Text 188, 72 rader
Skriven 2004-09-11 07:39:58 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 700
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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@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 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@mpipks-dresden.mpg.de)

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 * Origin: Big Bang (1:106/2000.7)