Number 221, April 10, 1995 by Phillip F. Schewe and Ben Stein
A TECHNIQUE FOR GENERATING HIGH-FREQUENCY, SINGLE-ENERGY PHONONS has
been devised, making it feasible to perform phonon spectroscopy, the study
of how matter absorbs and radiates phonons. Phonons are discrete bundles
of acoustical or thermal energy in solids. Researchers at Utrecht University
in the Netherlands (contact H.W. de Wijn, dewijn@fys.ruu.nl) produce a
directional beam of monochromatic phonons in a molybdate (PbMoO4) crystal
by shining a pair of interfering, narrow-frequency-band dye lasers on a
deposited gold film. The interfering lasers periodically create a deformation
or "strain" in the film. The periodic deformations produce a
phonon beam whose frequency is equal to the difference in frequency between
the two lasers. The beam then travels into the crystal, where studies of
its properties can be performed. The phonon frequency can be tuned by changing
the frequency of one of the lasers. The narrowness of the beam is limited
by a classical effect known as Fraunhofer diffraction, in which a series
of parallel rays that reach a small opening (the spaces in the crystal,
in this case) spread out by a small amount (on the order of 0.01 radians,
in the experiments). (E.P.N. Damen et al, Physical Review Letters, upcoming
article.)
EARTH'S INNER CORE MAY BE A SINGLE IRON CRYSTAL. Furthermore, the inner
core's magnetism might even influence the field shape we observe at the
surface. Lars Stixrude of the University of Gottingen in Germany and Ronald
Cohen of the Carnegie Institution of Washington have proposed a model in
which the 2400-km inner core consists of an immense collection of hexagonal
close-packed grains in a nearly perfect alignment. The fact that such a
crystalline structure would be mechanically anisotropic (that is, its elastic
properties would vary in different directions) could help to explain the
puzzling observation that seismic waves take longer to propagate through
the planet in the plane of the equator than they do along the spin axis.
Some scientists have reservations about this model. For example, how would
the inner core's magnetism work its way outward past the turbulent motion
of the fluid outer core? (Science, 31 March 1995.)
OBJECTS AND FEATURES IN THE HEAVENS must be named according to a systematic
protocol laid down by the International Astronomical Union (IAU). For example,
features on Venus must bear female names: craters may be named after mortal
women, but ridges must be named for sky goddesses. Uplands are named after
goddesses of love and plains after mythological heroines. On Mercury valleys
are named after radio telescopes while scarps take the names of famous
ships of discovery. Features on Uranus' moon Puck are named after mischievous
spirits, while features on Neptune's moons are all watery spirits. Little
did Virgil know that persons and places in his epic poem Aeneid would 2000
years later be affixed to maps of Saturn's moon Dione. And so on. (Sky
& Telescope, May.)
THE ADVANCED PHOTON SOURCE (APS) is nearing completion at Argonne. The
$811 million synchrotron light source uses 7-GeV positrons to produce short
pulses of hard x rays. One of the chief functions of the APS will be as
an x-ray camera, taking still photos of proteins and movies of molecules
on the move. APS's chief rivals are the 6-GeV European Synchrotron Radiation
Facility (ESRF) in Grenoble, France (finished last year) and an 8-GeV machine
in Japan, to be finished in 1998. (Science, 31 March.)
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