We present unconventional nodal crossings in a two-dimensional collinear altermagnet, which are enforced by crystal symmetries to lock spin polarization and valley degrees of freedom. Altermagnetism generates non-relativistic 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 that are strongly concentrated near the 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 reveal unique topological nodes arising from the interplay between crystal symmetry constraints and intrinsic time-reversal symmetry breaking, whose associated topological responses are relevant for spintronic applications.
Zhang et al. (Thu,) studied this question.