Electron Doping a Kagome Spin Liquid

Abstract

Herbertsmithite, ZnCu3(OH)6Cl2 , is a two-dimensional kagome lattice realization of a spin liquid, with evidence for fractionalized excitations and a gapped ground state. Such a quantum spin liquid has been proposed to underlie high-temperature superconductivity and is predicted to produce a wealth of new states, including a Dirac metal at 1/3 electron doping. Here, we report the topochemical synthesis of electron-doped ZnLixCu3(OH)6Cl2 from x=0 to x=1.8 (3/5 per Cu2+ ). Contrary to expectations, no metallicity or superconductivity is induced. Instead, we find a systematic suppression of magnetic behavior across the phase diagram. Our results demonstrate that significant theoretical work is needed to understand and predict the role of doping in magnetically frustrated narrow band insulators, particularly the interplay between local structural disorder and tendency toward electron localization, and pave the way for future studies of doped spin liquids.

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Plain Language Summary

Despite the discovery of superconductivity over a century ago, the phenomenon still lacks a general theory, particularly in the high-temperature limit (i.e., above 30 K). In 1987, Anderson theorized that a complex magnetic state, known as a quantum spin liquid, could predicate high-temperature superconductivity. In particular, high-temperature superconductivity could be induced by introducing charge carriers into a quantum spin liquid material. However, this prediction has not been experimentally realized to date; no one has been able to successfully chemically dope a quantum spin liquid until now. Here, we introduce electrons by chemically inserting lithium ions into the leading candidate spin liquid, Herbertsmithite, and investigate the doped material using a variety of experimental techniques.

Herbertsmithite, ZnCu3(OH)6Cl2 , has been studied extensively over the past decade for its magnetically frustrated kagome structure, a lattice of corner-sharing triangles in two-dimensional planes. This structure has been experimentally shown to possess the predicted hallmarks of the spin liquid state such as fractional spin excitations. We intercalate lithium into Herbertsmithite to yield ZnLixCu3(OH)6Cl2 , where x ranges from 0 to 1.8. We then use x-ray powder diffraction and x-ray photoelectron spectroscopy, among other techniques, to investigate the resulting doped material, which appears to be black. We find that electron doping continuously suppresses the magnetism in the material without the appearance of superconductivity or related metallic phases at temperatures as low as T=1.8K . Additionally, our heat capacity measurements reveal an interesting trend of excess entropy that is consistent with a singlet-triplet excitation of localized electron pairs.

We expect that our findings will pave the way for additional studies of doped spin liquids and revised theories of superconductivity.