Number 607, October 2, 2002
by Phil Schewe, James Riordon, and Ben Stein
A Clean Air Act At the Quantum Scale
A clean air act at the quantum scale takes place inside state-of-the-art
fuel cells. Environmentally friendly technologies have become a necessity
for dealing with increasing levels of air pollution. For example, catalytic
converters reduce the amount of toxic species in automobile exhausts.
Meanwhile, researchers are intensively developing potential new sources
of low-emission power generation, such as solid-oxide fuel cells, which
use a solid material to supply migrating ions for an electricity-producing
chemical reaction.
To a large extent, many of these devices exploit an amazing property
of solid cerium oxide (CeO2) to release oxygen under oxygen-poor
conditions. To shed oxygen, cerium ions in cerium oxide gain electrons,
and a series of "reduced" compounds, with Ce2O3
as an end product, is formed.
In turn, the final product Ce2O3 easily takes
up oxygen under oxygen-rich conditions. However, the fundamental microscopic
origin of this phenomenon has not been elucidated--until now.
Researchers from Chalmers and Uppsala Universities in Sweden now offer
a detailed quantum-mechanical description of how this reaction occurs.
The pivotal transition from CeO2 to Ce2O3,
they show, results from the formation of an oxygen vacancy, in which
an oxygen atom leaves a spot it normally occupies on the cerium oxide
crystal lattice. In order to vacate the CeO2 lattice, oxygen
has to leave behind two electrons, so that it can transform from an
ion with a charge of -2 to a free oxygen atom.
Quantum effects make this process possible. They enable the electrons
to "localize" on two nearby cerium ions, which initially have
a charge of +4. Gaining these electrons allows the two Ce ions each
to acquire a charge of +3 and allow a series of "reduced"
compounds and eventually Ce2O3 to form.
This makes the oxygen storage-and-release ability of solid cerium oxide
a remarkable example of the quantum process of electron localization,
directly manifesting itself in a macroscopic property used in many modern
environmental friendly applications. (N.V.
Skorodumova, S.I. Simak, B.I. Lundqvist, I.A. Abrikosov, B. Johansson,
Physical Review Letters, 14 Oct 2002; contact Sergei Simak, Uppsala
University, 011-46-18-471-5739, Sergei.Simak@fysik.uu.se).
Arctic Europa
Modeling of tidal processes on Europa, making use of observations recorded
with the Galileo spacecraft, suggest that water could surge near the
surface. This water would originate in the ocean thought to reside beneath
the icy surface layer on Europa and well up in cracks caused by Europa's
ongoing tidal battle with Jupiter. Thus the cracks might afford an avenue
for an exchange of material and liquid between ocean and surface.
According to Richard Greenberg (University of Arizona, greenberg@lpl.arizona.edu),
if living organisms existed at Europa they might be able to survive
as close as a few tens of centimeters from the surface especially if
they live in a crack where they could be bathed daily by water delivered
by tides. Exploring for such biological samples would not then require
deep drilling.
The nearness of water on Europa would therefore be more like that in
Earth's Arctic basin, with ocean lying beneath riven and relatively
thin ice sheets, rather than the Antarctic, where lake water is surmounted
by kilometers-thick glacier. (Greenberg et al., Reviews of Geophysics, 6 September 2002.)
The Semiconductor Laser is 40 Years Old
DVDs, barcode scanners, high-speed fiber-optic telecommunications--these
multi-billion-dollar technological tokens of the early 21st century
all depend upon the semiconductor laser, which was invented 40 years
ago in much humbler settings. One of the laser's most prominent children,
the CD player, is also celebrating its 20th anniversary this autumn
in the consumer market.
In this design, also known as a "diode" laser, electrons
and positively charged holes meet at a semiconductor interface to annihilate
each other and create light.
Semiconductors can convert electricity into light so efficiently that
some physicists scoffed at early reports and complained that this design,
very different from the original solid-state and gas lasers, required
breaking the second law of thermodynamics in order to work as advertised.
But starting in September 1962, scientists reported functioning diode
lasers from four independent laboratories--GE (at two different research
centers), IBM, and MIT's Lincoln Lab, where the corporate ethos of the
day allowed physicists to pursue research on esoteric topics even without
likely applications or a guarantee of success.
The four groups' results appeared within three months of each other
in the journals Physical Review Letters
and Applied Physics Letters,
the latter journal then in its first year of publication.
Technological development of the semiconductor laser continues to this
day. Examples include quantum cascade lasers (see Updates 181,
322,
359),
multi-wavelength lasers from a single material (Update 407),
surface emitting lasers (Update 132,
217,
229)
and blue lasers (Update 50).
Within the decade, blue lasers might replace red lasers in DVD players,
enabling a six-fold increase in information on the same-sized disk.
Hand in hand with diode lasers is the visible LED, also invented in
1962. The low-power, high efficiency LEDs have found their way into
traffic signals and automobile and bus tail lights. Recently available
white-light LEDs have become powerful enough to replace incandescent
front headlights in some automobile models, a development that even
some of the most optimistic LED designers would have found ludicrous
a quarter of a century ago. (Also see AIP/OSA
news release.)
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