Dirac points and Weyl phase in a honeycomb altermagnet

We present unconventional nodal crossings in a two-dimensional (2D) collinear altermagnet, which are enforced by crystal symmetries to lock spin polarization and valley degrees of freedom. The altermagnetism generate nonrelativistic spin-splitting in honeycomb antiferromagnets, guaranteeing novel band degeneracies between bands sharing identical spin configurations yet different sublattices. Inspired by the $XPS_{3}$ (X=Mn, Fe, Ni) materials, we demonstrate distinctive Berry curvature distributions concentrating intensely at Weyl nodes, which further generalize the locking between valleys and Berry curvature. Topological phase transitions are characterized by the high Chern numbers preserving the non-intersecting flows of Wannier centers over occupied bands, where degeneracy lifting contributes to unconventional spin textures to induce the valley Hall effect. Our results yield unique topological nodes via leveraging the crystal symmetry constraints with the intrinsic time-reversal symmetry breaking, where corresponding topological responses enable the potential of advancing spintronics. Our results yield unique topological nodes without the spin-orbit coupling (SOC) achieved by combining crystal symmetry constraints with inherent time-reversal symmetry breaking, whose associated topological responses enable promising applications in spintronics.