The Kondo effect – an intricate quantum phenomenon involving the spins of many electrons surrounding a magnetic atom – is already quite intriguing by itself. But an even higher level of complexity is reached when two coupled atoms are together Kondo-screened. Depending on the competition between the exchange interaction and the screening strength, combined with an external magnetic field, a variety of different correlated phases can be realised. So far, some of these phases existed only on paper.

Today, Anna Spinelli and coworkers from the Otte Lab show in Nature Communications that the complete phase diagram of the two-impurity Kondo problem in a magnetic field can be covered experimentally. In order to achieve this, a range of different pairs of Co atoms was designed and assembled through atom manipulation. This work forms the basis for advanced experiments on e.g. Kondo chains and Kondo lattices.

Phase diagram of the two-impurity Kondo problem. Horizontal axis: exchange interaction ranging from ferromagnetic (left) to antiferromagnetic (right). Vertical axis: applied magnetic field. The two-impurity Kondo screening phase was not reached experimentally before.