Dirac points and Weyl phase in a honeycomb altermagnet

Abstract: We present a graphene derivative displaying novel topological phenomena without the spin-orbit coupling (SOC) in a collinear altermagnet. Band degeneracies in spin-split honeycomb antiferromagnets produce unique distribution patterns of Berry curvature and give rise to topological boundary states with unconventional spin textures. The asymmetric Berry curvature due to spin splitting can be created by lifting the degeneracies, which concentrate intensely at Weyl points contributing to carrier dynamics with a large valley Hall effect. The altermagnetism breaks the intrinsic time-reversal symmetry, guaranteeing the formation of pairs of Weyl points associated with the internal spin structure chiralities. We investigate the evolution of Dirac and Weyl crossings, in which topological phase transitions are characterized by the high Chern numbers preserving the non-intersecting flows of Wannier centers over occupied bands. Our results yield unique degeneracy-protected states by leveraging the symmetry constraints, where corresponding experiments are illustrated to demonstrate the potential of advancing valleytronic devices via unconventional topological responses.