Text 729, 95 rader
Skriven 2006-02-28 19:17:23 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 767
===============
PHYSICS NEWS UPDATE                                                            

The American Institute of Physics Bulletin of Physics News
Number 767   February 28, 2006  by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi
                
ATOM WIRES.  The smallest wire width in mass produced electronic
devices is about 50 nm, or about 500 atoms across.  The ultimate
limit of thinness would be wires only one atom wide.  Such wires can
be made now, although not for any working electronic device, and it
is useful to know their properties for future reference.  Paul
Snijders and Sven Rogge from the Kavli Institute of Nanoscience at
the Delft University of Technology and Hanno Weitering from the
University of Tennessee build the world's smallest gold necklaces by
evaporating a puff of gold atoms onto a silicon substrate which has
first been cleared of impurities by baking it at 1200 K.  The
crystalline surface was cut to form staircase corrugations.  Left to
themselves, the atoms then self-assemble into wires (aligned along
the corrugations) of up to 150 atoms each (see figure at
www.aip.org/png ). Then the researchers lower the probe of a
scanning tunneling microscope (STM) over the tiny causeway of gold
atoms to study the nano-electricity moving in the chain; it both
images the atoms and measures the energy states of the atoms'
outermost electrons.  What they see is the onset of charge density
waves---normally variations in the density of electrons along the
wire moving in pulselike fashion.  But in this case (owing to the
curtailed length of the wire) a standing wave pattern is what
results---as the temperature is lowered.  The wave is a quantum
thing; hence certain wavelengths are allowed.  In other words, the
charge density wave is frozen in place, allowing the STM probe to
measure the wave (the electron density) at many points along the
wire.  Surprisingly, two or more density waves could co-exist along
the wire.  The charge density disturbance can also be considered as
a particlelike thing, including excitations which at times possess a
fractional charge.  (Snijders et al., Physical Review Letters, 24
February 2006; p.c.snijders@tnw.tudelft.nl)     

FORMATION OF LARGE FLUID VORTICES: CORPORATE MERGER OR
HOSTILE-TAKEOVER?  Large, energetic vortex structures commonly form
in irregular or turbulent two-dimensional flows. Familiar examples
are Jupiter's Red Spot or hurricanes and typhoons on Earth. What is
the mechanism that transfers energy from small-scale vortices to
these often long-lived, large-scale circulation patterns? Many
suggestions have been made, such as a merger of small vortices into
larger ones. According to this scenario, the process is similar to
the consolidation or merger of many small corporations into a
mega-corporation. In a new paper, researchers verify by experiment
and simulation a quite different mechanism based on elongation and
thinning of small-scale vortices, stretched like taffy by
large-scale strain. This process weakens the velocity of the small
vortices and transfers their kinetic energy into the large-scales.
The thinning mechanism allows the large vortices to drain the energy
of the smaller ones, squeezing them dry. Thus, the process is more
like a hostile takeover of many small corporations by a larger one
that strips their assets and liquidates them. According to the
authors, the work provides quantitative models of how a population
of small-scale vortices sustains on the large-scale circulations.
These results will help to model and predict formation of
large-scale vortices in atmospheres and oceans (Chen et al.,
Physical Review Letters, 3 March 2006; contact Gregory Eyink, Johns
Hopkins,
eyink@ams.jhu.edu; for an example experimental image, one that shows
Van Gogh-like fluid swirls, see www.aip.org/png                                
                                        

AN ION TEMPERATURE OF 2 TO 3 BILLION KELVIN, hotter than the
interior of any known star, has been achieved in New Mexico.  This
temperature record was set recently in a test shot at the Z Pinch
device at Sandia National Lab where an immense amount of electrical
charge is stored in a device called a Marx generator.  Many
capacitors in parallel are charged up and then suddenly switched
into a series configuration, generating a voltage of 8 million volts
(a process captured in a famous photograph; see www.aip.org/png ).
This colossal electrical discharge constitutes a current of 20
million amps passing through a cylindrical array of wires, which
implodes.  The imploding material reaches the record high
temperature and also emits a large amount of x-ray energy
(http://www.aip.org/pnu/2004/split/702-1.html).  Why the implosion
process should be so hot and why it generates x rays so efficiently
(10-15% of all electrical energy is turned into soft x rays) has
been a mystery.  Now Malcolm Haines (Imperial College) and his
colleagues think they have an explanation.  In the hot fireball
formed after the jolt of electricity passes through, they believe,
the powerful magnetic field sets in motion a myriad of tiny vortices
(through instabilities in the plasma), which in turn are damped out
by the viscosity of the plasma (ionized atoms).  In the space of
only a few nanoseconds, a great deal of magnetic energy is converted
into the thermal energy of the plasma.  Last but not least, the hot
ions transfer much energy to the relatively cool electrons, energy
which is radiated away in the form of x rays.  (Haines et al.,
Physical Review Letters, 24 February 2006; contact Malcolm Haines at
m.haines@imperial.ac.uk)

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