Spectral evidence for Dirac spinons in a kagome lattice antiferromagnet

TL;DR

This study provides experimental spectral evidence for Dirac spinons in a kagome lattice antiferromagnet, supporting the existence of a Dirac quantum spin liquid state with novel spin excitations.

Contribution

First experimental confirmation of Dirac spinons in a kagome antiferromagnet, linking spectral data with theoretical Dirac quantum spin liquid models.

Findings

Spectral conical spin excitations consistent with Dirac spinons

Agreement between spectral measurements and spin liquid model predictions

Differences in excitation locations suggest an unexpected origin of Dirac spinons

Abstract

Emergent quasiparticles with a Dirac dispersion in condensed matter systems can be described by the Dirac equation for relativistic electrons, in analogy with Dirac particles in high-energy physics. For example, electrons with a Dirac dispersion have been intensively studied in electronic systems such as graphene and topological insulators. However, charge is not a prerequisite for Dirac fermions, and the emergence of Dirac fermions without charge degree of freedom has been theoretically predicted to be realized in Dirac quantum spin liquids. These quasiparticles carry a spin of 1/2 but are charge-neutral, and so are called spinons. Here we show that the spin excitations of a kagome antiferromagnet, YCu$_3$(OD)$_6$Br$_2$[Br$_{0.33}$(OD)$_{0.67}$], are conical with a spin continuum inside, which is consistent with the convolution of two Dirac spinons. The predictions of a Dirac spin liquid model with a spinon velocity obtained from the spectral measurements are in agreement with the low-temperature specific heat of the sample. Our results thus provide spectral evidence for the Dirac quantum spin liquid state emerging in this kagome lattice antiferromagnet. However, the locations of the conical spin excitations differ from those calculated by the nearest neighbor Heisenberg model, suggesting the Dirac spinons have an unexpected origin.