Destruction of superconductivity in disordered thin films can be described by a Quantum Phase Transition toward an insulating phase. This Superconductor-to-Insulator Transition (SIT) is usually associated with the competition between superconductivity, Coulomb interactions and localization.
Amorphous NbxSi1-x thin films are a model system to study this transition as disorder can be tuned by different means such as a modification of the composition x, of the thickness, or by applying a heat treatment procedure. In this 2D superconducting system, contrary to theoretical predictions regarding the SIT, our group has recently observed metallic states in between the superconducting and the insulating ones. This observation has raised the question of the mechanisms at play near the SIT in this material, as well as of the nature of the possible ground states.
In this seminar, after a brief introduction on the SIT, I will detail the general features of the destruction of superconductivity in a-NbxSi1-x. Previous work had concentrated on the characterization of the superconducting and metallic states, which I will mention. I will then expose new results obtained through the analysis of DC transport properties evolution, as the metallic phase evolves toward an insulator. These results have enabled us to grab a more thorough picture of these enigmatic 2D metallic ground states. Finally, I will present a new setup that I have developed in order to probe the SIT through high frequency measurements (GHz) as well as its application to the determination of superconducting thin films properties.
