Two-Site Kondo Effect in Atomic Chains
  Linear CoCu_nCo clusters on Cu(111) are fabricated by means of atomic manipulation. They represent a two-site Kondo system with tunable interaction. Scanning tunneling spectroscopy reveals oscillations of the Kondo temperature T_K with the number n of Cu atoms for n>=3. Density functional calculations show that the Ruderman-Kittel-Kasuya-Yosida interaction mediated by the Cu chains causes the oscillations. Calculations find ferromagnetic and antiferromagnetic interaction for n=1 and 2, respectively. Both interactions lead to a decrease of T_K as experimentally observed. 
N. N\'eel, R. Berndt, J. Kr\"oger, T. O. Wehling, A. I. Lichtenstein,   M. I. Katsnelson
cond-mat.mtrl-sci