Quasiparticle Screening near a Bosonic Superconductor-Insulator Transition Revealed by Magnetic Impurity Doping

Experiments show that the Cooper pair transport in the insulator phase that forms at thin film superconductor to insulator transitions (SIT) is simply activated. The activation energy T_{0} depends on the microscopic factors that drive Cooper pair localization. To test proposed models, we investigated how a perturbation that weakens Cooper pair binding, magnetic impurity doping, and phase frustration affects T_{0}. The data show that T_{0} decreases monotonically with doping in films tuned farther from the SIT and increases and peaks in films that are closer to the SIT critical point. The observations provide strong evidence that the bosonic SIT in thin films is a Mott transition driven by Coulomb interactions that are screened by virtual quasiparticle excitations. This dependence on underlying fermionic degrees of freedom distinguishes these SITs from those in microfabricated Josephson Junction arrays, cold atom systems, and likely in high temperature superconductors with nodes in their quasiparticle density of states.