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

TL;DR

This study investigates how magnetic impurity doping affects the activation energy in thin film superconductors near the superconductor-insulator transition, revealing insights into the bosonic transition mechanism and the role of fermionic excitations.

Contribution

It provides experimental evidence that the bosonic SIT is a Mott transition driven by Coulomb interactions, influenced by fermionic quasiparticles, which is a novel insight into the transition's nature.

Findings

Activation energy $T_0$ decreases with doping in films farther from SIT.

$T_0$ peaks near the critical point of SIT.

Results support a Coulomb-driven Mott transition model.

Abstract

Experiments show that the Cooper pair transport in the insulator phase that forms at thin film superconductor to insulator transitions (SIT) is simply activated. This activated behavior depends on the microscopic factors that drive the localization of the Cooper pairs. To test proposed models, we investigated how a perturbation that weakens Cooper pair binding, magnetic impurity doping, affects the characteristic activation energy, $T_0$. 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. These observations 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. This dependence on underlying fermionic degrees of freedom distinguishes these SITs 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.