Two dimensional topological semimetals robust against to spin–orbital coupling

The two-dimensional (2D) topological semimetals (TSMs), which can host nontrivial Berry phases and topologically protected edge states, have attracted growing attention in recent years. Based on their band degeneracies, dispersion characteristics, and symmetry-protection mechanisms, these semimetallic phases can be classified into Dirac, Weyl, and nodal-ring types. In this review, we first introduce the research background of TSMs, tracing their development from early studies of graphene. We then survey theoretical and experimental progress on 2D Dirac and Weyl semimetals, followed by a discussion of nodal-ring semimetals. Finally, we provide a perspective on future research directions, including accelerating the candidate materials identification through the state-of-the-art calculation technique and enriching the classification of 2D TSMs based on a broader symmetry group landscape, as well as their new physical effects. We hope that this review serves as a comprehensive guide to current research on 2D TSMs.