A tunable two-impurity Kondo system in an atomic point contact

If a magnetic impurity is introduced into a non-magnetic metallic environment, its spin interacts with the conduction electrons of the host. Below a characteristic temperature TK, this leads due to the Kondo effect to a non-magnetic singlet ground state, where the spin of the magnetic impurity is completely screened [1]. As soon as two or more magnetic impurities are in proximity to each other, a competition between Kondo screening and magnetic interaction among the spins sets in and the ground state depends sensitively on their respective magnitude. Many of the peculiar properties of correlated electron materials are attributed to this competition between screening of local spins and magnetic interaction of neighbouring spins. Depending on which interaction dominates, the properties can vary, e.g., between antiferromagnetic ordering and heavy fermion behaviour, between being an insulator or a metal. Certain materials can be tuned continuously between the aforementioned phases through a quantum phase transition (QPT) as a function of an external control parameter, such as doping, pressure or magnetic field. The physics close to the quantum critical point is often described based on models in which the formation of dimers governs the physics. Thus the study of a model system consisting of a dimer, as presented here, with magnetically coupled spins can serve as a test ground for these theories.