ld not ordinarily expect the Kondo effect to play a role in wires and contacts made entirely of ferromagnetic metals like iron, cobalt and nickel. Yet that is precisely what co-authors Maria Reyes Calvo and Carlos Untiedt found occurring in experiments in Untiedt's laboratory in Alicante, Spain, in 2008. Calvo, a graduate student, was working with single-atom ferromagnetic contacts that were created by lowering and raising the tip of a scanning tunneling microscope onto a surface.
Untiedt knew that Natelson worked on similar-sized systems that were created in a wholly different way, by laying down metals on a flat surface. So Untiedt arranged for a travel grant from the Spanish government and Natelson agreed to oversee Calvo's recreation of the study at Rice.
"Reyes was a very quick study, and within just a few weeks she had mastered our technique for making single-atom junctions," Natelson said. "She conducted dozens of experiments on junctions made of cobalt and nickel, and we saw the characteristic Kondo effect in the conductance, just as she had seen in Spain."
Co-authors Joaqun Fernndez-Rossier and Juan Jos Palacios, both of the University of Alicante, and David Jacob of Rutgers University. provided a theoretical framework to help explain the unexpected effect. Natelson said the team's discovery is yet another example of the unique types of effects that characterize nanotechnology.
"The fact that this atom is all by itself at the surface is what makes it behave so differently, and it shows that engineers need to be mindful of surface effects in anything they design at this level," Natelson said.
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