OF INDIVIDUAL CARBON NANOTUBES :
Because of their nanometer diameters and micron lengths, nanotubes are
difficult to study singly. A team of scientists at NEC (Princeton) and
the University of Illinois has, nevertheless, measured the Young's Modulus
(indicative of a material's elasticity) of an assortment of nanotubes by
noting the amount by which the tips of the freestanding tubes are blurred
in microscopic pictures taken at various temperatures. (Higher temperatures
lead to more thermally induced vibration.) The researchers found that the
fibers had an average modulus of 1.8 TA (1.8 x 10**12 N/m**2), higher than
that of steel. This extreme stiffness, along with low density, might make
carbon nanotubes an excellent ingredient in building materials. (M.M.J.
Treacy et al., Nature, 20 June 1996.) Electrical properties are potentially
no less important. Another NEC experiment, this time in conjunction with
Micrion Europe (Munich, Germany) and Fundamental Research Laboratories
(Tsukuba, Japan), has succeeded in connecting four tungsten probes to individual
nanotubes. This affords a more controlled study of conductivity than was
possible in previous two-probe experiments. The four-probe tests have yielded
nanotubes with a wide range of properties, from metallic to semiconducting.
The researchers conclude that a tube's conductivity depends greatly on
its diameter and on the helicity of carbon atoms making up the layers of
the tubes. (T.W. Ebbesen et al., Nature, 4 July 1996.)
GANYMEDE IS COVERED WITH A DIRTY CRUST OF WATER ICE. The Galileo spacecraft,
now patrolling Jupiter's neighborhood, recently swung past Ganymede, the
largest moon in the solar system. Galileo recorded the presence of a self-generated
magnetic field at the moon, and new high-resolution pictures testify to
the presence of tectonic-like forces at work, giving Ganymede's surface
a network of fault lines. (JPL press release, July 11, 1996.)
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