The two-impurity Kondo problem.

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. The group of Prof. Kern Klaus (LSEN - Laboratory of Nanoscale Science), 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. 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.

Jakob Bork et al., Nature Physics, doi:10.1038/nphys2076 (2011)