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Number 371, May 13, 1998 by Phillip F. Schewe and Ben Stein
MAGNETIC ROTATION OF NUCLEI, a fundamentally new phenomenon in nuclear physics, may expand the very notion of rotation for objects at the quantum scale. In many cases, an atomic nucleus is shaped like a perfectly uniform sphere and has the same properties when viewed from any direction. According to the observation-based worldview of quantum mechanics, rotation does not exist under such conditions; think of trying to detect rotation in a white cue ball spinning against a black background. Physicists eventually discovered, however, that some nuclei have distorted shapes, such as the football-shaped "superdeformed" nuclei first detected in the 1980s. Such nuclei radiate sequences of gamma rays corresponding to the nucleus slowing down its rotation. In the past few years researchers have discovered that similar gamma ray sequences are being emitted by nuclei (such as lead-198) known to be nearly spherical. So why then are such nuclei emitting these sequences? Stefan Frauendorf of FZ-Rossendorf in Germany has suggested that the gammas come from a re-shuffling of the internal arrangement of just a few protons and neutrons. In the case of lead- 198, 2 protons and 4 neutrons are like low-lying satellites skimming an "Earth" of 192 non-participating nucleons. The circulating protons and neutrons generate angular momentum in different directions, specifiying in effect an orientation for the nucleus, allowing rotation to occur even though the nucleus remains spherical. In addition, the 6 itinerant nucleons would create a sizeable magnetic dipole, leading to the name "magnetic rotation." (Paper delivered by Rod Clark (LBL, 510-486-4243) at the April APS meeting; lay language writeup at the APS Virtual Press Room)
THE EARTH VIBRATES CONTINUOUSLY even without help from earthquakes. A collaboration of scientists from UC Santa Barbara and Tokyo Institute of Technology has analyzed gravimeter data from 1983 to 1994 and found 61 days to be seismically "quiet" enough for the purpose of searching out Earth's natural oscillation modes. They identify several such modes in the 2 to 7 milli-Hz range (that is, vibrations with periods of hundreds of seconds). The acceleration of material in the solid Earth produced by these spheroidal waves is tiny, on the order of nano-gals, or 10-9 cm/sec2. The researchers suspect that the cause of the vibrations is atmospheric turbulence. (Tanimoto et al., Geophysical Research Lett., May 15; contact Toshiro Tanimoto, UC Santa Barbara, toshiro@magic.geol.ucsb.edu.)
A CARBON NANOTUBE TRANSISTOR, an electronic device based on a single rolled-up sheet of carbon atoms, has been built by researchers in the Netherlands (Cees Dekker, Delft Institute of Technology, dekker@qt.tn.tudelft.nl), providing a demonstration of room-temperature, carbon-based electronics at the single-molecule scale. In the device, a semiconducting carbon nanotube (only about 1 nm in diameter) bridges two closely separated metal electrodes (400 nm apart) atop a silicon surface coated with silicon dioxide. Applying an electric field to the silicon (via a gate electrode) turns on and off the flow of current across the nanotube, by controlling the movement of charge carriers onto it. Although carbon nanotubes are robust and durable molecules, they can't yet be made uniformly. While this can provide disadvantages (a slight deviation from the desired radius can give the nanotube metallic properties), it can also bring about advantages--such as the possibility of a metal-semiconductor junction made completely of carbon nanotubes. (S.J. Tans et al., Nature, 7 May 1998; image at Physics News Graphics
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