Number 700, September 10, 2004
by Phil Schewe and Ben Stein
Making Stellar Magnetic Fields in a Jar
An experiment at the University of Maryland reports the first experimental
observation of a magnetorotational instability---essentially the creation
of an induced magnetic field amid the turbulence of a rotating electrically
conducting fluid immersed in a separate magnetic field.
In the Maryland experiment a baseball-sized copper ball is rotated
within a vessel containing liquid sodium. With this setup, the researchers
try to simulate the ingredients shared in common by Earth's core, the
outer envelopes of stars, and the accretion disk surrounding black holes.In
each case a conducting fluid, differential rotation (inner parts of
the fluid rotating faster than outer parts), and potent magnetism add
up to interesting physics. Until now there had been only theories and
simulations of this physical environment.
Now, the Maryland experiment actually demonstrates that an organized
magnetic field (see figures at Maryland
website) can arise even from a hydrodynamic turbulent fluid. According
to Daniel Lathrop, one of the scientists involved, the new test allows
researchers to study the interplay between moving fluids, the ways in
which turbulence can occur, and how the fluid rotation can be braked.
(Sisan et al., Physical Review Letters,
10 September; contact Lathrop at dpl@complex.umd.edu, 301-405-1594)
Can Chemical Environment Affect Nuclear Properties?
A new experiment shows that the decay lifetime of radioactive beryllium-7
changes by almost 1% when placed inside a carbon-60 molecule. This is
perhaps the largest shift yet seen in a chemically induced modification
of a nuclear lifetime. The Be-7 is unstable and one way for it to decay
is for the nucleus to capture one of its own electrons, process in which
a proton is turned into a neutron.
Now if the Be atom lies in the cavity within a C60 molecule
(in which case it is referred to as endohedral Be, or abbreviated further,
Be@C60) the surrounding halo of carbon-based electrons apparently
modifies the wave-functions of the beryllium-associated electrons and
the associated "phase space" so that the rate at which electrons are
captured by the Be nucleus is speeded up.
Previous attempts to modify nuclear lifetimes through chemical means
have resulted in shifts that were at the 0.15% level. The researchers
from Tohoku University and Yokohama National University (Japan) doing
the present experiment believe that it would be premature to suggest
that this approach can be used to mitigate the problems of storing radioactive
materials, but, in the near term the use of endohedral fullerenes (cargo-carrying
C60 molecules) might lead to specialized radio-therapies
or tracers for tagging metabolic pathways in the body. (Ohtsuki
et al., Physical Review Letters, 10 September 2004; Ohtsuki@LNS.tohoku.ac.jp)
Atom-Hole BECs
Atom-hole BECs, condensates of atom-hole pairs held in an "optical
lattice" made of crossed laser beams, might contribute to the now-popular
program of putting quantum weirdness to use in information processing
and to the study of superfluids through the use of tailored interactions.
Chaohong Lee, a physicist at the Max Planck Institute for the Physics
of Complex Systems in Dresden, has suggested his model of atom-hole
condensates in analogy with electron-hole clouds in semiconductors.
When an electron is sprung from its niche in a semiconductor crystal,
the hole remaining behind can itself move around and act as if it were
a positively charged object. Indeed, a nearby electron and hole can
behave as a sort of pair. These pairs, or "excitons," can condense into
a single quantum state. In light emitting diodes (LEDs) the coalescence
of holes and electrons results in light emission.
Lee believes the same can happen to supercold Fermi atoms (those with
a half-integral amount of spin) lodged in all, or nearly all, the interstices
of an optical lattice. In his model two species---with different magnetic
polarizations---of the same element would be loaded in the trap. Then,
by altering an applied magnetic field, interactions among the trapped
atoms, and the potential depth of the optical lattice could be manipulated
so as to favor atom-hole pair formation and even condensation. Like
the electron-hole partners meeting to create light, the atom-hole mates
might also be made to render light in novel ways.(Physical
Review Letters, upcoming article; 49-871-2124, chlee@mpipks-dresden.mpg.de)
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