From a superconductor to a Mott insulator: The case of granular aluminum

Understanding the nature of the metal-to-insulator in films consisting of nano-scale aluminum grains embedded in an insulating matrix, namely granular aluminum, is essential both on the fundamental level and for applications such as quantum circuits. We show how the evolution of the system from the metallic side (and superconducting at low temperatures) to the insulator side, through a Mott transition, involves rich physics such as the Kondo effect, pseudo-gap effects, collective modes, and a BCS–BEC crossover. We focus on samples prepared on a liquid nitrogen-cooled substrate, having a grain size of about 2 nm, and suggest that the role of the grain size and hence quantum size effects are essential to explain some of the observed phenomena.