From density functional theory within the generalized gradient approximation we predict a structure of stanene with dumbbell units (DBs), and show that it is a two-dimensional topological insulator with an inverted band gap which can be tuned by compressive strain. Furthermore, we propose that the boron nitride sheet and reconstructed (2×2) InSb(111) surfaces are ideal substrates for the experimental realization of DB stanene, maintaining its nontrivial topology. Combined with standard semiconductor technologies, such as magnetic doping and electrical gating, the quantum anomalous Hall effect, Chern half metallicity, and topological superconductivity can be realized in DB stanene on those substrates. These properties make the two-dimensional supported stanene a good platform for the study of quantum spin Hall insulators as well as other exotic quantum states of matter. , have attracted much attention in recent decades due to their exotic electronic properties. Especially for twodimensional (2D) graphene, its great success has inspired the "graphene age" in the fields of physics, chemistry, and material science. For direct integration on silicon based electronic devices, the search for other 2D group IV materials has triggered enormous interest. Since the first proposal of low-buckled (LB) silicene [5], a dumbbell configuration of silicene [6] and a flat germanene structure [7] have been synthesized. In contrast to graphene and LB silicene, the bond lengths in LB 2D layers of tin (called stanene) are much larger, and the relatively weak π -π bonding cannot stabilize the planar configuration, resulting in instability of free-standing LB stanene (see the Supplemental Material [8] for details).Following the initial discovery of the quantum spin Hall (QSH) effect in 2D topological insulator (TI) HgTe quantum wells [9,10], much attention has been focused on the search for 2D TIs with larger energy gaps. Recently 2D stanene has been proposed as a promising material to realize the QSH insulator, with gaps as large as 0.3 eV, with appropriate chemical functionalization [11]. Owing to time-reversal (TR) symmetry, spin-filtered helical edge states propagate along the edges with dissipationless spin and charge currents, leading to promising potential applications in spintronics and fault-tolerant quantum computation [12].In this Rapid Communication, we propose a structure of stanene with dumbbell units (DBs) (free-standing or supported on an insulating substrate). Based on ab initio structure optimization, phonon dispersion, and band structure calculations, we predict that this structure can be stable and behave as a 2D TI. The band inversion induced by spin-orbit coupling * Corresponding author: dwh@phys.tsinghua.edu.cn † Corresponding author: angel.rubio@ehu.es (SOC) occurs at the point and the gap size can be tuned by applying compressive strain. We further show that in the realistic growth on the substrate, for example, the boron nitride (BN) sheet or reconstructed InSb(111) surface on which α-tin(111) thin films have b...

Based on first-principles calculations, we predict Bi 2 TeI, a stoichiometric compound that is synthesized, to be a weak topological insulator (TI) in layered subvalent bismuth telluroiodides. Within a bulk energy gap of 80 meV, two Dirac-cone-like topological surface states exist on the side surface perpendicular to the BiTeI layer plane. These Dirac cones are relatively isotropic due to the strong interlayer coupling, distinguished from those of previously reported weak TI candidates. Moreover, with chemically stable cladding layers, the BiTeI-Bi 2 -BiTeI sandwiched structure is a robust quantum spin Hall system, which can be obtained by simply cleaving the bulk Bi 2 TeI.

The low energy physics of both graphene and surface states of three-dimensional topological insulators is described by gapless Dirac fermions with linear dispersion. In this work, we predict the emergence of a "heavy" Dirac fermion in a graphene/topological insulator hetero-junction, where the linear term almost vanishes and the corresponding energy dispersion becomes highly non-linear. By combining ab initio calculations and an effective low-energy model, we show explicitly how strong hybridization between Dirac fermions in graphene and the surface states of topological insulators can reduce the Fermi velocity of Dirac fermions. Due to the negligible linear term, interaction effects will be greatly enhanced and can drive "heavy" Dirac fermion states into the half quantum Hall state with non-zero Hall conductance.

Based on first-principles calculations, we find that LiZnBi, a metallic hexagonal ABC compound, can be driven into a Dirac semimetal with a pair of Dirac points by strain. The nontrivial topological nature of the strained LiZnBi is directly demonstrated by calculating its Z 2 index and the surface states, where the Fermi arcs are clearly observed. The low-energy states as well as topological properties are shown to be sensitive to the strain configurations. The finding of Dirac semimetal phase in LiZnBi may intrigue further researches on the topological properties of hexagonal ABC materials and promote new practical applications.