PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 718 February 2, 2005
by Phillip F. Schewe, Ben Stein

COMPLEX HYBRID STRUCTURES, part vortex ring and part soliton, have been
observed in a Bose-Einstein condensate (BEC) at the Harvard lab of Lene
Vestergaard Hau.  Hau previously pioneered the technique of slowing and
then stopping a light pulse in a BEC consisting of a few million atoms
chilled into a cigar shape about 100 microns long.  In the new experiment,
for the first time, two such light pulses are sent into the BEC and
stopped.  The entry of these pulses into the BEC set in motion tornado-like
vortices.  These swirls are further modulated by solitons, waves which can
propagate in the condensate without losing their shape.  The resultant
envelope can act to isolate a tiny island of superfluid BEC from the rest
of the sample.  The dynamic behavior of the structures can be imaged with a
CCD camera by shining a laser beam at the sample (see figure at
www.aip.org/png ).  Never seen before, these bizarre BEC excitations
sometimes open up like an umbrella.  Two of the excitations can collide and
form a spherical shell (the vortex rings taking up the position of constant
latitudes).  Two such rings, circulating in opposite directions, will
co-exist for a while, but after some period of pushing and pulling, they
can annihilate each other as if they had been a particle-antiparticle pair.
 Hau (hau{at}physics.harvard.edu, 617-496-5967) and her colleagues, graduate
student Naomi Ginsberg (ginsber{at}fas.harvard.edu) and theorist Joachim Brand
(at the Max Planck Institute for the Physics of Complex Systems, Dresden),
have devised a theory to explain the strange BEC excitations and believe
their new work will help physicists gain new insights into the superfluid
phenomenon and into the breakdown of superconductivity. (Ginsberg, Brand,
Hau, Physical Review Letters, 4 February; lab website
http://www.deas.harvard.edu/haulab/mainframe.htm )

ROD-SHAPED NUCLEI, even slablike nuclei, might occur amid the cataclysm of
a supernova.  This is when nuclear matter---normally hard, spherical, and
dense (3 x 10^14 g/cm^3)---can thin out, to an average density only half
that of normal nuclear matter.  The nuclear "rods" would still be
densely packed in the star (like a liquid crystal) and the rods might
coalesce into slabs, says Gentaro Watanabe, temporarily at the NORDITA lab
in Denmark.  He and his colleagues at the Japan Atomic Energy Research
Institute, the University of Tokyo, the RIKEN lab, and Keio University,
have modeled alternative nuclear shapes in an effort to address the subtle
problems in simulating supernovae.  One of these problems is that shock
waves stall in the stellar core.  The Japanese researchers expect that
incorporating effects of "pasta" phases (the collective name for
rod or slab nuclei) in core collapse simulations would help them to model
the explosion more realistically.  The "pasta" phases would be
formed in the central region of the collapsing core, while the region where
the shock waves propagate and stall is much further out.  Neutrinos from
central region contribute "neutrino heating" and would help the
shock waves to revive.  This scenario is more tenable if the pasta phases
are present, and not just uniform nuclear matter.  (Watanabe et al.,
Physical Review Letters, 28 January 2005; contact, gentaro#nordita.dk )
                                
CONTROLLING BRAIN WAVES.  A new study conducted at George Mason University
confirms predictions that electrical fields can be used to modify waves
traveling through brain tissue.  This is perhaps the first example of
electric modification of neuronal thresholds to control wave movement.
Indeed, it is one of the first times waves have been controlled in an
excitable medium through changing thresholds.  The researchers begin with a
section of rat brain; the tissue consists of 6 layers of 2-dimensional
sheets of neurons.  A neural wave is initiated at one end of the network
and the signal is observed at the other end.  By using electrical fields,
the excitability of individual neurons can be modified.  Doing this can
slow down, speed up, or stop any wave propagating through the sample. 
Previously neural waves had only been modified by pharmacological means. 
This action can be negated only by washing out the drug used, which takes
seconds, whereas the electric method takes only microseconds to have an
effect.  One potential application for modifying brain waves would be in
mitigating epileptic seizures. (Richardson et al., Physical Review Letters,
21 January 2005; lab website, www.neuraldynamics.org; contact Bruce
Gluckman, bgluckma{at}gmu.edu, 703-993-4384 or Steven Schiff, sschiff{at}gmu.edu)
 Part of the George Mason contingent also was involved in the recent
discovery of true spiral waves in the sensory cortex of the brain (Huang et
al J Neurosci 24: 9897-9902, 2004).

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