June 7, 1997
by I. Peterson
How materials known as high-temperature superconductors can carry electric current without resistance has baffled scientists ever since the discovery of these compounds about a decade ago. Now, researchers have obtained an important clue in the form of experimental evidence strongly favoring an unconventional explanation of superconductivity in thallium barium copper oxide. John R. Kirtley and C.C. Tsuei of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., and their coworkers report their results in the May 29 Nature. |
 Scanning magnetometer image shows a sinlge magnetic vortex trapped at a point in the superconducting film where two differently oriented crystals meet. The peak's characteristics indicate the presence of d-wave symmetry. |
In conventional, low-temperature superconductors like niobium, electrons overcome their mutual repulsion and pair up to pass unhindered through the host material. This pairing is facilitated by vibrations of the material's crystal lattice.
Quantum theory describes the pair by means of a single wave function, which mathematically specifies the probability distribution showing where the two electrons are most likely to be. In a conventional superconductor, the electrons' wave function is spherical, indicating that a pair has an equal chance of moving in any direction. Such a pairing is said to display s-wave symmetry.
In copper oxide superconductors, lattice vibrations alone are not strong enough to maintain the necessary electron pairing at elevated temperatures. Some theorists have proposed that magnetic interactions between the electrons and copper atoms play a key role in forging electron pairs.
In this case, an electron pair would instead have a wave function with d-wave symmetry, resembling a four-leaf clover that has its lobes aligned along the crystal axes.
Researchers performed a number of experiments aimed at detecting d-wave pairing. The results pointed to the presence of d-wave symmetry, but they couldn't unambiguously rule out an additional contribution from s-wave pairing (SN: 3/9/96, p. 156).
Kirtley and his coworkers looked for d-wave pairing in a thin film of a thallium barium copper oxide that has a crystal structure known as tetragonal, which is difficult to make but highly symmetrical.
In particular, the crystal geometry requires that the electron pairing be either s-wave or d-wave. "It can't be a combination of the two," Kirtley says.
The results indicate that electron pairs in thallium barium copper oxide display d-wave symmetry. "This is the first time that an experiment has shown that s-wave behavior in electrons is not critical to high-temperature superconductivity," says Jui H. Wang of the State University of New York at Buffalo, a member of the team that fabricated the material.
The identification of a superconductor displaying pure d-wave symmetry serves as a starting point for understanding the more complicated, mixed states that appear to characterize other high-temperature superconductors.
Tsuei, C.C., et al. 1997. Pure d(x^2 - y^2) order-parameter symmetry in the tetragonal superconductor Tl2Ba2CuO6. Nature 387(May 29):481.
Buchanan, M., and B. Daviss. 1997. The heat is on. New Scientist (May 3):26.
Peterson, I. 1996. Electron pairs and waves. Science News 149(March 9):156.
John R. Kirtley
IBM T.J. Watson Research Center
P.O. Box 218
Yorktown Heights, NY 10598
J.H. Wang
Superconducting Materials Laboratory
State University of New York
Buffalo, NY 14260
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