PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 793 September 20, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi www.aip.org/pnu
STRONGER HURRICANES LINKED TO CLIMATE CHANGE. A new study of
climate data suggests that global warming is causing the Atlantic
Ocean to generate deadlier hurricanes. Hurricanes have become
stronger in recent decades, in apparent correlation with the raise
in atmospheric temperatures. Indeed James Elsner of Florida State
University in Tallahassee reports in Geophysical Research Letters
that there is in fact a clear cause-and-effect link. Less than three
weeks after Hurricane Katrina, a study published in Science showed
that, while the number of tropical cyclones had not increased
between 1970 and 2004, their strength had surged: Category-4 or -5
hurricanes where more than 50 percent more frequent in the second
half of that period than in the first (Webster et al., Science,16
September 2005,
http://www.sciencemag.org/cgi/content/abstract/309/5742/1844).
The same period saw a rise in global atmospheric temperatures--
widely attributed to the accumulation of greenhouse gases such as
CO2--and in sea-surface temperatures in the Atlantic, where
hurricanes are born. Some climatologists believe that global
(atmospheric) warming is causing the oceans' temperatures to rise,
and that warmer sea surfaces can in turn add to a hurricane's
strength. But others attributed nature's increased wrath to a
long-term cyclic fluctuation in sea temperatures called the Atlantic
Multidecadal Oscillation. Opinions also varied on whether a warmer
atmosphere can significantly make the oceans warmer, and on the
extent to which sea temperatures contribute to hurricane strength.
Elsner ([EMAIL PROTECTED], 850-644-8374) used an elaborate
statistical method (first devised by economics Nobel Prize winner
Clive Granger) to answer the first of those two questions. He
examined spikes in global atmospheric temperature (using satellite
and ground-based data collected by the Intergovernmental Panel on
Climate Change) and compared them to seasonal changes in average
sea-surface temperatures for the entire northern-hemisphere part of
the Atlantic (based on National Atmospheric and Oceanic
Administration data). His analysis showed that the spikes in
atmospheric temperature mostly tended to come right before
hurricane-season spikes in oceanic temperature, suggesting that the
first were causing the second. Global warming could indeed be
causing stronger hurricanes. (Geophysical Research Letters, 23
August)
ROOM-TEMPERATURE SPIN HALL EFFECT. A new experiment by David
Awschalom and his colleagues at UC Santa Barbara plus collaborators
from Penn State shoots a stream of electrons through a sample of
ZnSe, a non-magnetic semiconductor, and segregates the electrons in
such a way that those with spins pointing up are steered to the left
while those with downward pointing spins deflect to the right. They
also demonstrated that they could polarize the electrons (orient
their spins) using only electric fields at room temperature as well,
a great boon for prospective spintronics circuitry that would
fashion a new form of electronics in which both charge and spin
provide ways of storing and processing data. Strangely, Awschalom's
new results---showing a spin current all the way up to room
temperature---is conducted not in GaAs, where most previous
observations of the spin Hall effect have been made, but in ZnSe,
which should not be as efficient at electrically polarizing spins.
Awschalom ([EMAIL PROTECTED], 805-893-2121) says that the
evidence that the spin Hall effect is strong even in a material
where it should be weak will kindle further the interesting
controversy swirling around interpretations of the spin Hall effect.
The new experiment is a spin equivalent of the conventional Hall
effect known since the 19th century. In the old Hall effect
electrons, moving longitudinally through a sample under the force of
an applied electric field will, if exposed to a vertically oriented
magnetic field, be deflected slightly to one side of the sample.
Two years ago physicists showed that a kind of Hall effect could be
used to steer spins (to be more exact, electrons polarized with
spins up or down) so that even while no pileup of electric charges
at the edge of the sample would occur a net pileup of spins would
occur (see Physics Today, Feb 2005). In another recent experiment,
Awschalom and colleagues showed that the spins wouldn't just pile
up; they could be led off into a wire and constitute a polarized
current, where they would be to a spintronic circuit of spin
transistors what an ordinary current is to ordinary electronics.
(Two articles in Physical Review Letters: Sih et al., in the 1
September 2006 issue and Stern et al. in the 22 September 2006
issue)
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