Tailoring Nanotubes
Superconductivity Theory Comes Up Short
Demonstrating Teleportation
Tailoring Nanotubes
Nanotubes are tiny, hollow tubes of carbon atoms with diameters of a few
nanometers, or approximately 1/10,000th that of a human hair. Discovered in
1991 by NEC Corporation's Sumio Iijima, they have become hot items in the
submicroscopic world. The attraction: the nanotubes, depending on their
structure, can be metallic or semiconducting and potentially can be used as
ultrathin wires or as components of novel ultrasmall electronic devices.
Recently, Tobias Hertel, Richard Martel and Phaedon Avouris at IBM Research
have made an important advance toward that goal by devising procedures for
changing nanotubes' positions, shapes and orientations, as well as cutting
them. Their approach allows for an effective use of nanotubes to fabricate
structures and test potential applications. "Previously, getting nanotubes
in the right positions for applications has relied on chance," explains
Avouris. "Now we can manipulate them in a controlled fashion."
The team, based at the Thomas J. Watson Research Center, examined the
threadlike nanotubes on silicon surfaces, using an atomic force microscope
(AFM), invented by IBM Fellow and Nobel laureate Gerd Binnig of IBM's
Zurich Research Laboratory. With the tip of an AFM, accurately measured
short-range repulsive forces can be applied to atoms and molecules.
Measuring the applied force necessary to move or bend nanotubes reveals the
magnitude of the natural forces that normally keep them in place on
surfaces.
The work showed that van der Waals forces -- physical attractions always
present between nearby atoms and molecules -- hold nanotubes firmly against
silicon or other substrates. While nanotubes prefer to be straight, forces
applied by the AFM tip can change their shape; the strong interaction with
the substrate freezes in these otherwise unstable configurations. The
Watson team found that, in general, a nanotube tends to conform to the
surface on which it sits, by bending and becoming slightly flattened so as
to increase the van der Waals interaction. Those changes can cause the
properties of nanotubes on surfaces to differ from those of free nanotubes,
which are straight with circular cross-sections. "The simple observation
that a nanotube's shape depends on the substrate it's put on has
implications for the electrical properties of structures and devices based
on nanotubes," says Avouris.
By applying particularly large forces, the researchers were able to take
the process a stage further and cut the nanotubes. However, that happened
only when the nanotubes were anchored to the surface more firmly than
normal. The anchoring in their studies was found to involve chemical bonds
between surface defects and the nanotubes.
The ability to position nanotubes, and to tailor their properties by
changing their shapes and lengths, could aid in building novel devices --
for example, carbon-based field effect or single electron transistors. Such
devices could allow components to shrink beyond the limits of current
silicon-based technology.
Superconductivity Theory Comes Up Short
Physicists have sought to explain the mechanism of superconductivity in
high-temperature superconductors ever since their discovery in 1986 by IBM
scientists at the Zurich Research Laboratory. One explanation, proposed by
a number of researchers, including Nobel laureate Philip Anderson of
Princeton University, suggests that superconductivity results from
increased coupling between adjacent atomic layers when cuprate compounds
enter the superconducting state.
It has been generally agreed that a test of this interlayer tunneling (ILT)
model requires measurement of the strength of the coupling between layers
in two high-temperature cuprates, one thallium-based and the second
mercury-based. Initial measurements using far-infrared optical techniques
at the University of Groningen (Netherlands) on the first of these cuprates
found no direct evidence for the interlayer coupling.
Only indirect measurements of magnetic susceptibilities have been possible
on the second cuprate. But a team from IBM's Thomas J. Watson Research
Center hit on a more direct method. This involves imaging so-called
Josephson vortices -- tubes of magnetic field between the atomic layers.
The team's first study, reported in Science (February 20, 1998), casts
strong doubt on the ILT theory's validity.
The team used a scanning SQUID microscope to image the vortices. This instrument, developed at Watson, can map out extremely small magnetic fields. "Thes
e vortices have never been quantitatively measured before," explains Watson
scientist John Kirtley. Adds Kathryn Moler, a Princeton physicist who
collaborated on the project with Kirtley: "The scanning SQUID microscope is
the only instrument that would allow us to do these experiments."
The results fail to support the theory. "The lengths of the vortices are
about 20 times what the ILT theory predicts," says Kirtley. "This is a
serious disagreement."
In a companion paper in Science, Anderson suggests that the Watson
result is unique to the particular thallium-based compound studied by
Kirtley, Moler and colleagues from the Argonne National Laboratory and the
University of Chicago. However, says Moler, more recent studies of the
second, mercury-based high-temperature superconductor have also revealed "a
discrepancy, this time of about a factor of 10."
"More than 10 years after the discovery of high-temperature
superconductivity," says Kirtley, "we finally have sufficient control of
the materials and the analytical techniques to make definitive tests of
candidate mechanisms for high-temperature superconductivity."
Demonstrating Teleportation
FIVE YEARS AGO, the world of theoretical physicists appeared to intersect
with that of science fiction,
when IBM Fellow Charles H. Bennett of the Thomas J. Watson Research Center
and five other scientists
proposed the theory of quantum teleportation.
In their scheme, information about a specific property of a particle such
as a photon of light could be
transferred along both a classical and a quantum mechanical channel. As a
result, a photon with exactly
the same property would be produced at a remote location.
Now, experimenters at Austria's University of
Innsbruck have provided the first demonstration of quantum teleportation.
Anton Zeilinger's
group transferred a property of a single photon -- its angle of
polarization -- to another, independent photon.
The transfer could take place, the Austrian scientists state in the
journal Nature (December 11, 1997),
even if the two photons were literally a galaxy apart. Another scientific
team in Rome has reportedly confirmed
the result.
Meanwhile, Bennett and his colleagues have continued to develop their
understanding
of teleportation. A key goal has been to understand how to deal with noisy
teleportation channels, which could
cause enough transmission errors to scramble any transfer. "The big
theoretical progress in the field over the past
two years," says Bennett, "has been the discovery of quantum error
correction procedures."
That work, by several international groups, including IBM researchers,
has shown that teleportedinformation can be reliably reconstructed even in
the presence of noise.
The ultimate goal of the various efforts in quantum information theory, which
include teleportation, error-correction and other related issues, is a
quantum computer.
Theoretically, such a machine
could work much faster than conventional computers.
The recent research has shown, says Bennett, that "at least in principle,
quantum computers don't have to be infinitely reliable. They just have to
get over a threshold of errors."
That does not, of course, make their emergence immediate or inevitable. "It
may turn out," Bennett adds, "that
quantum computers, like fusion power, prove possible in principle, but not
practical for a long time."
