Text 636, 60 rader
Skriven 2005-10-18 07:02:24 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 744
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Physics News Update
Number 744, September 6, 2005
by Phil Schewe and Ben Stein

Atom-Molecule Dark States
Physicists at the University of Innsbruck have demonstrated that atom pairing
(molecule formation) in Bose-Einstein condensates (BECs) using photoassociation
is coherent. Coherent pairing of atoms (locking them into a particular quantum
relationship) has been observed before using a tuned magnetic condition---a
Feshbach resonance---between the atoms. But molecules made that way are only
feebly attached. By contrast the process of photoassociation---i.e. using light
to fuse two atoms into one molecule---allows more deeply bound molecule states
to be established. The trouble is that the same laser light can also be
absorbed to dissociate the molecules rather than only perform its associative
task. The counter measure used by the Innsbruck researchers (contact Johannes
Hecker Denschlag, 43-512-507-6340, johannes.denschlag@uibk.ac.at) is to create
a "dark state" in which the light cannot be absorbed. A dark state is a special
quantum condition: it consists of three quantum energy levels, two stable
ground sta
tes and one excited level. If laser light at the two frequencies needed for the
transitions from both the ground states to the excited state are present
simultaneously, the two excitations (from the two lower energy states) can
destructively interfere with each other if there is phase coherence between the
ground states. (Homely example: offer one cookie to two children and, if they
fall into the right kind of arguing, the cookie goes uneaten.) The consequence
is that no light gets absorbed and the molecules are stable. Such
"electromagnetically induced transparency" has been observed before for
transitions within atoms (PNU 343) but the Innsbruck scientists are the first
to use it for a transition between a BEC of atoms and molecules. In their
experiments, the same (two-color) laser light that creates the dark state is
also the light that photoassociates rubidium atoms into molecules. Johannes
Hecker Denschlag says that atom-molecule dark states are a convenient tool to
analyze the atom-molecule system a
nd to optimize the conversion of atomic into molecular BECs. BECs of ultracold
molecules represent, because of their many internal degree of freedom
(vibrational and rotational), a new field of research beyond atomic BECs.
(Winkler et al., Physical Review Letters, 5 August 2005; lab website,
www.dark.ultracold.at)

Measuring Nanotubes' Conductivity
How well nanotubes conduct electricity depends a lot on their environment.
Hongjie Dai and his colleagues at Stanford have made the first electrical
measurements of currents flowing under high voltage (high bias) through
single-walled carbon nanotubes suspended like miniature power lines. They
discovered that in suspended form a micron-scale-long nanotube could carry
about 5 micro-amps of current, whereas lying in the plane of a substrate the
same tube can carry about 25 micro-amps. The reason for the better in-the-plane
performance is that the substrate helps to dampen "optical phonons,"
high-energy vibrations of the nanotube atomic lattice. Dai (650-723-4518,
hdai1@stanford.edu) believes that with careful engineering of the interface
between a nanotube and a substrate, maximum currents could be raised to higher
levels than previously possible, which might make carbon nanotubes useful for
applications in high-power transistors and even nanoscale transmission lines.
To make the kind of transmission lines you see in the countryside out of
nanotubes, you'd have to develop a process for producing km-length carbon
tubes, which is not feasible for the foreseeable future. (Pop et al., Physical
Review Letters, upcoming article)

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