PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 743   August 29, 2005  by Phillip F. Schewe, 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{at}sci.ccny.cuny.edu, Yoko Suzuki, yoko{at}sci.ccny.cuny.edu),
CUNY-Lehman College (Eugene Chudnovsky (eugene.chudnovsky{at}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 magne
tic 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{at}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; website at physics.berkeley.edu/research/ultracold)

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