Helping Out A High-temperature Superconductor

“Intheory, superconducting materials can conduct an enormous amount ofelectric current. But when incorporated into actual devices, certainfactors tend to limit the current,” said Brookhaven materials scientistQiang Li, a co-author on the paper. “We studied these factors and foundthat one, which we call ‘substrate roughness,’ can actuallysignificantly increase the current-carrying capacity.”

Thesuperconducting material studied here consists of the elements yttrium,barium, copper, and oxygen. Dubbed YBCO, it is a member of a class ofcopper- and oxygen-containing superconductors called “cuprates.”Cuprates are “high-temperature” superconductors because theysuperconduct at temperatures much “warmer” than conventionalsuperconductors (although still very cold) — for example, -300°F ratherthan -440°F. This difference, while not huge, is enough to makecuprates more viable for practical applications than materials thatmust be kept much colder.

In many of these applications, YBCOfilms are deposited onto a ‘normal’ metal surface (the “substrate”),forming components known as coated conductors. One of the factorswidely thought to degrade the performance of coated conductors is theroughness of the metal surface.

To verify this, Li and hiscolleagues set out to study and measure how the roughness of thesubstrate affects the current-carrying capacity of YBCO.

Theresearchers deposited a YBCO layer onto a substrate prepared with twodistinct areas: a rough, corrugated region with nanometer(billionth-of-a-meter) sized ridges and grooves, and a smooth region.This configuration allowed the group to directly compare the behaviorof the YBCO film on both surface types. They were able to do this usingelectrical-transport measurement techniques, which track the amount ofsupercurrent passing through the material, and “magneto-optical”imaging, a technique used to study superconductors by following theirmagnetic behavior.

“What we found is remarkable and surprising,”said lead author Zuxin Ye, a graduate student under Li’s supervision.“Rather than limiting the current, the nanoscaled corrugated surfaceproduces more than a 30 percent increase in the supercurrent carried bythe YBCO films. This suggests that metal substrates with some degree ofroughness at the nanoscale might help improve the performance ofhigh-temperature superconductors.”

The work is the result of acollaboration between scientists in Brookhaven Lab’s Materials ScienceDepartment, the Condensed Matter Physics group within the PhysicsDepartment, and the Lab’s Center for Functional Nanomaterials. It wassupported by the Office of Basic Energy Sciences within the U.S.Department of Energy’s Office of Science.