Quasiparticle screening near a bosonic superconductor-insulator transition revealed by magnetic impurity doping

While bosonic superconductor-insulator transitions (SIT) have been clearly observed in a number of thin film systems, the mechanisms driving the Cooper pair localization remain to be established. These SITs feature thermally activated Cooper pair transport in the insulating phase with an activation energy T₀ that grows continuously from zero at the critical point. Some models attribute this behavior to disorder effects that give rise to Anderson localization of Cooper pairs while others invoke Coulomb interaction effects that drive a Mott transition. I will describe experiments on ultrathin, nanoporous a-Bi films that focus on how T₀ depends on the pairbreaking effects induced by magnetic impurity doping. I will discuss how the data provide strong evidence that the bosonic SIT in thin films is a Mott transition driven by Coulomb interactions that are screened by virtual quasi-particle excitations[1]. The dependence of these SITs on on underlying fermionic degrees of freedom distinguishes them from those occuring in micro-fabricated Josephson Junction Arrays, cold atom systems, and likely in high temperature superconductors with nodes in their quasiparticle density of states.
Work done in collaboration with: Xue Zhang, James Joy, Wu Chunshu, and Jimmy Xu