The two-impurity Kondo problem

(Nanowerk News) Two magnetic atoms, one attached to the tip of a scanning tunnelling microscope and one adsorbed on a metal surface, each constituting a Kondo system, have been proposed as one of the simplest conceivable systems potentially exhibiting quantum critical behaviour.
Kondo effect
The group of Prof. Kern Klaus (LSEN - Laboratory of Nanoscale Science) at EPFL, in collaboration with researchers from France, Denmark, Hungary, Germiny and China, have succeeded in implementing this concept experimentally for cobalt dimers clamped between a scanning tunnelling microscope tip and a gold surface ("A tunable two-impurity Kondo system in an atomic point contact").
Control of the tip–sample distance with subpicometre resolution enables them to tune the interaction between the two cobalt atoms with unprecedented precision. Electronic transport measurements on this two-impurity Kondo system revealed a rich physical scenario, which is governed by a crossover from local Kondo screening to non-local singlet formation due to antiferromagnetic coupling as a function of separation of the cobalt atoms.
Source: EPFL