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Nanoscale imaging reveals
unexpected behaviors in high-temperature superconductors
Princeton, NJ, 30 May 2007: Recent
discoveries regarding the physics of ceramic superconductors may help improve
scientists' understanding of resistance-free electrical power.
Tiny, isolated patches of superconductivity exist
within these substances at higher temperatures than previously were known,
according to a paper by
Princeton
scientists, who have developed new techniques to image superconducting behavior
at the nanoscale.
Superconductivity, the ability to carry
electrical current without resistance, could revolutionize electrical power
transmission if the property ever appeared in a material at close to room
temperature. Even the so-called high-temperature ceramic superconductors
discovered two decades ago must be cooled to more than minus 100 degrees Celsius
to function.
Using a special customized microscope, the
Princeton team has discovered that traces of superconductivity remain present
inside these ceramic materials even when they are warmed up above the critical
temperature where they lose their resistance. Though the entire sample is too
warm to exhibit superconductivity, disconnected regions within it possess Cooper
pairs -- the coupled electrons that carry current through a superconductor --
which previously were only known to appear below the critical temperature at
which a material superconducts.
The regions are only a few nanometers wide, but
they appear in some materials at up to 50 degrees above the critical
temperature. Ali Yazdani, senior author of the research paper, said that
understanding why these minuscule patches of superconductivity exist at higher
temperatures -- and how to create a material that exhibits the property
everywhere -- may be the key to enhancing superconductivity.
"Our measurements show that Cooper pairs survive
in local patches of the material at temperatures far above the critical
temperature," said Yazdani, a professor of physics at Princeton. "Within these
tiny regions, there are particular arrangements of atoms that favor formation of
electron pairs at very high temperatures. These patches are a precursor to
superconductivity and important to enhancing it."
The paper appears in the May 31 edition of
Nature. Other members of the research group are Princeton graduate students
Kenjiro Gomes and Aakash Pushp and postdoctoral fellow Abhay Pasupathy, as well
as Shimpei Ono and Yoichi Ando of the Central Research Institute of Electric
Power Industry in Tokyo.
For more than two decades, scientists have worked
to explain and enhance the performance of copper-oxide based ceramics, which two
decades ago were discovered to superconduct at temperatures far warmer than any
other known materials -- though still requiring temperatures that are quite
chilly by human standards. High-temperature superconductivity in ceramics has
defied a widely accepted explanation and is considered one of the major puzzles
in physics.
The key to the puzzle is to determine how
electrons, which are negatively charged and normally repel one another,
mysteriously change their attitude toward each other and form Cooper pairs.
Below the critical temperature, the pairs form everywhere in a material, and can
then act in concert as a "superfluid" to carry electric current through it
without resistance.
"In lower temperature superconductors, electrons
pair up and form a superfluid at the critical temperature -- end of story,"
Yazdani said. "In ceramics, however, our team is finding that electron pairing
occurs over a wide range of temperatures, and their pairing is a function of
highly localized chemistry in the sample, often in patches only a few atoms
wide."
Investigation on this tiny scale was made
possible by a state-of-the-art scanning tunneling microscope the Princeton team
designed especially to map superconducting properties on the scale of single
atoms while they changed the temperature. The team was able to apply their
technique systematically to a large number of high quality copper-oxide
superconducting samples.
Unlike an optical microscope that uses light to
magnify, the scanning tunneling microscope uses a beam of electrons from a sharp
tip to image the sample. The beam served a double purpose for the experiments:
Not only does it provide images of a sample down to scales of just a few atoms
wide, the beam also is capable of breaking apart electron pairs if it is
energetic enough. By varying the energy of the electron beam, the team was able
to determine whether pairs had formed in a given spot within the material.
"We spent about two and a half years looking at
many different samples at different temperatures to decipher the story," Yazdani
said. "We were motivated to search for pairing at high temperatures because of
the work of others, most notably that of my colleague Phuan Ong."
The researchers hope to use their experimental
results to shed light on what controls the pairing temperature on the atomic
scale in ceramic superconductors, and also to determine what limits the Cooper
pairs' ability to get their act together to superconduct.
"This type of precision experiment performed
while varying temperature gives us a new window into the complex problem of
ceramic superconductors," Yazdani said. "If we can figure out the details of
what is happening at these local patches within the samples, it might be
possible to construct a material that performs better overall."
Such an accomplishment might revolutionize
technology for the power industry, said Mike Norman, a physicist in Argonne
National Laboratory's Materials Science Division, who was not affiliated with
the research.
"If we could raise the critical temperature by
making the sample more homogeneous, then superconductivity's application to
day-to-day technologies, such as power grids, becomes much more realistic,"
Norman said. "The nice thing with superconductors is that there is no power
loss, so they could be a major player in 'green' and 'efficient' technologies
for power transmission."
###
The National Science Foundation funded this work
through a grant from its Division of Materials Research and its support of the
Princeton Center for Complex Materials through its program for Materials
Research Science and Engineering Centers.
Abstract
Visualizing Pair Formation on the Atomic Scale in the High-Tc Superconductor
Bi2Sr2CaCu2O8+δ
By Kenjiro K. Gomes, Abhay N. Pasupathy, Aakash Pushp, Shimpei Ono, Yoichi Ando
and Ali Yazdani
Pairing of electrons in conventional
superconductors occurs at the superconducting transition temperature, Tc,
creating an energy gap Δ in the electronic density of states (DOS). In the
high-Tc superconductors, a partial gap in the DOS exists below a temperature T*
> Tc. A key question is whether the gap in the DOS above Tc is associated with
pairing, and what determines the temperature at which incoherent pairs form.
Here we report the first spatially resolved measurements of gap formation in a
high-Tc superconductor, measured on Bi2Sr2CaCu2O8+δ samples with different Tc’s
(hole concentration x = 0.12 to 0.22) using scanning tunneling microscopy (STM).
Over a wide range of doping (0.16 ≤ x ≤ 0.22) we find that pairing gaps
nucleate in nanoscale regions above Tc. These regions proliferate as the
temperature is lowered, resulting in a spatial distribution of gap sizes Δ in
the superconducting state. Despite the inhomogeneity, we find that every pairing
gap Δ develops locally at a temperature Tp, following the relation 2Δ/kBTp =
7.9±0.5. At very low doping (x ≤ 0.14), systematic changes in the DOS
indicate the presence of another phenomenon, which is unrelated and perhaps
competes with electron pairing. Our observation of nanometer-sized pairing
regions provides the missing microscopic basis for understanding recent reports
of fluctuating superconducting response above Tc in hole-doped high-Tc cuprate
superconductors.
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