Electron doping of proposed quantum spin liquid kagom\'e Zn-Cu hydroxyl-halides produces localized states in the band gap

Carrier doping of quantum spin liquids is a long-proposed route to the emergence of high-temperature superconductivity. Electrochemical intercalation in kagome hydroxyl-halide materials shows that samples remain insulating across a wide range of electron counts. Here we demonstrate through first-principles density functional calculations corrected for self-interaction the mechanism by which electrons remain localized in various Zn-Cu hydroxyl-halides, independently of the chemical identity of the dopant - the formation of polaronic states with attendant lattice displacements and a dramatic narrowing of bandwidth upon electron addition. The same theoretical method applied to electron doping in cuprate Nd2CuO4 correctly produces a metallic state when the initially formed polaron dissolves into an extended state. Our general findings explain the insulating behavior in a wide range of doped quantum magnets and demonstrate that new quantum spin liquid host materials are needed to realize metallicity borne of a spin liquid.