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CX2SA > TECH 09.12.05 02:41l 71 Lines 3598 Bytes #999 (0) @ WW
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Subj: How to create a crystal made..
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Sent: 051209/0032Z @:CX2SA.LAV.URY.SA #:4305 [Minas] FBB7.00e $:4305_CX2SA
From: CX2SA@CX2SA.LAV.URY.SA
To : TECH@WW
How to create a crystal made entirely of holes
==============================================
Computer experiments have revealed that crystal-like structures can be formed
entirely from the "holes" left behind as electrons move through a
semiconducting material.
Crystals are normally made from naturally repeating, three-dimensional patterns
of atoms. Synthesising crystals from electron holes could lead to novel
superconducting materials that function above the extremely cold temperatures
currently required, according to the researchers involved.
Physicists first predicted in the 1960s that exciting the electrons in
semiconductors might cause the holes left behind to form a crystalline
structure. But the conditions required to make this happen remained unclear.
Now a team led by Michael Bonitz at the Institute of Theoretical Physics and
Astrophysics at Kiel University, Germany, has used computer simulation to study
the behaviour of electron holes within a semiconductor and determine how they
can be coaxed into forming the unusual crystalline structures.
Three rules
-----------
The researchers discovered that a crystal of holes should form when three
criteria are met. First, the material must be cooled to about 10øC. Second,
after excitation, it must have a sufficiently high density of holes - about
1020 per cubic centimetre.
Finally, each hole must have an "effective mass" about 80 times that of an
electron in the material, meaning it must have the same effect on the electron
as a much heavier particle.
Under these conditions, the computer simulations suggest that a repeating
structure of holes will spontaneously emerge. Each hole repels other holes,
causing them to "crystallise" at a set distance from one another.
A video clip (4.8MB avi file) of the computer simulation shows holes (pink and
red) forming crystal-like patterns surrounded by a gas-like cloud of electrons
(yellow and blue). The variation in colour corresponds to each of the quantum
spin states of each electron or hole, but this is not important to the
formation of the crystal structure.
Real-world uses
---------------
Bonitz says it should be possible to meet these criteria in the laboratory,
using a laser to excite a semiconducting material or perhaps by compressing it.
He told New Scientist that such materials could find important real-world
applications: "It could mean superconductivity at higher temperatures."
Superconductivity normally occurs close to absolute zero (-273øC), so it can be
costly and complex to achieve. Some existing technologies already exploit
superconductors, for example magnetic resonance imaging (MRI) scanners and
power generators.
But superconductors that work at less extreme temperatures might also find uses
in computers, antennas and power lines.
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