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{at}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{at}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.

---