Spin-layer locked gapless states in gated bilayer graphene.

Gated bilayer graphene exhibits spin-degenerate gapless states with a topological character localized at stacking domain walls. These states allow for one-dimensional currents along the domain walls. We herein demonstrate that these topologically protected currents are spin-polarized and locked in a single layer when bilayer graphene contains stacking domain walls decorated with magnetic defects. The magnetic defects, which we model as π-vacancies, perturb the topological states but also lift their spin degeneracy. One gapless state survives the perturbation of these defects, and its spin polarization is largely localized in one layer. The spin-polarized current in the topological state flows in a single layer, and this finding suggests the possibility of effectively exploiting these states in spintronic applications.

Controlling the layer localization of gapless states in bilayer graphene with a gate voltage.

Experiments in gated bilayer graphene with stacking domain walls present topological gapless states protected by no-valley mixing. Here we research these states under gate voltages using atomistic models, which allow us to elucidate their origin. We find that the gate potential controls the layer localization of the two states, which switches non-trivially between layers depending on the applied gate voltage magnitude. We also show how these bilayer gapless states arise from bands of single-layer graphene by analyzing the formation of carbon bonds between layers. Based on this analysis we provide a model Hamiltonian with analytical solutions, which explains the layer localization as a function of the ratio between the applied potential and interlayer hopping. Our results open a route for the manipulation of gapless states in electronic devices, analogous to the proposed writing and reading memories in topological insulators.

Existence of nontrivial topologically protected states at grain boundaries in bilayer graphene: signatures and electrical switching.

Recent experiments [L. Ju, et al., Nature, 2015, 520, 650] confirm the existence of gapless states at domain walls created in gated bilayer graphene, when the sublattice stacking is changed from AB to BA. These states are significant because they are topologically protected, valley-polarized and give rise to conductance along the domain wall. Current theoretical models predict the appearance of such states only at domain walls, which preserve the sublattice order. Here we show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge in unexpected geometries, e.g., at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon-double pentagon defects that mix graphene sublattices. We demonstrate that gap states are preserved even with pentagonal defects. Remarkably, unlike previous predictions, the number of gap states changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect, linked to defect states, should be detectable in transport measurements and could be exploited in electrical switches.

Octagonal defects at carbon nanotube junctions.

We investigate knee-shaped junctions of semiconductor zigzag carbon nanotubes. Two dissimilar octagons appear at such junctions; one of them can reconstruct into a pair of pentagons. The junction with two octagons presents two degenerate localized states at Fermi energy (E(F)). The reconstructed junction has only one state near E(F), indicating that these localized states are related to the octagonal defects. The inclusion of Coulomb interaction splits the localized states in the junction with two octagons, yielding an antiferromagnetic system.

Effect of mechanical strain on the optical properties of quantum dots: controlling exciton shape, orientation, and phase with a mechanical strain.

We show how a nanomechanical strain can be used to dynamically reengineer the optics of quantum dots, giving a tool to manipulate mechanoexciton shape, orientation, fine structure splitting, and optical transitions, transfer carriers between dots, and interact qubits for quantum processing. Most importantly, a nanomechanical strain reengineers both the magnitude and phase of the exciton exchange coupling to tune exchange splittings, change the phase of spin mixing, and rotate the polarization of mechanoexcitons, providing phase and energy control of excitons.

Surface effects on capped and uncapped nanocrystals.

Surface effects significantly influence the functionality of semiconductor nanocrystals. A theoretical understanding of these effects requires an atomic-scale description of the surface. We present an atomistic tight-binding theory of the electronic and optical properties of passivated and unpassivated CdS nanocrystals and CdS/ZnS core/shell nanocrystals. Fully passivated dots, with all dangling bonds saturated, have no surface states in the fundamental band gap, and all near-band-edge states are quantum-confined internal states. When surface anion dangling bonds are unpassivated, an anion-derived, narrow (bandwidth 0.05 eV), surface-state band lies 0.5 eV above the valence band edge, and a broader (0.2 eV) band of back-bonded surface states exists in the gap just above the valence band edge. When surface cation dangling bonds are unpassivated, a broad band of mixed surface/internal states exists above the conduction band edge. Partial passivation can push internal levels above the internal levels of a fully passivated dot or into the band gap. Because of this sensitivity to passivation, explicit models for surface effects are needed to describe accurately internal states. Capping the CdS dot with ZnS reduces the effect of the surface on the internal electronic states and optical properties. Six monolayers of ZnS are needed to eliminate the influence of any surface states on the internal states.

Accelerating Scientific Discovery Through Computation and Visualization II.

This is the second in a series of articles describing a wide variety of projects at NIST that synergistically combine physical science and information science. It describes, through examples, how the Scientific Applications and Visualization Group (SAVG) at NIST has utilized high performance parallel computing, visualization, and machine learning to accelerate research. The examples include scientific collaborations in the following areas: (1) High Precision Energies for few electron atomic systems, (2) Flows of suspensions, (3) X-ray absorption, (4) Molecular dynamics of fluids, (5) Nanostructures, (6) Dendritic growth in alloys, (7) Screen saver science, (8) genetic programming.