Helical edge modes in a triangular Heisenberg antiferromagnet

An energy or action term proportional to a topological invariant does not change under continuous variations. Although such a term does not affect classical dynamics in the bulk, it may alter boundary conditions. The authors show here that a topological energy term, allowed in a Heisenberg antiferromagnet on a triangular lattice, leads to the formation of one-way propagating magnon edge modes. They offer a realistic microscopic superexchange model that yields this topological term and helical edge modes.