PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 700 September 10, 2004
by Phillip F. 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 atcomplex.umd.edu) 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{at}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{at}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{at}LNS.tohoku.ac.jp)

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 ne
arly 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{at}mpipks-dresden.mpg.de)

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