Text 432, 63 rader
Skriven 2005-05-06 10:15:54 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 730
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 730 May 5, 2005
by Phillip F. Schewe, Ben Stein

ROOM TEMPERATURE LIQUID SODIUM can occur but only under pressures of a million
atmospheres.  Melting is a mystery.  It happens when the thermal agitation
among atoms in a solid overcomes the inter-atom bonds.  Applying pressure to a
solid sample usually helps to negate the effect of thermal agitation and so the
melting temperature usually goes up with pressure.  In a few materials, such as
water, above a certain pressure the melting point begins to drop.  Now, the
most dramatic case yet seen of such a "negative melting curve" has been studied
by scientists at the Carnegie Institution of Washington looking at one of the
simplest metals known, sodium. What happens is this:  With zero pressure
applied, sodium melts at a temperature of 371 K.  As pressure is added, the
melting temperature goes up too, up to 1000 K at a pressure of 30 giga-pascals
(30 GPa), or about 300,000 atm.  Then strange things happen.  As the pressure
is taken up further, the melting point starts to drop, reaching a low of 300 K
(below its ambient melting point) at pressures of 118 GPa (see graph at
www.aip.org/png).  All previous materials exhibiting negative melting curves
have gone negative very reluctantly, over pressure ranges of a few GPa or
temperature ranges of a few K.  Sodium, by contrast, goes negative over a range
of 700 K and 80 GPa.  According to Carnegie researcher Eugene Gregoryanz
(e.gregoryanz@gl.ciw.edu), at a pressure of a million atmospheres his sodium
sample melts at room temperature.  The liquid is denser than the solid (water
shares this trait), and might have strange plastic or mechanical properties. 
It might even be superconducting under some circumstances, he says. 
(Gregoryanz et al., Physical Review Letters, upcoming article)

AN OPTICAL CONVEYOR BELT for moving sub-micron objects has been achieved by
collaborating physicists at the Institute of Scientific Instruments in Brno,
Czech Republic and at the University of St. Andrews in Scotland. Their set-up
used a special type of non-diffracting laser light that forms a very narrow
beam existing over long distance without changing its width.  Two such
counter-propagating laser beams establish up a lace-like standing wave pattern
which  can suspend and hold tiny polystyrene spheres of just the right size. 
The balls, which range in size from 400 nm to one micron, have a density
comparable to water. Previously, scientists have used such non-diffracting
"optical lace" beams to move particles with the force of radiation pressure,
but without the ability to stop them using only a single beam. The Czech and
Scottish researchers, by contrast, set up a light lace pattern with numerous
knots, corresponding to intensity maxima (antinodes) of the standing wave. 
Furthermore a particle can be confined near a knot and all the knots can then
be moved simultaneously over large distances by changing the  relative phases
of the counter-propagating laser beams. Moreover thanks to the self-hea
ling property of the non-diffracting beams, many particles can be confined
simultaneously in the standing wave structure (near the knots) without
significantly spoiling the beam properties. The positioning accuracy, related
to the precision of the phase shift and the optical trap depth (the size of the
knots), is at the micron level and will get better.  Pavel Zemanek
(zemanek@isibrno.cz) says that possible applications for his device include the
delivery of biological or colloidal microparticles or even ultracold atoms. 
(Cizmar et al., Applied Physics Letters, 25 April 2005; lab site at
http://www.isibrno.cz/omitec/index.php?swt.html ) (A few years we wrote about a
different kind of photon conveyor belt:
http://www.aip.org/pnu/1997/split/pnu321-1.htm )

CORRECTION: In the item on pyrofusion (Update 729, Item 1), the tungsten tip is
actually positively charged, so that it and the pyroelectric crystal both repel
the positive deuterium ions towards a solid deuterium-containing target.

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