New
Release -- Superconductor Week does not edit or endorse the following
news release:
More evidence for 'stripes' in
high-temperature superconductors
Upton, NY, Apr. 26: An international
collaboration including two physicists from the U.S. Department of Energy's
Brookhaven National Laboratory
has published additional evidence to support the existence of "stripes" in
high-temperature (Tc) superconductors. The report in the April 27, 2006, issue
of Nature strengthens earlier claims that such stripes -- a particular spatial
arrangement of electrical charges -- might somehow contribute to the mechanism
by which these materials carry current with no resistance. Understanding the
mechanism for high-Tc superconductors, which operate at temperatures warmer than
traditional superconductors but still far below freezing, may one day help
scientists design superconductors able to function closer to room temperature
for applications such as more-efficient power transmission.
In the material the scientists studied, as in all
materials, the atoms' negatively charged electrons repel one another. But by
trying to stay as far apart as possible, each individual electron is confined to
a limited space, which makes the electrons "unhappy" in the sense that it costs
energy. "It's like putting a bunch of claustrophobics into a crowded room," says
Brookhaven physicist John Tranquada, who leads the Lab's role in this work.
To achieve a lower-energy state, the electrons
arrange themselves with their spins aligned in alternating directions on
adjacent atoms, a configuration known as antiferromagnetic order. Through
chemical substitutions, the scientists can effectively "dope" the material with
electron "holes," or the absence of electrons, to allow the electrons/holes to
move more freely and carry current as a superconductor.
The big question is: How do those electrons/holes
arrange themselves?
"Our earlier research suggests that the holes
segregate themselves into stripes that alternate with antiferromagnetic
regions," Tranquada says. Their conclusion is based on observing a similar
magnetic signature in a well-known high-Tc superconductor and a material known
to have such charge-segregated stripes. Ironically, the stripes in the latter
material are observable only at a particular level of doping where the material
loses its superconductivity. But because the magnetic spectra were so similar,
Tranquada says, "We inferred that the stripes might also be present in the
superconducting materials, just more fluid, or dynamic -- and harder to
observe."
Since then, Tranquada's group has been looking
for additional experimental signatures to back up their controversial claim. In
the current experiment, they examined the effect of the stripes on vibrations in
the crystal lattice. Lattice vibrations, or phonons, are known to play a role in
pairing up the electrons that carry current in conventional superconductors.
At the Laboratorie Leon Brillouin, Saclay, in
France, the researchers bombarded samples of superconducting materials and the
same stripe-ordered non-superconductor with beams of neutrons and measured how
the beams scattered. Comparing the energy and momentum of the incoming beams
with those scattered by the samples gives the scientists a measure of how much
energy and momentum is transferred to the lattice vibrations.
Each of these vibrations, like a vibrating guitar
string, normally has a particular, well-defined frequency for a given
wavelength. But in the superconductor experiment, at a particular wavelength,
the scientists observed an anomaly: a wider range of frequencies in the lattice
vibrations.
"It's as if a musician were able to make a single
guitar string produce a chord," Tranquada says.
The scientists observed this anomalous signature
most clearly in samples with observable stripe order -- that is, the special
material that loses its superconductivity with a particular level of doping. But
they also saw it in samples of good superconductors.
"Seeing this feature in both stripe-ordered
samples and in good superconductors without static stripes leads us to believe
that the signature is indicating the presence of dynamic stripes," Tranquada
says.
"This result suggests that stripes are common to
copper-oxide superconductors and may be important in the mechanism for high-Tc
superconductivity," he adds. To further support their case, Tranquada notes that
the anomalous signature goes away in cases where the superconducting material is
either under- or over-doped. In this case, the material no longer acts as a
superconductor, and may no longer have stripes, he says.
Return
to industry news releases |
