Chiral Dirac-like fermion in spin-orbit-free antiferromagnetic semimetals

TL;DR

This paper predicts a new type of massless Dirac-like quasiparticle, called flavor Weyl fermion, in certain antiferromagnetic semimetals with negligible spin-orbit coupling, expanding the understanding of topological states in quantum materials.

Contribution

It introduces the concept of flavor Weyl fermions linked by SU(2) isospin symmetry in spin-orbit-free antiferromagnetic systems, which cannot be explained by conventional space group symmetries.

Findings

Discovery of flavor Weyl fermions with linear dispersion and Chern number 2.

Identification of CoNb3S6 as a candidate material for flavor Weyl semimetal.

Prediction of topologically protected Fermi arc networks in these systems.

Abstract

Dirac semimetal is a phase of matter, whose elementary excitation is described by the relativistic Dirac equation. In the limit of zero mass, its parity-time symmetry enforces the Dirac fermion in the momentum space, which is composed of two Weyl fermions with opposite chirality, to be non-chiral. Inspired by the flavor symmetry in particle physics, we theoretically propose a massless Dirac-like equation yet linking two Weyl fields with the identical chirality by assuming SU(2) isospin symmetry, independent of the space-time rotation exchanging the two fields. Dramatically, such symmetry is hidden in certain solid-state spin-1/2 systems with negligible spin-orbit coupling, where the spin degree of freedom is decoupled with the lattice. Therefore, the existence of the corresponding quasiparticle, dubbed as flavor Weyl fermion, cannot be explained by the conventional (magnetic) space group framework. The four-fold degenerate flavor Weyl fermion manifests linear dispersion and a Chern number of 2, leading to a robust network of topologically protected Fermi arcs throughout the Brillouin zone. For material realization, we show that the transition-metal chalcogenide CoNb3S6 with experimentally confirmed collinear antiferromagnetic order is ideal for flavor Weyl semimetal under the approximation of vanishing spin-orbit coupling. Our work reveals a counterpart of the flavor symmetry in magnetic electronic systems, leading to further possibilities of emergent phenomena in quantum materials.