Topological excitations and the dynamic structure factor of spin liquids on the kagome lattice

TL;DR

This paper explains neutron scattering observations in a kagome lattice quantum spin liquid by proposing that topological vison excitations form flat bands, accounting for the flat, momentum-independent excitation continuum observed.

Contribution

It introduces a model where topological visons in a Z2 spin liquid form flat bands, providing a new explanation for experimental neutron scattering results in Herbertsmithite.

Findings

Visons form flat bands on the kagome lattice.

The dynamic structure factor matches experimental data above a low energy cutoff.

One of the studied Z2 spin liquids explains the observed features well.

Abstract

Recent neutron scattering experiments on the spin-1/2 kagome lattice antiferromagnet ZnCu3(OH)6Cl2 (Herbertsmithite) provide the first evidence of fractionalized excitations in a quantum spin liquid state in two spatial dimensions. In contrast to existing theoretical models of spin liquids, the measured dynamic structure factor reveals an excitation continuum which is remarkably flat as a function of frequency and has almost no momentum dependence along several high-symmetry directions. Here we show that many experimentally observed features can be explained by the presence of topological vison excitations in a Z2 spin liquid. These visons form flat bands on the kagome lattice, and thus act as a momentum sink for spin-carrying excitations which are probed by neutron scattering. We compute the dynamic structure factor for two different Z2 spin liquids and find that one of them describes Herbertsmithite well above a very low energy cutoff.