Posted May 22, 2003 Atlanta
When the clusters are cooled to below 20 degrees Kelvin, electrical charges in them suddenly shift, creating structures known as dipoles.
"This is very strange, because no metal is supposed to be able to do this," said Walter de Heer, a professor in the School of Physics at the Georgia Institute of Technology and co-author of the paper published on the topic in Science. "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."
This ferroelectric phenomenon has so far been observed in clusters of niobium, vanadium and tantalum - three transition metals that in bulk form become superconducting at about the same temperature that the researchers observe formation of dipoles in the tiny clusters. De Heer believes this discovery will open up a new field of research - and provide clues to the mystery of superconductivity.
In bulk metals -- and even in niobium clusters at room temperature -- electrical charge is normally distributed equally throughout the sample unless an electric field is applied. But in the clusters of up to 200 niobium atoms created by de Heer and collaborators Ramiro Moro, Xiaoshan Xu and Shuangye Yin, that changes when the particles are cooled to less than 20 degrees Kelvin.
The Georgia Tech researchers discovered this "spontaneous symmetry breaking" while searching for signs of superconductivity in the nanometer-scale clusters. It was completely unexpected - and de Heer admits he has no explanation for it.
"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."
For the smallest clusters, the strength of the dipole effect varies dramatically according to size. Clusters composed of 14 atoms display strong effects, while those made up of 15 atoms show little effect. Above 30 atoms, clusters with even numbers of atoms display stronger dipole effects than clusters with odd numbers of atoms.
"Structure matters greatly to this process," de Heer said. "A small change can affect the position of the phase transition rather profoundly, and the exact arrangement of atoms really does matter to these systems."
He attributes the size sensitivity to the quantum size regime, which is related to restrictions on how electrons can move in very small clusters.
De Heer sees strong "circumstantial evidence," but no solid proof, that the phenomenon is connected to superconductivity in these metals.