Physics News Update
The American Institute of Physics Bulletin of Physics News
Number 314, March 28, 1997 by Phillip F. Schewe and Ben Stein
STARDUST WITH RED GIANT FINGERPRINTS. Heavier elements are assembled
out of lighter elements inside stars. For roughly half the nuclei heavier
than iron-56, nucleosynthesis proceeds slowly (hence the name "s process")
by the addition of one neutron at a time. The observed and calculated (based
on s-process models) abundances of elements mostly agree; one notable exception
is the isotopically anomalous neodymium found in carbonaceous meteorites.
Partly to investigate this discrepancy and partly to accommodate the latest
theories which hold that the s process is prevalent at solar energies of
6-8 keV (corresponding to temperatures of 69-93 million K) rather than
30 keV as was previously thought, scientists at Oak Ridge have performed
difficult new scattering experiments in the lab. Using neutrons from the
Oak Ridge Electron Linear Accelerator (ORELA) they have re-measured with
much greater precision the likelihood of a neutron being captured by the
nuclides 142Nd and 144Nd and have translated this
into reaction rates within stars. Their findings (contact Klaus Guber,guber@mail.phy.ornl.gov)
help to explain the Nd anomaly, confirming that the 142Nd isotopic
distribution in certain microscopic dust grains in some meteorites originated
in low mass red giant stars under the auspices of the s process. This work
may have implications for "r-process" models, which prescribe
how even heavier nuclei are made rapidly in supernovas, and for theories
of how our solar system came about. (K. Guber et al., Physical
Review Letters, 7 April.)
A ONE-SIZE-FITS-ALL UNIVERSAL SUBSTRATE for semiconductors has been
created, potentially allowing researchers to deposit crystals of many previously
incompatible materials onto a semiconductor surface. If the unit cell of
a crystal differs in dimensions by as little as 1% from the dimensions
of the surface onto which it is deposited, defects can form which prevent
the proper functioning of the material. Yu-Hwa Lo (607- 255-5077) and his
colleagues at Cornell have created a thin gallium arsenide film whose crystal
axis is rotated slightly relative to that of the gallium arsenide substrate
onto which it was deposited. The resulting surface, known as a "compliant
surface," became more receptive to bonding with crystals of different
materials, with the interface between the substrate and the deposited crystals
forming a "twist boundary." Onto the GaAs surface, the researchers
successfully deposited crystals of InGaP, GaSb, and InSb. The lattice mismatches
between crystal and surface were as high as 15 percent, but the density
of defects was reduced by a factor of 105 compared to that for
regular substrates. Moreover, if gallium nitride (mismatch of 20%) could
be deposited onto this surface, the researchers believe that high-quality
blue and ultraviolet semiconductor lasers might result. They expect that
their approach could allow computer chips of many different types to exist
on the same motherboard. (Applied
Physics Letters, 31 March 1997; more text and pictures in Cornell
News Release)
THE ANAPOLE MOMENT OF A NUCLEUS IS DETECTED. Parity violation---the
differentiation between left and right---was first observed (1957) in transitions
between nuclear states. Later certain transitions in atoms too were seen
to violate the conservation of parity. Now an experiment at the University
of Colorado not only makes the most accurate measurement of this effect
in cesium atoms but also observes, for the first time, the anapole moment
for a nucleus, the internal electromagnetic moment in the nucleus which
comes about because of the weak force. (C.S. Wood et al., Science,
21 March.)
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