Chiral Dirac fermion in a collinear antiferromagnet

Abstract

Abstract The Dirac equation combines the two cornerstones of modern physics—quantum mechanics and relativity. There are several manifestations of the Dirac equation in condensed matter systems, such as the quasiparticle dispersion in graphene1, topological insulators2-4, Dirac semimetals (DSMs)5-9, Weyl semimetals10-12, and d-wave high-temperature superconductors13. In a DSM, 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 antiferromagnets, there exists a type of chiral “Dirac-like” fermion, whose dispersion manifests four-fold degenerate crossing points formed by doubly degenerate linear bands, with topologically protected Fermi arcs14. Such unconventional chiral fermion, protected by a hidden SU(2) symmetry in the hierarchy of an enhanced crystallographic group, namely spin space group15-17, is not experimentally verified yet. Here, by combining neutron diffraction, angle-resolved photoemission spectroscopy and first-principles calculations, we reveal the existence of the Fermi-arc surface states induced by chiral Dirac-like fermions in collinear antiferromagnet CoNb3S6, which caught great interest due to its surprisingly large AHE18-23. Our transport measurements and theoretical calculations provide a scenario that large Berry curvature embedded in the chiral fermions and weak symmetry breaking are responsible for the emergent AHE. Our work evidences the existence of chiral Dirac-like fermion in CoNb3S6, paving an avenue for exploring new emergent phenomena in quantum materials with unconventional quasiparticle excitations.