Aug. 27, 2009

Superconductivity 'fingerprint' found at higher temperatures

New measurements at Cornell have shown that "high-temperature" superconductors may have the potential to go even higher, offering the possibility of creating room-temperature superconductors, or at least superconductors that will work with conventional refrigeration.

Such materials could lead to far more efficient electric generators, lossless power transmission and other energy-saving applications.

Superconductors conduct electricity with zero resistance, but only when cooled to very low temperatures. Recently developed materials called cuprates, consisting of copper oxide doped with other elements, superconduct up to temperatures as "high" as 150 kelvins (-123 C or -253 F).

In these superconductors electrons join up in pairs that somehow can move through a crystal lattice without bouncing off of atoms and slowing down. Theory and some experiments have suggested that these "Cooper pairs" are still formed in a temperature range up to 1.5 times the critical temperature at which superconductivity occurs.

But previous measurements were indirect and could be explained in other ways, said J.C. Séamus Davis, the J.G. White Distinguished Professor of Physical Sciences at Cornell and a senior scientist at Brookhaven National Laboratory. The new results show a definitive signature, Davis said. The research, a collaboration among Cornell, Brookhaven and scientists in Japan, is reported in the Aug. 28 issue of the journal Science (Vol. 325, Pg. 1099).

"For two decades people have wondered if [the behavior in this range] is related to superconductivity, and we have proved that it is," Davis said. "I think we have demonstrated very clearly that the electrons are in pairs up to a higher temperature."

Davis and his team worked with a cuprate containing bismuth, strontium and calcium that superconducts when cooled below 37 kelvins (-236 C or -393 F). They used an unusually sensitive scanning tunneling microscope (STM) mounted on massive supports to eliminate outside vibration to scan across a cuprate sample in steps smaller than the width of an atom. An STM scans with a probe so sharp that its tip is a single atom, and measures current flowing between the tip and a surface. By measuring current flow under a range of conditions, the researchers could determine the energy states of electrons below the probe.

The experiment offered serious technical challenges. Thermal motion of the atoms introduces noise in the signal, so the researchers chose an "underdoped" cuprate that superconducts at a very low temperature. To further increase the signal-to-noise ratio, measurements were taken very slowly, taking 10 days to scan a 45-nanometer square, possible only because of the extreme stability of the specially built STM. Special care was also taken in the manufacture of the STM's tiny probes.

"We found that the characteristic signature passes unchanged from the superconducting state into the parent state -- up to temperatures of at least 55 K, or 1.5 times the transition temperature," Davis said. But in that range, the scans showed that the physical orientation of the pairs becomes disorganized or "out of phase." Davis likens superconductivity to a group of dancers lined up for a Virginia reel, but carrying basketballs. If each couple tosses a ball to the next in a regular rhythm, the balls can move swiftly along the line. But if the dancers are scattered around the floor, say doing a polka, the balls will go every which way and make little progress.

"After you pass through the critical temperature at which superconductivity is lost you still have the same fingerprint," Davis said. "The theoretical explanation is that the process of making the pairs survives but the process of aligning them periodically is destroyed. If we could understand why these pairs lose their lockstep, that knowledge would be sufficient to engineer materials that keep that and then we would have high-T superconductors."

The research was supported by the U.S. Department of Energy, the U.S. Office of Naval Research, the U.S. Army Research Office, the Japanese Ministry of Science and Education and the Japan Society for the Promotion of Science.