Two-dimensional double kagome lattice nitrogene: A direct band gap semiconductor with nontrivial corner state

The kagome lattice, characterized by its flat band and Dirac electrons, continues to attract enduring attention in condensed matter physics as an ideal structure for studying electron correlation and various quantum phases. Despite recent experimental advances, two-dimensional (2D) kagome-structured materials with significant flat-band characteristics still remain rare. Based on first-principles calculations, we predict that nitrogen atoms can assemble into a single-layer double kagome lattice (DKL), which possesses the characteristics of an intrinsic direct band gap semiconductor, boasting a substantial band gap of 3.460 eV. The DKL structure results in a flat valence band with a high effective mass and a conduction band with a small effective mass that comes from Dirac electrons. These distinctive band edges lead to a significant disparity in carrier mobilities, with electron mobility being four orders of magnitude higher than that of holes. The presence of a flat band in DKL-nitrogene can be further discerned through the enhanced optical absorption and correlated effects as exemplified by hole-induced ferromagnetism. Interestingly, DKL-nitrogene exhibits inherent second-order topological states, confirmed by a nontrivial second Stiefel-Whitney number and the presence of 1D floating edge states and 0D corner states within the bulk band gap. Additionally, the robust N-N bonds and the lattice's bending structure ensure thermodynamic stability and mechanical stiffness. These attributes make it exceptionally stable for potential applications in nanodevices.