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Magnetic forces may turn some nanotubes into metals

WHEN PLACED inside a magnetic field, the basic electrical properties of semiconducting carbon nanotubes change. The phenomenon, published in the journal Science, could cause semiconducting nanotubes to transform into metals in even stronger magnetic fields and is unique among known materials.

Lead researcher Junichiro Kono, an assistant professor of electrical and computer engineering at Rice University said scientists found that the 'band gap" of semiconducting nanotubes shrank steadily in the presence of a strong magnetic force. The research, 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 and helps confirm quantum mechanical theories offered more than 4 decades ago.

"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 armour 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 nanotubes."

By their very nature, semiconductors be non-conducting, like plastics and other insulators or they can either conduct electricity, in the same way metals do. 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.

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" to "on" semiconducting materials like silicon and gallium arsenide are the mainstays of the computer industry. "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."

Hollow cylinders of pure carbon that are just one atom thick, nanotubes come in dozens of different varieties, each with a subtle difference in diameter or physical structure. Of these varieties roughly two third are semiconductors and the rest metals.

Kono's group placed solutions of nanotubes inside a chamber containing strong magnetic fields in experiments, performed at National High Magnetic Field Laboratory (NHMFL) at Florida State University. Lasers were shined at the samples, and conclusions were drawn based upon an analysis of the light that was emitted and absorbed by the samples. "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 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.

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