Number 718, February 2, 2005
by Phil Schewe and Ben Stein
Complex Hybrid Structures
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. 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@physics.harvard.edu, 617-496-5967) and her colleagues, graduate
student Naomi Ginsberg (ginsber@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/
)
Rod-Shaped Nuclei
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 1014 g/cm3)---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. (Watanabeet 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. (Richardsonet al., Physical Review Letters, 21 January 2005; lab website,www.neuraldynamics.org;
contact Bruce Gluckman, bgluckma@gmu.edu, 703-993-4384 or Steven Schiff,
sschiff@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 (Huanget al J Neurosci 24: 9897-9902, 2004).
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