Super-cold metals show 'strange' behaviour

U.S. researchers say the "strange" behavior of the metal niobium at ultra-cold temperatures answers the question, "When is a metal not a metal?" Georgia Tech researchers, writing in the journal Science, report surprising behavior exhibited by nanometer-scale clusters of the metal niobium when they are cooled to below minus 424 degrees Fahrenheit. At those temperatures, electrical charges in the clusters suddenly shift, creating structures known as dipoles. "This is very strange, because no metal is supposed to be able to do this," Georgia Tech physicist Walter de Heer said. "These clusters become spontaneously polarized, with electrons moving to one side of the cluster for no apparent reason. "One side of each cluster becomes negatively charged, and the other side becomes positively charged. The clusters lock into that behavior and stay that way." The phenomenon has been observed in clusters of niobium, vanadium and tantalum, three transition metals that become superconductors at the same extremely cold temperatures, de Heer said. The Georgia Tech researchers discovered it while searching for signs of superconductivity in the nanometer-scale clusters. It was completely unexpected, de Heer admits. "When this happens, these particles that are made out of metal atoms no longer behave as if they were metallic," he said. "Something changes the particles from a metal into something else." The dipole discovery could open up a new field of research and provide clues to the mystery of superconductivity, he said.