Number 743 , August 29, 2005
by Phil Schewe and Ben Stein
Magnetic Burning
A new experiment suggests that the fast flipping
of the magnetic orientation of some molecules in a solid sample
resembles the propagation of a flame front
through a material being burned, and that the "magnetic burning"
process can be used to study flammable substances without actually
having flames present. In a chemical fire---say, the burning of the
pages of a book---the flame front marks a dividing point: ahead of
the front is intact unburned material, while behind the front is
ash, the state of material that has been oxidized in the combustion
process. Now, consider the magnetic equivalent as studied by a
collaboration of scientists from CUNY-City College (Myriam Sarachik,
sarachik@sci.ccny.cuny.edu, Yoko Suzuki, yoko@sci.ccny.cuny.edu),
CUNY-Lehman College (Eugene Chudnovsky
(eugene.chudnovsky@lehman.cuny.edu), the Weizmann Institute, and the
University of Florida. A crystal of manganese12-acetate (Mn12-ac)
molecules, each with a net spin of 10 units, is quite susceptible to
magnetic influence. Turning on a strong external magnetic field
opposed to the prevailing magnetic orientation of the crystal can
cause a sudden reversal of spins of the molecules. The reversal
propagates along a front through the crystal (which can be thought
of as a stack of nanomagnets) just as a flame moves through a solid
in the case of a conventional combustion. In the magnetic case, much
heat will be generated as the spins get flipped (the heat energy
being equal to the difference in energy of the
before and after spin states), but there will be no destructive
burning (see movie at
http://www.sci.ccny.cuny.edu/~sarachik/MagBurn.mpeg ). The "ash"
consists of the molecules in their new spin state. In summary,
magnetic burning in molecular magnets has several of the qualities
of regular burning (a flame front and combustion) but not the
destructiveness. Myriam Sarachik says that magnetic burning might
offer a more controlled way of learning how to control and channel
flame propagation. (Suzuki et al., Physical Review Letters,
upcoming article; http://www.sci.ccny.cuny.edu/~sarachik/)
BEC in a Circular Waveguide
Bose Einstein condensates (BECs), in
which trapped, chilled atoms fall into a single corporate quantum
state, have been achieved for several elements of the periodic table
and in a variety of trap geometries. Physicists at UC Berkeley have
now, for the first time, produced a BEC in a ring-shaped trap about
1 millimeter across. By using
an extra magnetic field, in addition to those used to maintain the
atoms in the trap to start with, the whole trap can be "tilted," so
as to accelerate the atoms up to velocities of about 50-150 mm/sec
(or equivalently to energies of about 100 pico-electron-volts per
nucleon, as compared to the TeV energies sought for particle
physics). After this initial "launch" phase, the atoms are
allowed to drift around the ring; they do this not in clumps (as you
would have with particles in a colliding-beam storage accelerator)
but in a continuously expanding stream. However, starting from the
BEC state, the atoms are really more like coherent atom waves
smeared out around the
ring; they move ballistically and without emitting synchrotron
radiation. According to Dan Stamper-Kurn (dmsk@berkeley.edu),
potential applications for BEC rings would become possible if parts
of the circulating condensate could be made to interfere with other
parts. From
such an interferometer one could devise gyroscopes or high-precision
rotation sensors. Other possible realms of study: quantized
circulation, fluid analogues of general relativity, and fluid
analogues of SQUID detectors and other superconducting devices.
(Gupta et al., Physical Review Letters, upcoming article; lab
website at physics.berkeley.edu/research/ultracold )
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