Compensation driven superconductor-insulator transition

TL;DR

This paper develops a theory for the superconductor-insulator transition in doped materials with strong compensation, focusing on how impurity screening and pairing nature influence the phase diagram.

Contribution

It introduces a new phase diagram model considering strong coupling superconductivity and the role of compact Cooper pairs in doped, compensated superconductors.

Findings

Transition occurs at higher density for compact Cooper pairs.

Screening of charged impurities determines localization or delocalization of carriers.

Phase diagram features depend on pairing type and impurity concentration.

Abstract

The superconductor-insulator transition in the presence of strong compensation of dopants was recently realized in La doped YBCO. The compensation of acceptors by donors makes it possible to change independently the concentration of holes n and the total concentration of charged impurities N. We propose a theory of the superconductor-insulator phase diagram in the (N,n) plane. It exhibits interesting new features in the case of strong coupling superconductivity, where Cooper pairs are compact, non-overlapping bosons. For compact Cooper pairs the transition occurs at a significantly higher density than in the case of spatially overlapping pairs. We establish the superconductor-insulator phase diagram by studying how the potential of randomly positioned charged impurities is screened by holes or by strongly bound Cooper pairs, both in isotropic and layered superconductors. In the resulting self-consistent potential the carriers are either delocalized or localized, which corresponds to the superconducting or insulating phase, respectively.