Radiotracer Diffusion Measurements of Isotope Motion in a Metal Alloy above the Glass-Transition Temperature Support Mod
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1 µm wavelength, and an expected band narrowing of a factor of 10. ROSALIA SERNA
Nanoclusters of Niobium Display Nonmetallic Properties at Ultracold Temperatures While searching for signs of superconductivity in nanometer-scale clusters of the metal niobium, researchers at the Georgia Institute of Technology found that the material stops behaving as a metal when the clusters—of up to 200 niobium atoms—are cooled to low temperature. The electrical charges in the clusters suddenly shift, forming dipoles, as the temperature is lowered below a transition temperature that depends on cluster size. “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. “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.” In bulk metals—including niobium clusters at room temperature—electrical charge is normally distributed equally throughout
the sample unless an electric field is applied to them. 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 provide insights into superconductivity. For the smallest clusters, as reported in the May 23 issue of Science by de Heer and collaborators R. Moro, X. Xu, and S. Yin, 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. De Heer attributes the size sensitivity to the quantum size regime, which is related to restrictions on how electrons can move in very small clusters. To produce and study the clusters, the researchers use a custom-built apparatus that includes a detector able to count and characterize several million particles per hour. First, a laser beam is used to vaporize the niobium, creating a cloud of metal-
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