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 at
complex.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@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, 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)