Superconductor with Magnetic Traits Discovered

Knoxville — A collaboration of German, Japanese and University of Tennessee physicists have discovered magnetic properties in a substance that may force theorists to rethink the nature of superconductors.

The surface of a superconducting material called strontium ruthenate becomes magnetic when a crystal of it is broken apart, said Dr. Ward Plummer, a condensed-matter physicist at UT and Oak Ridge National Laboratory who was a principal in the team.

“This development and this class of materials totally challenge all the assumptions about condensed materials,” Plummer said. The findings are published in a recent issue of Science.

Plummer worked with Dr. Rene Matzdorf, a Humboldt Fellow who came to UT, and Drs. Yoshi Tokura and Kiyo Terakura, who are at the Joint Research Center for Atom Technologies in Tsukuba, Japan, and at the University of Tokyo.

High-temperature superconductors are materials, usually containing copper, that conduct electricity with very little resistance. Previously, scientists had assumed that magnetism and superconductivity would not coexist because a magnetic field would create a current in the superconductor which would cancel out the magnetism.

The researchers worked on strontium ruthenate, a compound of strontium, ruthenium and oxygen, with no copper. They found that, when a crystal of the substance is broken open, the surfaces of the compound become increasingly magnetic as temperature drops. The substance was studied at 50 degrees Kelvin, or about 370 F. below zero.

“Ruthenium shouldn’t be magnetic under normal circumstances,” Plummer said. “We found that the surface is structurally different from the bulk of the material.”
He said the discovery might foreshadow the creation of a switch that would allow a superconductor to be switched on or off with a magnetic field, but that development might take years to perfect.

“I don’t expect any great devices to come out in my scientific lifetime,” he said. “At face value, this is an intellectual curiosity, but it’s also a good test vehicle for theoretical physicists.”

Scientists Zhong Fang and Tsuyoshi Kimura from the research center collaborated on the work. In the United States, Ismail from ORNL and Jiandi Zhang from Florida International University were co-investigators.