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news release:
New insights into high-temperature
superconductors
Washington, DC, 26 February 2007:
Scientists at the Carnegie Institution's
Geophysical Laboratory in
collaboration with a physicist at the
Chinese University
of Hong Kong have discovered that two different physical parameters —pressure
and the substitution of different isotopes of oxygen (isotopes are different
forms of an element) —have a similar effect on electronic properties of
mysterious materials called high-temperature superconductors. The results also
suggest that vibrations (called phonons), within the lattice structure of these
materials, are essential to their superconductivity by binding electrons in
pairs. The research is published in the February 26 - March 2
on-line
edition of the Proceedings of the National Academy of Sciences.
Superconductors are substances that conduct
electricity — the flow of electrons — without any resistance. Electrical
resistance disappears in superconductors at specific, so-called, transition
temperatures, Tc's. The early conventional superconductors had to be cooled to
extremely low (below 20 K or –253ºC) temperatures for electricity to flow
freely. In 1986 scientists discovered a class of high-temperature
superconductors made of ceramic copper oxides that have much higher transition
temperatures. But understanding how they work and thus how they can be
manipulated has been surprisingly hard.
As Carnegie's Xiao-Jia Chen, lead author of the
study explains: "High-temperature superconductors consist of copper and oxygen
atoms in a layered structure. Scientists have been trying hard to determine the
properties that affect their transition temperatures since 1987. In this study,
we found that by substituting oxygen-16 with its heavier sibling oxygen-18, the
transition temperature changes; such a substitution is known as the isotope
effect. The different masses of the isotopes cause a change in lattice
vibrations and hence the binding force that enables pairs of electrons to travel
through the material without resistance. Even more exciting is our discovery
that manipulating the compression of the crystalline lattice of the high-Tc
material has a similar effect on the superconducting transition temperature. Our
study revealed that pressure and the isotope effect have equivalent roles on the
transition temperature in cuprate superconductors."
Superconducting materials can achieve their
maximum transition temperatures at a specific amount of "doping," which is
simply the addition of charged particles (negatively charged electrons or
positively charged holes). Both the transition temperature and isotope effect
critically depend on the doping level. For optimally doped materials, the higher
the maximum transition temperature is, the smaller the isotope effect is.
Understanding this behavior is very challenging.
The Carnegie / Hong Kong collaboration found that if phonons are at work, they
would account both for the magnitude of the isotope effect, as a function of the
doping level, and the variation among different types of cuprate
superconductors. The study also revealed what might be happening to modify the
electronic structures among various optimally doped materials to cause the
variation of the superconducting properties. The suite of results presents a
unified picture for the oxygen isotope effect in cuprates at ambient condition
and under high pressure.
"Although we've known for some time that
vibrations of the atoms, or phonons, propel electrons through conventional
superconductors, they have just recently been suspected to be at work in
high-temperature superconductors," commented coauthor Viktor Struzhkin. "This
research suggests that lattice vibrations are important to the way the high-Tc
materials function as well. We are very excited by the possibilities arising
from these findings."
This work was supported by the Office of Basic
Energy Science and National Nuclear Security Administration of the US Department
of Energy and the Hong Kong Research Grants Council. This research was conducted
by X. J. Chen, V. V. Struzhkin, Z. G. Wu, R. J. Hemley, and H. K. Mao (Carnegie
Institution); and H. Q. Lin (The Chinese University of Hong Kong).
The Carnegie Institution of Washington (www.carnegieinstitution.org),
a private nonprofit organization, has been a pioneering force in basic
scientific research since 1902. It has six research departments: the Geophysical
Laboratory and the Department of Terrestrial Magnetism, both located in
Washington, D.C.; The Observatories, in Pasadena, California, and Chile; the
Department of Plant Biology and the Department of Global Ecology, in Stanford,
California; and the Department of Embryology, in Baltimore, Maryland.
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