Emergence of spin singlets with inhomogeneous gaps in the kagome lattice Heisenberg antiferromagnets Zn-barlowite and herbertsmithite

The kagome Heisenberg antiferromagnet formed by frustrated spins arranged in a lattice of corner-sharing triangles is a prime candidate for hosting a quantum spin liquid (QSL) ground state consisting of entangled spin singlets(1). However, the existence of various competing states makes a convincing theoretical prediction of the QSL ground state difficult(2), calling for experimental clues from model materials. The kagome lattice materials Zn-barlowite (ZnCu3(OD)(6)FBr)(3-5) and herbertsmithite (ZnCu3(OD)(6)Cl-2)(6-10) do not exhibit long-range order and are considered the best realizations of the kagome Heisenberg antiferromagnet known so far. Here we use Cu-63 nuclear quadrupole resonance combined with the inverse Laplace transform(11-13) to locally probe the inhomogeneity of delicate quantum ground states affected by disorder(14-17). We present direct evidence for the gradual emergence of spin singlets with spatially varying excitation gaps, but even at temperatures far below the super-exchange energy scale their fraction is limited to similar to 60% of the total spins. Theoretical models(18,19) need to incorporate the role of disorder to account for the observed inhomogeneously gapped behaviour.

Automated summary

The kagome Heisenberg antiferromagnet is a prime candidate for hosting a quantum spin liquid ground state, but the existence of competing states makes theoretical prediction difficult, calling for experimental clues from model materials. Zn-barlowite and herbertsmithite are considered the best realizations of the kagome Heisenberg antiferromagnet known so far, and Cu-63 nuclear quadrupole resonance is used to locally probe the inhomogeneity of delicate quantum ground states affected by disorder.