Exotic quantum critical point in a two-site charge Kondo circuit*

A central feature of heavy fermion materials, the emergence of coherence in a magnetic lattice collectively screened by a surrounding sea of mobile electrons, has yet to be realized in a simple model. We here experimentally implement a tunable nanoelectronic circuit comprising two quantum islands, each of whose charge state acts like a local spin that can be screened via a Kondo interaction. Coupling these two islands together realizes a novel model which captures the essence of competition between local and collective screening of magnetic moments, a variant of the prototypical two-impurity Kondo model. We tune our device to a quantum critical point and show experimentally that the dependence of conductance on deviation from this point matches non-trivial universal predictions from numerical renormalization group calculations. This work is a necessary first step in scaling up such circuits from individual sites to networks or lattices.