Number 277, July 1, 1996 by Phillip F. Schewe and Ben Stein
AN EXCITON ANALOG OF A LASER may be possible. In a semiconductor crystal,
an electron can be excited from a valence state to a conduction state.
The "hole" left behind can be filled by another nearby electron,
but this only moves the hole to a new location. In effect, the hole acts
like a positively charged object which can move about and bind itself to
a negatively-charged electron. This tiny "atom" is called an
exciton. Under special conditions excitons can condense into clouds. Insofar
as excitons are bosons (particles with a spin of 0 or 1), they can even
coalesce into a single quantum state, namely a Bose-Einstein condensate.
Physicists at the University of Ottawa (Emery Fortin, fortin@physics.uottawa.ca)
and the Ecole Polytechnique in Palaiseau, France have shown that it is
possible to amplify a coherent beam of energy based not on light waves,
as in a laser, but on excitons. In their experiment a laser creates an
ongoing bath of excitons in a small semiconductor sample. A second laser
establishes a small moving cloud of excitons (350-400 microns in size).
Through a process of stimulated scattering the moving cloud gains new excitons
as it proceeds; it grows not in size but in density. In other words, the
directed beam of excitons is undergoing amplification. Because "exciton
mirrors" do not yet exist, this amplification process occurs only
over a single pass through the sample, and consequently there is not the
repeated buildup of signal one gets in a laser. Furthermore, because excitons
subsist of electrons and holes in the crystal, a beam of excitons could
never be extracted from the sample and exist as an external beam as laser
beams do. Still, the exciton beam in this present experiment may offer
new ways of studying the coherent transfer of energy and charge. (A. Mysyrowicz
et al., upcoming article in Physical Review Letters.)
100-MILLION K TEMPERATURES IN THE SUN'S CORONA have been detected by
the Solar and Heliospheric Observatory (SOHO) satellite. Measurements at
ultraviolet wavelengths show that oxygen ions at one point above the sun's
north pole simmered at a temperature far higher than had been seen before
in the corona; at a distance of 0.9 solar radii, the oxygen-ion temperature
was 100 million K. These results, reported a few weeks ago at the American
Astronomical Society meeting in Madison, WI add to the mystery of why the
corona should be so much hotter than the solar surface. (Science, 21 June
1996.)
A LARGE LAKE BENEATH THE ANTARCTIC ICE SHEET has been pinpointed by
British and Russian scientists. Lake Vostok, as it is called, was first
detected in the 1970s using radio-echo signals received on an orbiting
satellite and has now been mapped with much greater accuracy. The equivalent
of Lake Ontario in size and as deep as 500 m in places, this body of fresh
water is warmed from below by the heat of the Earth and insulated above
from the Antarctic chill by a 4-km-thick roof of ice. A drilling team near
the Russian Vostok Station (studying layers in the ice sheet) would have
broken through to the sub-glacial lake, but operations have been halted
for fear of contaminating the water, which might harbor unique bacteria.
Methods for analyzing the ultra-pure, ancient water are being pondered.
(A.P. Kapitsa et al., Nature 20 June 1996.)
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