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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 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.
At Brookhaven's National Synchrotron Light Source
and other facilities on site, Brookhaven physicist Christopher 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?
Studies conducted by Brookhaven physicist John
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.
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. |
