Electron Doping of Proposed Kagome Quantum Spin Liquid Produces Localized Electrons in Band Gap States but Not Free Electrons

Cu₃ZnX with X=[(OH)₆Cl₂] or X=[(OH)₆BrF] are Kagome Heisenberg antiferromagnets that display some of the expected quantum spin liquid (QSL) fingerprints, creating the hope that electron doping would enable the long sought high-temperature superconductivity. However, successful insertion of electrons does not necessarily mean free electrons with the ensuing shifted Fermi energy. Z. Kelly et al. recently found that insertion of as much 0.6 Li ions per Cu²⁺ into Cu₃Zn[(OH)₆Cl₂] does not show the expected metallic conductivity. We have used the modern theory of doping to enquire what happens to electrons inserted into such lattices, particularly the possible interplay between local structural disorder and localization tendencies. Using approaches that correct the self-interaction error we find that whereas the d⁹ electrons of Cu²⁺ in the undoped Cu₃Zn(OH)₆BrF are spread over a broad energy range inside the valence band, upon adding an electron, this broad distribution of levels is bunched into a narrow range of highly localized d¹⁰ states inside the band gap. This suggests that the Cu-X manifold has an intrinsic tendency to localize added electrons into a polaronic state.