Number 603, September 9, 2002
by Phil Schewe, James Riordon, and Ben Stein
How Jupiter Got Its Stripes
A new study of turbulence in the atmosphere around a rotating sphere
is helping to explain the dramatic stripes on Jupiter, Saturn, and the
other giant planets. On Earth, turbulence caused by solar heating and
friction with the ground disrupts atmospheric flows and dissipates the
energy provided by the sun that might otherwise lead to the formation
of circulating, global cloud bands. In the thin atmospheres of gas giants,
however, energy dissipation is small, and some of the sun's energy is
gradually collected in stable, global jets that trap clouds and form
planetary stripes.
Researchers at the University of South Florida and Ben-Gurion University
of the Negev (Israel) have now developed a model that shows how planetary
rotation and nearly two-dimensional atmospheric turbulence may combine
to create large scale structures.
Scientists have long suspected that the interaction between planetary
rotation and large-scale turbulence governs the banded circulations
on giant planets. The new research has quantified the phenomenon, leading
to an equation that characterizes the distribution of energy among different
scales of motion, and to simple formulae that describe basic energetic
features of giant planets' circulations.
The model helps explain the paradoxical observation that the outer
planets have stronger atmospheric flows, even though the energy provided
by the sun to maintain such flows decreases with increasing distance
from the sun. The researchers (B. Galperin, bgalperin@marine.usf.edu,
727-553-1101) have found that the atmospheres of distant planets dissipate
even less energy than their warmer sisters.
Although the outer planets receive less energy from the sun, they keep
more of the energy they receive. As a result, the model shows why Neptune
has the strongest atmospheric circulation of all the gas giants even
though it is the farthest of the bunch from the sun. (S.
Sukoriansky, B. Galperin, N. Dikovskaya, Physical Review Letters,
16 September 2002.)
Atoms Light Up Very Rapidly Near Nanotubes
Just as the sharp point of a lightning rod modifies the electrical
properties of space above a building, so too will certain highly curved
(on a nanoscopic scale) surfaces modify the electromagnetic properties
of physical vacuum in their vicinity. This changes the behavior of an
atom near nanobodies (quantum dots, nanospheres, nanocylinders, etc.).
Generally called the Purcell effect, the phenomenon happens because
an excited electron inside the outside atom strongly senses the modified
structure of physical vacuum near surfaces in its vicinity.
New calculations performed by physicists at the Belarusian State University
in Minsk show that due to unique conducting properties of carbon nanotubes
the fluorescence rate of an excited atom or molecule in their vicinity
should be enhanced by as much as million, a much greater effect than
for other geometries studied. The Purcell effect has been observed in
many of these other cases, and the Belarusian scientists (contact Prof.
Sergei Maksimenko, maksim@bsu.by) hope to find collaborators to test
their nanotube hypothesis. The hope is to exploit the enhanced spontaneous
decay rate to control the behavior of nuclei, atoms, or organic molecules
outside or inside nanotubes. (Bondarev
et al., Physical Review Letters, 9 September 2002.)
Photonics plus Spintronics
First came solid-state electronics, producing the field effect transistor
(FET), in which a tiny voltage applied to a gate enables a much larger
current to flow through a circuit. Next came optoelectronics, producing
the light emitting diode (LED), in which electrons and holes (the spaces
vacated by electrons) are made to combine and produce useful light (unfortunately
this does not include silicon, an infamous non-light-emitter, at least
until recently). Then came spintronics, producing circuit elements such
as magnetoresistive sensors, in which an electron polarization (the
direction of an electron's magnetic moment) is an important variable.
Now scientists would like to combine optical and magnetic features in
a single technology.
Some steps have already been taken: dilute magnet semiconductors (DMS),
materials doped with magnetic metal atoms, can be made ferromagnetic;
that is, they can be magnetized and will stay magnetic providing you
stay below the curie temperature (which is to magnets what the transition
temperature is to superconductors). Furthermore, polarized electrons
have been used to make polarized photons in the dilute magnet materials.
The latest advance is to make a silicon-compatible spintronics material
that functions at room temperature. Arthur Hebard (afh@phys.ufl.edu,
352-392-8842) and his colleagues at the University of Florida show that
the semiconductor gallium phosphide (GaP) doped with manganese becomes
and stays magnetic above room temperature.
These results suggest that the related compounds, InGaP and AlInGaP,
which are already used in light emitting diode applications, might also
become magnetic when doped with Mn and thus be useful as polarized light
emitters. This should lead handily to spin-LEDs and spin-FETs (requiring
much small operating voltages than conventional FETs).
More promising still is possibility of integrating doped-Ga-P spin-FETs
and LEDs with silicon technology, the reigning industry standard material.
Finally, it should be noted that a result like this, involving the fine
tailoring of a material with dopant elements, necessitated a strong
collaboration between the physics department at Florida (Hebard) and
the department of materials science and engineering (Cammy Abernathy
and Steve Pearton) (Theodoropoulou
et al., Physical Review Letters, 2 Sept.)
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