Two-Dimensional Quadrupole Topological Insulator in γ-Graphyne.

Two-dimensional quadrupole topological insulator (2D QTI), as a new class of second-order topological phases, has been experimentally confirmed in various artificial systems recently. However, its realization in electronic materials has seldom been reported. In this work, we predict that the experimentally synthesized gamma-graphyne is a large-gap (similar to 0.2 eV) 2D QTI. Three characterized features for 2D QTI are simultaneously observed in gamma-graphyne: quantized finite bulk quadrupole moment, gapped topological edge states, and in-gap topological corner states. Intriguingly, we found that gapped topological edge states exist on armchair edge with C C (but not C C) termination, and in-gap topological corner states exist at corner with 120 degrees (but not 60 degrees) termination, which can be explained by different edge-hopping textures and corner chiral charges. Moreover, the robustness of in-gap topological corner states is further identified by varying edge-disorder and system-size calculations. Our results demonstrate a realistic electronic material for large-gap 2D QTI, which is expected to draw immediate experimental attention.