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January 3, 2003
Contact: Pete Genzer, 631 344-3174
Note: this is an informational posting only, not a Brookhaven press release.

New Insights Into Superconducting Copper-Oxide Compounds

Working in the field of high-temperature superconductors, researchers from Brookhaven National Laboratory, Princeton University, and several institutions in Japan have determined the upper range of magnetic field at which copper-oxide compounds can be superconducting. Their results are reported in the January 3, 2003, issue of Science magazine. (A link to the paper is here.)

Superconductivity can be destroyed by applying a sufficiently large magnetic field. The size of the field that is required to destroy the superconductivity depends, at least in part, on the strength of the pairing between the charge carriers in the superconducting state. In copper-oxide compounds, the magnetic field required to destroy the superconductivity is generally too large to measure - until now.

Using facilities at the National High Magnetic Field Lab at Florida State University, the research team analyzed specially grown single copper-oxide crystals to determine the critical magnetic field. This is the first time that a systematic study has been done of the upper critical field relative to carrier density. The team concluded that the strength of the pairing is quite large when the carrier density is small, and it decreases as the carrier density increases.

For samples with a low carrier density, the temperature at which the sample becomes superconducting is relatively low. In conventional superconductors, such as the niobium and niobium-tin used to form the magnets for magnetic resonance imaging (MRI) devices, the superconducting transition temperature is limited by the pairing strength. In contrast, the new results indicate that some other effect must control the transition temperature in these copper-oxide superconductors, since pairing strength alone would allow a much higher transition temperature.

To understand the new effect, one must understand that the paired charge carriers behave, because of quantum mechanics, like waves. In order for the charge carriers to form a superconducting state, their vibrations must be synchronized, or "in phase." When the carrier density is low, the transition temperature can be limited by fluctuations in the phases of the electronic waves. This limitation on superconductivity due to "phase fluctuations" was first suggested theoretically by the late Victor Emery at Brookhaven and Steven Kivelson of the University of California, Los Angeles, in 1995.

The fact that the superconductivity in copper-oxide compounds can survive in high magnetic fields makes these materials highly desirable for building new high-field magnets for use in applications such as medical imaging devices and particle accelerators. The challenge is to form the long, continuous, bendable wires required to build a magnet out of these brittle ceramic compounds. The search for ways to overcome this challenge is being actively pursued by many groups around the world.