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Date: Thu, 22 Mar 2001 15:21:45 -0500 (EST)
From: AIP listserver <physnews@aip.org>
To: physnews-mailing@aip.org
Subject: update.531
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 531 March 22, 2001 by Phillip F. Schewe, Ben Stein,
and James Riordon
A CARBON NANOTUBE INTEGRATED CIRCUIT, with a
thousand nanotubes acting like transistors, has been devised by
Phaedon Avouris of IBM (914-945-2722, avouris@us.ibm.com).
Nanometer-wide tubes made of carbon chickenwire have for some
years expected to become an active ingredient in electronics (see
Scientific American for December 2000). Besides their strong
mechanical properties, nanotubes have a variety of attractive
electrical properties. Nanotubes, for example, can sustain current
densities hundreds of times greater than that of common metals,
and are created in both metallic and semiconducting form.
Speaking at last week's APS meeting in Seattle, Avouris described
how, in a mixed batch of nanotubes, one can short out the metallic
nanotubes (with a surge of voltage) while leaving the
semiconducting ones intact for use as circuit elements. Other
nanotube highlights from the same meeting: David Tomanek of
Michigan State (517-355-9702, tomanek@pa.msu.edu) said that
experimental measurements of nanotube heat conductivity went as
high as 3000 watts/m/K, almost as high as that of diamond. He
predicted that nanotube performance would reach levels of 6600
watts/m/K. The ability to conduct heat will come in handy for
future circuits needing to dispose of lots of heat from tight places.
Mathieu Kociak of the CNRS lab, University of Paris-South (33-
169-155-342, kociak@lps.u-psud.fr) announced the first
observation of superconductivity in nanotube ropes (see also
Kociak et al., Physical Review Letters, 12 March 2001; get text at
http://www.aip.org/physnews/select). "This represents the first
observation of superconductivity in a system with such a small
number of conduction channels," said Kociak, referring to the
meager material substrate over which the supercurrent must flow,
namely the aggregate of essentially two-dimensional surfaces of
nanotubes. The researchers hope to raise the transition
temperatures, presently only 300-400 mK, through judicious
doping. John Hafner of Harvard reported using single nanotubes
(with diameters of .9-2.8 nm) as extensions on the ends of atomic
force microscope probes. Not only does this narrow the probe
profile, resulting in greater spatial resolution when imaging a
variety of biomolecules (such as immunoglobulins) but, when used
to seek out specific molecules on a sample surface, the nanotube
probe could help in studying tip-sample adhesion. Hafner referred
to this approach as "chemical force microscopy" (CFM). Finally,
Masako Yudasaka of the NEC lab in Japan (81-(0)298-50-1190,
yudasaka@frl.cl.nec.co.jp) reported on the enormous pressures that
arise when C60 molecules are encased inside nanotubes (an
arrangement called "peapods" the force on the C60 is only a
nano-Newton, but by dividing by the area of the tube, one arrives
at a pressure of .1 giga-Pascal. In other words the buckyball can
act like a piston for facilitating novel forms of tailored chemistry.
Yudasaka also described her work with nanotubes that flare out
like cones (typical size: 2 nm small diameter, length of 50 nm, and
opening angle of 20 degrees). These "nano-horns" might be useful
for absorbing gases (replacing other forms of activated carbon in
filters).
MOLECULAR BEACONS FOR CANCER. Aiming to detect
cancers early, safely, and inexpensively, Britton Chance of the
University of Pennsylvania (215-898-4342,
chance@mail.med.upenn.edu) and his colleagues have created
"molecular beacons," tiny capsules that are opened by specific
biochemical activity related to a tumor. At the APS March
Meeting, Chance described molecular beacons designed to detect
1-2 mm sub-surface breast tumors inexpensively and without
ionizing radiation. Injected into the body, the capsules remain
sealed until opened by specific enzymes associated with breast
cancer. The beacons then fluoresce near-infrared light in response
to light beaming from a small device outside the body. That same
device then detects the signal from the beacons. (The beacons emit
enough near-infrared light so that some of it gets through the
body.) The device is designed to cost only several thousand
dollars, Chance said, and is based on off-the-shelf CD and cell-
phone technology. The molecular beacon has successfully been
tested in mice, and human tests are planned. The technique does
not require uncomfortable compression of the breast, which is
what often is required for women under 40 years of age who
receive mammograms. Self-tests for breast cancer may eventually
be possible with this technique, Chance said.
USING THE MOON AS A COSMIC RAY DETECTOR. Some
of the ultrahigh-energy cosmic ray neutrinos striking the Moon's
soil are expected to set up shock waves of Cerenkov radiation, the
light given off by particles (in this case charged particles spawned
by neutrinos) traveling faster than light itself in that medium (see
schematic drawing at http://www.aip.org/mgr/png). For the
cosmic rays of greatest interest, those with an energy above 10^20
eV, the Cerenkov radiation peaks in the microwave region of the
electromagnetic spectrum. To test the validity of this "Askaryan
effect" (named for the Armenian-Russian scientist Gurgen
Askaryan), a consortium of scientists (David Saltzberg, 310-206-
4542, saltzberg@physics.ucla.edu; Peter Gorham,
peter.w.gorham@jpl.nasa.gov) have shot gamma rays into a bed of
sand at the SLAC Final Focus Test Beam. Sure enough, the
expected coherent microwaves appeared. The scientists are
pointing two JPL radio telescopes (part of the Deep Space
Network) toward the Moon to look for such radiation from cosmic
ray neutrinos. Right now their calibration involves pointing at
distant quasars, but it would be nice to have some source of
microwave pulses on the Moon itself, a luxury not possible at
present. Some of the more optimistic estimates place the number
of possible 10^20 eV cosmic ray neutrino events as high as one
every 10 to 20 hours or so. (Saltzberg et al., Physical Review
Letters, 26 March; text at http://www.aip.org/physnews/select;
http://www.physics.ucla.edu/~moonemp/public/lunacee2/index.ht
ml) By the way, in this week's issue of Nature, members of the
AMANDA collaboration report the observation of cosmic-ray
neutrinos, also via the emission of Cerenkov radiation, but in this
case the detectors are buried in Antarctic ice (Andres et al., Nature,
22 March 2001.)
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