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.

No abstract

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.

The electronic states of a single Bi(111) bilayer and its edges, suggested as a two dimensional topological insulator, are investigated by scanning tunneling spectroscopy (STS) and first-principles calculations. Well-ordered bilayer films and islands with zigzag edges are grown epitaxially on a cleaved Bi2Te2Se crystal. The calculation shows that the band gap of the Bi bilayer closes with a formation of a new but small hybridization gap due to the strong interaction between Bi and Bi2Te2Se. Nevertheless, the topological nature of the Bi bilayer and the topological edge state are preserved only with an energy shift. The edge-enhanced local density of states are identified and visualized clearly by STS in good agreement with the calculation. This can be the sign of the topological edge state, which corresponds to the quantum spin Hall state. The interfacial state between Bi and Bi2Te2Se is also identified inside the band gap region. This state also exhibits the edge modulation, which was previously interpreted as the evidence of the topological edge state [F. Yang et al., Phys. Rev. Lett. 109, 016801 (2012)].

Discovery of two-dimensional topological insulator such as Bi bilayer initiates challenges in exploring exotic quantum states in low dimensions. We demonstrate a promising way to realize the Kane-Mele-type quantum spin Hall (QSH) phase and the quantum anomalous Hall (QAH) phase in chemically-modified Bi and Sb bilayers using first-principles calculations. We show that single Bi and Sb bilayers exhibit topological phase transitions from the band-inverted QSH phase or the normal insulator phase to Kane-Mele-type QSH phase upon chemical functionalization. We also predict that the QAH effect can be induced in Bi or Sb bilayers upon nitrogen deposition as checked from calculated Berry curvature and the Chern number. We explicitly demonstrate the spin-chiral edge states to appear in nitrogenated Bi-bilayer nanoribbons.

We investigate the topological surface states in heterostructures formed from a three-dimensional topological insulator (TI) and a two-dimensional insulating thin film, using first-principles calculations and the tight-binding method. Utilizing a single Bi or Sb bilayer on top of the topological insulators Bi 2 Se 3 , Bi 2 Te 3 , Bi 2 Te 2 Se, and Sb 2 Te 3 , we find that the surface states evolve in very peculiar but predictable ways. We show that strong hybridization between the bilayer and TI substrates causes the topological surface states to migrate to the top bilayer. It is found that the difference in the work function of constituent layers, which determines the band alignment and the strength of hybridization, governs the character of newly emerged Dirac states.