Haiyue Huang | Searcharxiv

arXiv2026

Bound States in Second-order Topological Graphitic Structures

Quadrupole insulators are a class of second-order topological insulators (SOTIs) that host zero-dimensional corner states within a two-dimensional bulk. Despite their unique properties, their realization in electronic systems on realistic material platforms remains rare. In this work, we present a general design principle to obtain quadrupole insulators based on two-dimensional graphitic structures. By engineering the positions and connections of zigzag edges, we identify four topological classes of graphitic structures. We show that topologically protected massless corner state emerge at the intersection of domains belonging to different topological classes. Crucially, by tuning the smoothness of the domain wall, we further demonstrate the appearance of additional massive localized states with non-zero angular momentum. Our results provide a practical framework for realizing experimentally accessible SOTIs and uncover the coexistence of both massless and massive bound states in two dimensions.

cond-mat.mes-hall↗

arXiv2024

Topological Solitons in Square-root Graphene Nanoribbons Controlled by Electric Fields

Graphene nanoribbons (GNRs) are unique quasi-one-dimensional (1D) materials that have garnered a lot of research interest in the field of topological insulators. While the topological phases exhibited by GNRs are primarily governed by their chemical structures, the ability to externally control these phases is crucial for their potential utilization in quantum electronics and spintronics. Here we propose a class of GNRs featured by mirror symmetry and four zigzag segments in a unit cell that has unique topological properties induced and controlled by an externally applied electric field. Their band structures manifest two finite gaps which support topological solitons, as described by an effective square-root model. To demonstrate the experimental feasibility, we design and synthesize a representative partially zigzag chevron-type GNR (pzc-GNR) with the desired zigzag segments using a bottom-up approach. First-principles calculations on pzc-GNR reveal band inversions at the two finite gaps by switching the direction of the electric field, which is in accordance with predictions from the square-root Hamiltonian. We show different topological phases can be achieved by controlling the direction of the field and the chemical potential of the system in square-root GNRs. Consequently, upon adding a step-function electric field, solitons states can be generated at the domain wall. We discuss the properties of two types of soliton states, depending on whether the terminating commensurate unit cell is mirror symmetric.

cond-mat.mes-hall↗