May 21, 2004 -- A new study, published in today's
issue of the journal Science, finds that the basic
electrical properties of semiconducting carbon nanotubes
change when they are placed inside a magnetic field.
The phenomenon is unique among known materials, and
it could cause semiconducting nanotubes to transform
into metals in even stronger magnetic fields.
Scientists found that the "band gap" of
semiconducting nanotubes shrank steadily in the presence
of a strong magnetic force, said lead researcher Junichiro
Kono, an assistant professor of electrical and computer
engineering at Rice University. The research, which
involved a multidisciplinary team of electrical engineers,
chemists and physicists, helps confirm quantum mechanical
theories offered more than four decades ago, and it
sheds new light on the unique electrical properties
of carbon nanotubes, tiny cylinders of carbon that
measure just one-billionth of a meter in diameter.
"We know carbon nanotubes are exceptionally strong,
very light and imbued with wonderful electrical properties
that make them candidates for things like 'smart'
spacecraft components, 'smart' power grids, biological
sensors, improved body armor and countless other applications,"
said paper co-author Richard Smalley, director of
Rice's Carbon Nanotechnology Laboratory. "These
findings remind us that there are still unique and
wonderful properties that we have yet to uncover about
By their very nature, semiconductors can either conduct
electricity, in the same way metals do, or they can
be non-conducting, like plastics and other insulators.
This simple transformation allows the transistors
inside a computer to be either "on" or "off,"
two states that correspond to the binary bits -- the
1's and 0's -- of electronic computation.
Semiconducting materials like silicon and gallium
arsenide are the mainstays of the computer industry,
in part because they have a narrow "band gap,"
a low energy threshold that corresponds to how much
electricity it takes to flip a transistor from "off"
"Among nanotubes with band gaps comparable to
silicon and gallium arsenide, we found that the band
gap shrank as we applied high magnetic fields,"
said physicist Sasa Zaric, whose doctoral dissertation
was based upon the work. "In even stronger fields,
we think the gap would disappear altogether."
Nanotubes, hollow cylinders of pure carbon that are
just one atom thick, come in dozens of different varieties,
each with a subtle difference in diameter or physical
structure. Of these varieties roughly one third are
metals and the rest are semiconductors.
In the experiments, which were performed at the National
High Magnetic Field Laboratory (NHMFL) at Florida
State University, Kono's group placed solutions of
nanotubes inside a chamber containing very strong
magnetic fields. Lasers were shined at the samples,
and conclusions were drawn based upon an analysis
of the light that was emitted and absorbed by the
"The behavior we observed is unique among known
materials, but it is consistent with theoretical predictions,
and we believe we understand what's causing it,"
said Kono. "Our data show, for the first time,
that the so-called Aharonov-Bohm phase can directly
affect the band structure of a solid. The Aharonov-Bohm
effect has been observed in other physical systems,
but this is the first case where the effect interferes
with another fundamental solid-state theorem, that
is, the Bloch theorem. This arises from the fact that
nanotubes are crystals with well-defined lattice periodicity.
I wouldn't be surprised to see a corresponding effect
in other tubular crystals like boron nitride nanotubes."
Kono said the discovery could lead to novel new experiments
on one-dimensional magneto-excitons, quantum pairings
that are interesting to researchers studying quantum
computing, nonlinear optics and quantum optics. Kono
said it's too early to predict what types of applied
science might flow from the discovery.
The NHMFL experiments were conducted in fields up
to 45 Tesla in strength -- the strongest continuous
magnetic field in any lab in the world. Kono said
he is arranging for additional tests in stronger magnetic
fields. He has already met with research groups in
France, Tokyo and at New Mexico's Los Alamos National
Laboratory, each of which has facilities that use
brief pulses of power to create short-lived magnetic
fields that are exceptionally strong.
The research was supported by the Welch Foundation,
the Texas Advanced Technology Program, the National
Science Foundation, the NHMFL and the State of Florida.
Other co-authors included NHMFL's Xing Wei, and Rice's
Robert Hauge, Gordana Ostojic, Jonah Shaver, Valerie
Moore and Michael Strano. Rice's team represented
the Carbon Nanotechnology Laboratory, the Center for
Nanoscale Science and Technology, the Center for Biological
and Environmental Nanotechnology and the Rice Quantum