Heterostructures of Dirac materials such as graphene and topological insulators provide interesting platforms to explore exotic quantum states of electrons in solids. Here we study the electronic structure of graphene-Sb 2 Te 3 heterostructure using density functional theory and tight-binding methods. We show that the epitaxial graphene on Sb 2 Te 3 turns into quantum spin-Hall phase due to its proximity to the topological insulating Sb 2 Te 3 . It is found that the epitaxial graphene develops a giant spin-orbit gap of about ~20 meV, which is about three orders of magnitude larger than that of pristine graphene. We discuss the origin of such enhancement of the spin-orbit interaction and possible outcomes of the spin-Hall phase in graphene.

MXenes are a large family of two-dimensional (2D) early transition metal carbides that have shown great potential for a host of applications ranging from electrodes in supercapacitors and batteries to sensors to reinforcements in polymers. Here, on the basis of first-principles calculations, we predict that MoMCO (M = Ti, Zr, or Hf), belonging to a recently discovered new class of MXenes with double transition metal elements in an ordered structure, are robust quantum spin Hall (QSH) insulators. A tight-binding (TB) model based on the d-, d-, and d-orbital basis in a triangular lattice is also constructed to describe the QSH states in MoMCO. It shows that the atomic spin-orbit coupling (SOC) strength of M totally contributes to the topological gap at the Γ point, a useful feature advantageous over the usual cases where the topological gap is much smaller than the atomic SOC strength based on the classic Kane-Mele (KM) or Bernevig-Hughes-Zhang (BHZ) model. Consequently, MoMCO show sizable gaps from 0.1 to 0.2 eV with different M atoms, sufficiently large for realizing room-temperature QSH effects. Another advantage of MoMCO MXenes lies in their oxygen-covered surfaces which make them antioxidative and stable upon exposure to air.

Two-dimensional (2D) electrides are layered ionic crystals in which anionic electrons are confined in the interlayer space. Here, we report a discovery of nontrivial [Formula: see text] topology in the electronic structures of 2D electride YC. Based on first-principles calculations, we found a topological [Formula: see text] invariant of (1; 111) for the bulk band and topologically protected surface states in the surfaces of YC, signifying its nontrivial electronic topology. We suggest a spin-resolved angle-resolved photoemission spectroscopy (ARPES) measurement to detect the unique helical spin texture of the spin-polarized topological surface state, which will provide characteristic evidence for the nontrivial electronic topology of YC. Furthermore, the coexistence of 2D surface electride states and topological surface state enables us to explain the outstanding discrepancy between the recent ARPES experiments and theoretical calculations. Our findings establish a preliminary link between the electride in chemistry and the band topology in condensed-matter physics, which are expected to inspire further interdisciplinary research between these fields.

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The adsorption of alkali metals ͑AMs͒ on single layer graphene is studied using first principles methods. We observe a common trend in the binding distance, the charge transfer, and the work function ͑W͒ at certain coverage of AMs with increase in the proportion ͑adatom/C atom͒ of the graphene covered by the AM. A dip in these properties occurs at Ϸ 0.04 for all AMs except Li, for which it occurs at Ϸ 0.08. This behavior is due to a transition of adsorbed metals from individual atoms to two-dimensional metallic sheets that exert a depolarization effect. W of graphene exhibits asymmetric dependence on : a dip in the adatom layer side but saturation on the graphene side, which is in contrast to the case of bulk graphite.