Two-dimensional quantum spin Hall (QSH) insulators with reasonably wide band gaps are imperative for the development of various innovative technologies. Through systematic density functional calculations and tight-binding simulations, we found that stanene on α-alumina surface may possess a sizeable topologically nontrivial band gap (~0.25 eV) at the Γ point. Furthermore, stanene is atomically bonded to but electronically decoupled from the substrate, providing high structural stability and isolated QSH states to a large extent. The underlying physical mechanism is rather general, and this finding may lead to the opening of a new vista for the exploration of QSH insulators for room temperature device applications.

We predict a new class of 3D topological insulators (TIs) in which the spin-orbit coupling (SOC) can more effectively generate band gap. Band gap of conventional TI is mainly limited by two factors, the strength of SOC and, from electronic structure perspective, the band gap when SOC is absent. While the former is an atomic property, the latter can be minimized in a generic rock-salt lattice model in which a stable crossing of bands at the Fermi level along with band character inversion occurs in the absence of SOC. Thus large-gap TIs or TIs composed of lighter elements can be expected. In fact, we find by performing first-principles calculations that the model applies to a class of double perovskites ABiXO (A = Ca, Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, surface Dirac cones are robust against the presence of dangling bond at boundary.

We present first-principles analysis of the Stark effect of CO adsorbed on an atomically sharp silver asperity, and current versus potential (I–V) characteristics of the Ag-CO-Ag junction. The analysis supports the suggestion that CO-bridged plasmonic junctions represent rectifying nanoantennas at optical frequencies and that the CO vibrational spectrum serves as a molecular voltmeter [M. Banik et al. ACS Nano 2012, 6, 10343]. The Stark effect is principally controlled by the field-induced charge redistribution between the antibonding 2π*-orbitals of CO and the s-electrons of Ag. The Stark tuning rate of the CO stretch, 1.5 × 10–6 cm–1/V cm–1, is ∼25% larger on atomically sharp asperities than on flat Ag, and remains constant over a large window of applied fields (±0.8 V/Å). As such, both sign and strength of local electric field can be quantitatively determined by the vibrational shift of CO. The I–V curve of the Ag-CO-Ag junction is nonlinear, rendering it an effective rectifier with responsivity S = (∂2 I/∂V 2)/(∂I/∂V) = −2.8 μA/V at zero bias. A more explicit treatment of rectification at optical frequencies is presented through time-dependent density functional simulations of the coupled electronic and nuclear degrees of freedom of the junction, to include dynamical impedance in the confirmation of the optical rectenna. The computed impedance correctly predicts the experimentally observed sign and magnitude of the rectified optical field, as measured by the Stark effect.

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li2.15(3)Os0.85(3)O3 (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μB, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)−Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.

Based on first-principle calculations and direct density functional theory calculations of surface bands, we predict a new class of three-dimensional (3D) Z2 topological insulators (TIs) with larger bulk bandgaps up to 0.4 eV in double perovskite materials A2TePoO6 (A = Ca, Sr, and Ba). The larger nontrivial gaps are induced by the symmetry-protected band contact along with band inversion occurring in the absence of spin-orbit coupling (SOC) making the SOC more effective than conventional TIs. The proposed materials are chemically inert and more robust to surface perturbations due to its intrinsic protection layer. This study provides the double perovskite material as a rich platform to design new TI-based electronic devices.

Topological insulators hold great potential for efficient information processing and storage. Using density functional theory calculations, we predict that a honeycomb lead monolayer can be stabilized on an AlO (0001) substrate to become topologically non-trivial with a sizeable band gap (∼0.27 eV). Furthermore, we propose to use a hexagonal boron-nitride (h-BN) monolayer as a protection for the topological states of Pb/AlO and Sn/AlO. Our findings suggest new possibilities for designing and protecting two-dimensional TIs for practical applications.

The generally observed higher hole mobility relative to electron mobility in conjugated polymers is explained with the defects and adsorbed molecular oxygen. Adsorption of the extrinsic molecular oxygen leads to that electrons are bound more tightly than holes by the traps in the originally symmetric electronic system. Hence, the mobility imbalance emerges from the asymmetric binding energies. Besides, the defects are the favored adsorption sites because the intermolecular attraction is enhanced due to stronger induced dipoledipole interaction when gap defect levels appear.