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MED research, fullerene traps, simple superconductor

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  • David Forrest
    This Week in SCIENCE, Volume 291, Issue 5508, dated February 23 2001, is now available at: http://www.sciencemag.org/content/vol291/issue5508/twis.shtml ...
    Message 1 of 1 , Feb 22, 2001
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      This Week in SCIENCE, Volume 291, Issue 5508,
      dated February 23 2001, is now available at:

      http://www.sciencemag.org/content/vol291/issue5508/twis.shtml

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      Electrons Make the Long Jump
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      Direct electron transfer over distances of a few nanometers may be quite
      useful in molecular electronics, but the barriers that must be overcome to
      transfer electrons between donor and acceptor groups generally increase
      with increasing distance. In many cases, long-distance electron transfer
      requires an intermediate site, and the electrons actually "hop" rather than
      fire directly. Sikes et al. (p. 1519) present data from a thermal-jump
      experiment that indicates that direct electron transfer can occur over
      distances as great as 28 angstroms. In this case, electron transfer in self
      -assembled monolayers occurred through oligophenylenevinylene bridges that
      tethered ferrocene groups in solution to gold electrodes. The authors
      eliminate other possible mechanisms for electron transfer and suggest that
      these bridges may maintain greater planarity and thus greater orbital
      overlap than similar bridging groups that have been used previously.

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      Unlocking Rare Clues
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      The Permian-Triassic (P-T) extinction, the largest in Earth's history, was
      also abrupt, lasting perhaps a few hundred thousand years. The cause of the
      extinction has been widely debated; unlike the Cretaceous-Tertiary
      extinction, there has been no compelling evidence for an asteroid impact,
      such as an iridium anomaly or shocked mineral grains. Becker et al. (p.
      1530; see the news story by Kerr) have now isolated and analyzed fullerenes
      from two separate locations at the P-T boundary that seem to contain an
      extraterrestrial signature. Fullerenes can trap rare gases in their cages,
      and analyses of the gases in the fullerenes from the P-T boundary show that
      their isotopic composition is unlike that of Earth's atmosphere but
      reflects an extraterrestrial signature. Fullerenes were not identified in
      sediments above or below the boundary and might have been delivered to the
      Earth in a large comet or asteroid.



      SUPERCONDUCTIVITY:
      Material Sets Record for Metal Compounds

      Robert F. Service

      Magnesium diboride, one of the simplest compounds around,
      superconducts at nearly twice the temperature of its closest metallic
      rival, researchers announced at a meeting last month in Japan.
      Although some ceramics can superconduct at temperatures up to 96
      degrees higher, most metallic compounds are better at carrying
      current across gaps between grains of material and thus make better
      wires. Magnesium diboride could therefore become the superconductor
      of choice for a wide range of applications, such as the wires that
      make up the high-field magnets in magnetic resonance imaging machines.
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