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Unlocking the Secrets of High-Temperature
Superconductors
Denver, CO, 7 March 2007: Although
it was discovered more than 20 years ago, a particular type of high-temperature
(Tc) superconductor -- material that conducts electricity with almost zero
resistance -- is regaining the attention of scientists at the U.S. Department of
Energy's Brookhaven National Laboratory.
Copper-oxide compounds, called cuprates, operate at temperatures warmer than
traditional superconductors but still far below freezing. Understanding the
mechanism for these superconductors may one day help scientists design
superconductors able to function closer to room temperature for applications
such as more-efficient power transmission.
Discovered in 1986, the most perplexing of
these cuprate superconductors is "LBCO," named for the elements it contains:
lanthanum, barium, copper, and oxygen. After years of research on similar
materials, Brookhaven researchers have learned how to "grow" better samples of
LBCO, which has allowed for extensive studies on its intriguing properties.
Three Brookhaven physicists will discuss their most recent findings about LBCO
at the March 2007 meeting of the American Physical Society. The details of their
research are highlighted below.
A SUPERCONDUCTOR WITH INSULATING
PROPERTIES
One of the most perplexing findings
involving LBCO is that the high-temperature superconductor actually has distinct
insulating-like properties. Each barium atom has one fewer electron than
lanthanum, so increasing barium adds electron "holes," or the absence of
electrons, to the system. The more barium that is "doped" into the material, the
more holes, and the greater the superconductivity -- until the composition
reaches a point where there is exactly one barium atom for every eight copper
atoms, a state known as the 1/8 doping. Then, oddly, the superconductivity
disappears. Above this point, as more holes (barium atoms) are added, the
superconductivity reappears.
Brookhaven physicist Christopher Homes
discussed this odd phenomenon on March 5, 2007, at the Colorado Convention
Center.
At Brookhaven's National Synchrotron Light
Source and other facilities on site, Homes investigates LBCO's electronic
properties by shining various types of light onto an LBCO crystal and measuring
the intensity that is reflected back. This optical picture tells scientists
about the behavior of the charge carriers -- or holes -- in LBCO. Most materials
have a set number of carriers that scientists can count using these methods. As
a material becomes a superconductor, some of the holes lower their energy by
falling into a superconducting state that allows them to flow without
resistance. As these carriers condense, there is a characteristic change in the
optical conductivity. However, even though LBCO is not a superconductor at the
1/8 doping, the number of holes still decreases at low temperature. Homes and
other researchers attribute this feature to the formation of the so-called
"energy gap." In semiconductors, the charge gap blocks the flow of current
because of its isotropic nature (the gap spreads evenly in all directions).
Superconductors also have energy gaps, but in the cuprates these gaps have
different energies in different directions with respect to the copper-oxygen
chemical bonds.
"The more we look at this charge gap, the
more it looks like a superconducting gap," Homes said. "It has the same
magnitude, the same shape and symmetry. Yet, it doesn't have superconductivity."
Homes and other BNL researchers continue to tackle this mysterious problem in
order to understand why a material that wants to be a superconductor is behaving
like an insulator.
LOOKING FOR "STRIPES" IN HIGH-TC
SUPERCONDUCTORS
In LBCO, as in all materials, 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 costs
energy. 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. As mentioned above, scientists can 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
question is: How do these holes arrange themselves?
Brookhaven physicist John Tranquada
answered that question during his talk about superconducting "stripes" on March
5, 2007, at the Colorado Convention Center.
Studies conducted by Tranquada and other
Brookhaven researchers support the controversial theory that the holes segregate
themselves into stripes that alternate with antiferromagnetic regions in the
material.
"There's a lot of excitement in trying to
understand why these materials are superconducting, and there's plenty of
controversy surrounding it," Tranquada said.
Most recently, Tranquada's research group
examined the effect of the stripes on vibrations in the crystal lattice. Lattice
vibrations play a role in pairing up the electrons that carry current in
conventional superconductors. At the Laboratorie Leon Brillouin, Saclay, in
France, 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
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.
The scientists observed this anomalous
signature most clearly in samples with observable stripe order, but they also
saw it in samples of good superconductors without static stripes. This indicates
the presence of dynamic stripes -- meaning that the stripes can wiggle through
the crystal lattice -- and suggests that stripes might be important in the
mechanism for high-Tc superconductivity, Tranquada said.
PAVING THE WAY FOR CRYSTAL GROWTH
In order to study the properties of LBCO
superconductors, scientists need to produce large, single crystals of the
material -- a difficult task that wasn't possible until recently. At the
state-of-the-art crystal growth facility in Brookhaven's physics building,
physicist Genda Gu and his colleagues have perfected the process.
Gu discussed his crystal growth method
today at the Colorado Convention Center.
The crystals are grown in an infrared
image furnace, a machine with two mirrors that focuses infrared light onto a
feed rod, heating it to about 2,200 degrees Celsius (3,992 degrees Fahrenheit)
and causing it to melt. Under just the right conditions, Gu and his colleagues
can make the liquefied material recrystallize as a single uniform crystal. At
present, the most interesting form of LBCO has one barium atom for every eight
copper atoms, or a 1/8 "doping," at which point the material loses its
superconductivity. Achieving this high barium concentration is extremely
difficult and is the reason many scientists previously opted to use different
but related materials for their research on superconducting stripes and other
properties, Gu said.
"LBCO was the first high-temperature
superconductor discovered, but everyone switched over to studying other
materials for a while because they weren't able to grow single crystals with a
concentration of barium greater than 11 percent," Gu said. "Now, we can study
the whole class of high-Tc materials."
Each crystal takes about a month to make,
with precise control over growth temperature, atmosphere, and other factors.
Brookhaven is currently capable of making crystals with barium concentrations up
to 16.5 percent, a world record, Gu said.
The research conducted by Homes, Tranquada,
and Gu is funded by the Office of Basic Energy Sciences within the U.S.
Department of Energy's Office of Science.
ABOUT BROOKHAVEN LAB
One of ten national laboratories overseen
and primarily funded by the Office of Science of the U.S. Department of Energy
(DOE), Brookhaven National Laboratory conducts research in the physical,
biomedical, and environmental sciences, as well as in energy technologies and
national security. Brookhaven Lab also builds and operates major scientific
facilities available to university, industry and government researchers.
Brookhaven is operated and managed for DOE's Office of Science by Brookhaven
Science Associates, a limited-liability company founded by the Research
Foundation of State University of New York on behalf of Stony Brook University,
the largest academic user of Laboratory facilities, and Battelle, a nonprofit,
applied science and technology organization. Visit Brookhaven Lab's electronic
newsroom for links, news archives, graphics, and more:
http://www.bnl.gov/newsroom.
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