Chiral Dirac fermion in a collinear antiferromagnet

TL;DR

This paper reports the experimental observation of chiral Dirac-like fermions in a collinear antiferromagnet, revealing surface Fermi arcs linked to these quasiparticles, and suggests their topological origin protected by hidden spin symmetries.

Contribution

It provides the first spectral evidence of chiral Dirac-like fermions in a collinear antiferromagnet, combining ARPES, neutron diffraction, and first-principles calculations.

Findings

Observation of surface Fermi arcs in CoNb3S6

Identification of chiral Dirac-like fermions linked to spin symmetry

Evidence of topologically protected surface states

Abstract

In a Dirac semimetal, the massless Dirac fermion has zero chirality, leading to surface states connected adiabatically to a topologically trivial surface state as well as vanishing anomalous Hall effect (AHE). Recently, it is predicted that in the nonrelativistic limit of certain collinear antiferromagnets, there exists a type of chiral Dirac-like fermion, whose dispersion manifests four-fold degenerate crossing points formed by spin-degenerate linear bands, with topologically protected Fermi arcs. Such unconventional chiral fermion, protected by a hidden SU(2) symmetry in the hierarchy of an enhanced crystallographic group, namely spin space group, is not experimentally verified yet. Here, by angle-resolved photoemission spectroscopy measurements, we reveal the surface origin of the electron pocket at the Fermi surface in collinear antiferromagnet CoNb3S6. Combining with neutron diffraction and first-principles calculations, we suggest a multidomain collinear AFM configuration, rendering the the existence of the Fermi-arc surface states induced by chiral Dirac-like fermions. Our work provides spectral evidence of the chiral Dirac-like fermion caused by particular spin symmetry in CoNb3S6, paving an avenue for exploring new emergent phenomena in antiferromagnets with unconventional quasiparticle excitations.