Comment on "Topological excitations and the dynamic structure factor of spin liquids on the kagome lattice" (Punk, M., Chowdhury, D. & Sachdev, S. Nature Physics 10, 289-293 (2014))

TL;DR

This paper critiques a recent claim that only Z2 spin liquids explain flat excitation continua in kagome lattice spin liquids, highlighting an alternative successful theoretical approach.

Contribution

It challenges the exclusivity of Z2 spin liquids in explaining flat excitation bands, presenting an alternative theoretical model for quantum spin liquids on the kagome lattice.

Findings

Existing models do not account for all observed features.

Alternative models can produce flat excitation bands.

The critique broadens the understanding of spin liquid theories.

Abstract

The authors of a recent paper evidently take the view that the whole of progress made toward a theoretical understanding of the physics of quantum spin liquids (QSL) is associated with models of the kind proposed and applied in their present work. As motivation for this work, they observe that in contrast to existing theoretical models of both gapped and gapless spin liquids, which give rise to sharp dispersive features in the dynamic structure factor, the measured dynamic structure factor reveals an excitation continuum that is remarkably flat as a function of frequency. They go on to assert that "so far, the only theoretical model for a spin liquid state on the kagome lattice which naturally gives rise to a flat excitation band at low energies consists of the $Z_2$ spin liquids". Here we point out that there already exists a different and demonstrably successful approach to the QSL problem that does naturally feature a flat band.