Quantum impurity systems comprise a spin, an atom or ion, or a single molecule on a surface or in a bulk material that is coupled electronically or via bosonic fluctuation to its environment. We are investigating the competition between different interactions causing arising magnetic moments to be screened via Kondo effect or aligned via RKKY interaction. Such competing interactions are often associated with different ground states: The systems can be tuned by external parameters from one ground state to the other. If those states are topologically disconnected a quantum phase transition is found governing the transition between different renormalization group fixed points.
Realisations of such systems are adatoms on surfaces, 3d and 4f transition metal alloys, semiconductor quantum dots, gated molecules as proposed for molecular electronics and also measured in STM experiments.
Switching of such devices requires the understanding of non-equilibrium dynamic where we have developed a powerful numerical renormalization group approach (TD-NRG) to adress quenches as well as steady-state currents though such devices.