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

TL;DR

This study uses first-principles calculations to show that electron doping in kagome Zn-Cu hydroxyl-halides leads to localized polaronic states, preventing metallicity and highlighting the need for new quantum spin liquid materials.

Contribution

It reveals the mechanism of electron localization in doped kagome quantum magnets and explains why doping does not induce metallicity in these materials.

Findings

Electrons form polaronic states with lattice displacements.

Bandwidth narrows significantly upon electron addition.

Doped cuprate Nd2CuO4 becomes metallic when polarons dissolve.

Abstract

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.