Second-Order Topological Insulator in Two-Dimensional C2N and Its Derivatives

Quadrupole phase, as a novel high-order topological phase, exhibits nontrivial gapless states at the boundaries whose dimension is lower than bulk by two. However, this phase has not been observed experimentally in two-dimensional (2D) materials up to now. In this work, using first-principles calculations and tight-binding (TB) model, we propose that the experimentally synthesized C2N is a 2D quadrupole topological insulator with one-dimensional gapped edge states and zero-dimensional gapless corner states. C2N is found to have a large bulk gap of 2.45 eV and an edge gap of 0.32 eV, making it an excellent candidate to evidently present the nontrivial corner states in experiments. The robustness of the corner states against the edge disorders has been explicitly identified. Moreover, another three C2N-like materials are also found to host the nontrivial quadrupole phase including an experimentally synthesized material aza-fused microporous polymers (CMP). The four 2D quadrupole topological phases proposed in our present work provide excellent candidates for studying the novel high-order topological properties in future experiments.