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

While bosonic superconductor-insulator quantum phase transitions (SIT) have been clearly observed in a number of thin film systems, the mechanisms driving the Cooper pair localization remain to be established. The associated insulating phase exhibits thermally activated Cooper pair transport with an activation energy T₀ that grows continuously from zero at the SIT critical point. Some models attribute this behavior to disorder inducing Anderson localization 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]. This dependence on underlying fermionic degrees of freedom is likely to be found in strongly disordered thin film systems, like Indium Oxide and TiN, that exhibit emergent granularity in the Cooper pair density near their SITs. It also distinguishes this bosonic SIT from those in micro-fabricated Josephson Junction Arrays, cold atom systems, and likely in high temperature superconductors with nodes in their quasiparticle density of states. This work was done in collaboration with: Xue Zhang, James Joy, Wu Chunshu, and Jimmy Xu
[1] Xue Zhang et al., Physical review letters 122, 157002 (2019)