[tt] Physics News Update 866

Tue Aug 19 10:39:20 UTC 2008

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From: physnews at aip.org
Date: Wed, 4 Jun 2008 13:47:54 -0400
To: eugen at LEITL.ORG
Subject: Physics News Update 866
Reply-to: physnews at aip.org

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 866 June 4, 2008      www.aip.org/pnu
by Phillip F. Schewe and Jason S. Bardi

ON VERY THIN ICE
For the first time, scientists have obtained pictures of ice only a
few nanometers thick in the act of forming bulk ice at the coldest
of temperatures.  The new images, showing ice sheets about 50,000
times thinner than a human hair, add to our knowledge of water, that
remarkable molecular substance that is common on Earth, and found in
significant amounts in other places around the solar system.
Usually we think of ice as forming from tiny six-sided crystals.
The six-fold symmetry of classic snowflakes comes from the way water
molecules fit together.  But at very low temperatures, close to
absolute zero -- the coldest conceivable state of matter -- water
molecules don’t behave the way they do at warmer temperatures.  Like
people first waking from a sound sleep, water molecules in this
chilly environment can’t move very well.  If you spray them onto a
platinum surface they tend to stay where they land.  If you keep
adding water, the molecules form a solid called amorphous ice, with
all of the molecules sticking together wherever they can.  Because
of the extreme cold, the molecules don’t have enough energy to line
up to form a crystalline array.  Raise the temperature a bit, just
above 120 K, and now the molecules have a chance to creep around
enough to start assembling a proper crystal.  But these little blobs
still aren’t in the familiar hexagonal shape.  Instead the water
molecules form a cubic crystal structure.  To form common ice, with
its hexagonal structure, a little bit more warmth is required - a
still-frosty 160 K.
Konrad Thürmer (kthuermer at gmail.com) and Norman C. Bartelt, two
scientists at Sandia National Laboratory in Livermore, California,
were interested in exploring the early formation of ice as part of
their research into the initial stages of the growth of crystal
films.  The device they used to make the ice pictures is a scanning
tunneling microscope (STM), whose tip is scanned across the face of
the ice sheet, can be used to form an image of the ice.  The images
show ice crystallites much smaller than previously seen (see figures
at http://www.aip.org/png/2008/303.htm).  Earlier attempts to image
very thin ice sheets with an STM failed because it is difficult to
conduct electricity through ice.  But this time the scientists got
just enough electricity to flow across the gap from tip to sample -
partly by cranking up the voltage and partly by making a
stepping-stone path for the electricity to follow through the ice.
What did they find?  When the ice film was really thin, only about 1
nm thick on average, the water molecules formed into little islands
of ice.  When the thickness reached 4 or 5 nanometers, the ice
started to form larger joined chunks.  They believe that when one
tiny crystallite adding molecules on a downward going slope meets
with a crystallite with an upward-going slope, the two structures
start to pivot around each other. This accounts for the somewhat
corkscrew appearance of the patchwork quilt of merging ice sheets.
(Physical Review B, May 2008)

SUPERCONDUCTIVITY IN SUPER HARD DIAMOND.  A Study calculates that
boron-doped diamond (BC5) should be superconducting on up to
temperatures of 45 K, which, if borne out in experiments, would make
this class of material with the highest with the highest transition
temperature into a superconducting state mediated by the passing of
phonons.  (Calandra and Maui, Physical Review Letters, upcoming
article)

CORRECTION.  In Update 865, the nucleus neon-20 was said to
transform to neon-18 by emitting two protons.  Instead it should be
by the emission of two neutrons.

***********
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