Bandgap of Semiconducting Nanotubes Shrinks in High Magnetic Fields
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Figure 1. The peaks represent the amount of light emitted by semiconducting carbon nanotubes through interband photoluminescence. The bottom graph depicts light-emission activity in the absence of a magnetic field. The top graph shows a significant shift of light emission peaks to lower energies taken from nanotubes inside a 45-T field. (Figure courtesy of Sasa Zaric, Rice University.)
MRS BULLETIN/JULY 2004
As reported in the May 21 issue of Science (p. 1129), the researchers placed solutions of nanotubes inside a chamber containing very strong magnetic fields. Lasers illuminated the samples, and conclusions were drawn based upon analysis of the light absorbed and emitted by the samples during polarization-dependent magneto-absorption spectroscopy. Kono said, “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.” According to the researchers, the bandgap behavior of the nanotubes in a strong magnetic field derives from their quantum
properties. Because of the material’s tubular, crystalline structure, electrons are limited to moving around the surface of the tube rather than in the hollow center. Kono said that this discovery may lead to experiments on one-dimensional magnetoexcitons, quantum pairings that are interesting to researchers studying quantum computing, nonlinear optics, and quantum optics.
α-SiAlON Ceramics with High Transparency Obtained After Lu2O3 Addition The most widely studied SiAlON ceramics, for which the two major phases are α and β, are those stabilized with Y2O3. Oxide additions help to stabilize the α phase of the Si3N4-based solution by substituting some of the silicon and nitrogen in the Si3N4 lattice. Further addition of a stabilizing element, usually a rare earth, equilibrates the valence difference of the α phase. The smaller the size of the ion added, the larger the temperature
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RESEARCH/RESEARCHERS
and composition ranges where the α phase is stable. In compositions with 100% α phase, the grain-boundary phase is reduced as all of the additives can enter into the structure. Now, by using a novel stabilizing material, Lu2O3, a team of scientists from the Agency of Industrial Science and Technology (AIST) and the Fine Ceramics Research Association in Nagoya, Japan, have obtained a highly dense, transparent Lu-α-SiAlON ceramic. As reported in the April issue of the Journal of the American Ceramic Society (p. 714), M.I. Jones and K. Hirao of AIST, H. Hyuga of the Fine Ceramics Research Association, and colleagues hot-pressed a mixture of Si3N4, Al2O3, AlN, and Lu2O3 at 1950°C and 40 MPa for 2 h in a 0.9 MPa nitrogen atmosphere. X-ray
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