Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states

John P. Sheckelton,Yu-Sheng Chen,Guangyong Xu,Simon J. Teat,Jack Mingde Jiang,Jeffrey W. Lynn,Hong-Chen Jiang,Wei He,Adam A. Aczel,Yi-Fan Jiang,Jiajia Wen,Young S. Lee,Charles J. Titus,Yang Zhao,Rebecca W. Smaha,Suyin Grass Wang

doi:10.1038/s41535-020-0222-8

Abstract

The spin-frac{1}{2} kagome antiferromagnet is considered an ideal host for a quantum spin liquid (QSL) ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu4(OH)6FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin-frac{1}{2} kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a QSL. The presence of interlayer spins eventually leads to an interesting pinwheel q = 0 magnetic order. Partially Zn-substituted barlowite (Cu3.44Zn0.56(OH)6FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is similar to that of herbertsmithite, even though the Cu2+ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.

Highlights

Identifying the ground state for interacting quantum spins on the kagome lattice is an important unresolved question in condensed matter physics, owing to the great difficulty in selecting amongst competing states that are very close in energy
New high-symmetry barlowite: a pinwheel modulated kagome lattice should lead to different magnitudes of the magnetic exchange interaction, though the differences may be small
Structural investigation employing a combination of highresolution synchrotron powder and single-crystal X-ray diffraction (PXRD and SCXRD), as well as neutron powder diffraction (NPD) to definitively determine the structure of barlowite 1, which transforms to orthorhombic Pnma below T ≈ 265 K (Supplementary Figs. 2–4, Supplementary Tables 2, 3, 5–8, 10, and 11)

Summary

Introduction

Identifying the ground state for interacting quantum spins on the kagome lattice is an important unresolved question in condensed matter physics, owing to the great difficulty in selecting amongst competing states that are very close in energy. 1 2 systems the ground state does not achieve magnetic order and is believed to be a quantum spin liquid (QSL)[1,2,3,4,5,6,7,8,9]. Real kagome materials often have additional interactions that relieve the frustration and drive the moments to magnetically order[25,26], indicating that the ground state depends sensitively on small perturbations and lattice distortions. We present a material system in which small changes can be made to the kagome lattice, thereby revealing closely related QSL, VBC, and spin-ordered states. We make close connection with theory by performing numerical simulations for the quantum moments based on the specific symmetry of the material

Methods

Results

Conclusion