PHYSICS NEWS UPDATE --- Number 831 July 5, 2007 by Phillip F. Schewe, Ben Stein
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Thu, 05 Jul 2007 22:05:32 GMT
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 831 July 5, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu
ARE DOUBLY CHARGED PARTICLES LURKING IN HIGH-T SUPERCONDUCTORS?. One
of the greatest unsolved problems in condensed matter physics is
explaining how electrons pair up in the copper-oxide materials that
superconduct at temperatures above 100 K. Some theorists believe that
the place to start in straightening out this mystery is to understand
better how the cuprates behave at normal temperatures, long before
they become superconducting. University of Illinois physicist Philip
Phillips suggests that the solution might be the existence of a
previously overlooked doubly charged particle, one that mediates
interactions among electrons lying in planes filled with copper and
oxygen atoms. This particle would be distinct from a Cooper pair, the
charge carrier in a superconductor. The new particle would be a boson
that carries twice the charge of an electron, but is not made out of
elementary excitations. Nonetheless, it emerges from the strong
repulsions among the electrons and persists above and below the
superconducting transition temperature. It is ironic, and revealing,
that the cuprates (in their undoped state) are Mott insulators. In
ordinary insulators every possible electron state is filled (with two
electrons of opposite spin orientation). Under these circumstances, no
electrical current is possible and the material is insulating. In a
Mott insulator things are rather counterintuitive. Only half the
electronic states are occupied but still no electrical current flows
(http://www.aip.org/pnu/2003/split/645-2.html). This state of affairs
comes about because strong electron repulsions prevent any electron
motion. When extra electrons or holes are introduced into a Mott
insulator through the addition of dopant atoms, Mott insulators change
drastically. One change is that the allowed energy bands in the
material do not remain static, as in a semiconductor. This lack of
rigidity of the energy bands facilitates the appearance of the new
particle, says Philips (philip@xxxxxxxxxxxxxxxxxxxxxxxx). But what
kind of collective excitation is this? Concentrate for the moment on
the electrons in the sample. Semiconductors and most materials obey
the standard counting principle that the removal of an electron leaves
behind one empty state. In a doped Mott insulator, by contrast, each
hole leaves behind two empty states. This indicates that the electron
that is removed ultimately did not reside in a single electronic state
but must have been in a superposition of two states. The question is,
how does one describe the extra state. This question has now been
answered by a new theory by Philips and his Illinois colleagues (Leigh
et al., Physical Review Letters, upcoming article). Some experimental
results support this theory (see Graf et al., Phys Rev Lett, 9
February 2007 ). The Illinois work shows that the proposed charge-2e
particle binds to the hole and produces the missing state. Philips
believes that this particle is responsible for the normal state of the
cuprates, including the odd *pseudogap state,* the condition in which
some electrons in the material seem to be paired even at temperatures
above where superconductivity sets in.
OPTICAL FERRIS WHEEL. A new form of optical lattice, a ring-shaped
lattice which spins about, has been planned by physicists at the
University of Glasgow and the University of Strathclyde. In an optical
lattice a web of laser beams can hold a collection of atoms in place
in free space. If the frequencies of the two holographically generated
laser beams are different, the resultant lattice can be spun about. In
fact the laser pattern is created in this case through the use of a
hologram. The trapped atoms can reside in either discrete
lozenge-shaped parcels (in some cases positioned in the dark regions
that result from the interference of laser beams) or spread out over a
continuous ring shape (see movie at
http://www.physics.gla.ac.uk/Optics/projects/AM/). One goal of
pinioning atoms in a light-free dark zone is to reduce unwanted
warming of the atoms, which need to be ultracold in order to carry out
fundamental tests of interatomic forces. Furthermore, as theorists are
very interested in studying atoms lodged in infinitely long
one-dimensional strings, and since such strings are difficult to
create experimentally, the next best thing is bend the string around
on itself in the shape of a ring; hence the motivation for producing a
ring optical lattice. The Scottish haven*t yet installed atoms into
their ring but according to Sonja Franke-Arnold
(s.franke-arnold@xxxxxxxxxxxxxxxxx), she and her colleagues at
Strathclyde plan soon to inject a Bose-Einstein condensate (BEC) of
rubidium atoms. (Franke-Arnold et al., Optics Express, 9 July 2007;
the journal is public access and the text can be obtained at
http://www.opticsexpress.org/abstract.cfm?id=138976)
***********
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