Type-I and II Nodal Lines coexist in antiferromagnetic monolayer CrAs$_{2}$

Using first-principles calculations, we propose a stable two-dimensional semimetal CrAs2 monolayer, which hosts antiferromagnetic (AFM) type-I and II nodal lines in the absence of spin-orbital coupling (SOC). It is worth mentioning that the essential ype-I nodal line is firstly proposed in AFM two-dimensional materials, that is filling enforced by nonsymmorphic analogue symmetry T, and cannot be gapped even in the presence of SOC. The type-II nodal lines, formed by band inversion, are protected by the mirror symmetry Mz in the absence of SOC. Furthermore, the topological nontriviality is verified by edge states on the boundary. From the calculated exfoliation energies and phonon dispersion, we show that CrAs2 monolayer can be readily obtained in experiment from the bulk material and exists as freestanding twodimensional crystal. Our results reveal CrAs2 monolayer as a promising platform for exploring the interplay between magnetism and exotic topological phases in two-dimensional system.