Tunable Dirac Electron and Hole Self-Doping of Topological Insulators Induced by Stacking Defects

Via density functional theory based calculations we show that self-doping of the surface Dirac cones in three-dimensional Bi₂X₃ (X = Se, Te) topological insulators can be tuned by controlling the sequence of stacking defects in the crystal. Twin boundaries inside the Bi₂X₃ bulk drive either n- or p-type self-doping of the (0001) topological surface states, depending on the precise orientation of the twin. The surface doping may achieve values up to 300 meV and can be controlled by the number of defects and their relative position with respect to the surface. Its origin relies on the spontaneous polarization generated by the dipole moments associated with the lattice defects. Our findings open the route to the fabrication of Bi₂X₃ surfaces with tailored surface charge and spin densities in the absence of external electric fields. In addition, in a thin film geometry two-dimensional electron and hole Dirac gases with the same spin-helicity coexist at opposite surfaces

Scanning Tunneling Microscopy Study of the Structure and Interaction between Carbon Monoxide and Hydrogen on the Ru(0001) Surface

We use scanning tunneling microscopy (STM) to investigate the spatial arrangement of carbon monoxide (CO) and hydrogen (H) coadsorbed on a model catalyst surface, Ru(0001). We find that at cryogenic temperatures, CO forms small triangular islands of up to 21 molecules with hydrogen segregated outside of the islands. Furthermore, whereas for small island sizes (3–6 CO molecules) the molecules adsorb at <i>hcp</i> sites, a registry shift toward <i>top</i> sites occurs for larger islands (10–21 CO molecules). To characterize the CO structures better and to help interpret the data, we carried out density functional theory (DFT) calculations of the structure and simulations of the STM images, which reveal a delicate interplay between the repulsions of the different species

Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport

Controlling the spin of electrons in nanoscale electronic devices is one of the most promising topics aiming at developing devices with rapid and high density information storage capabilities. The interface magnetism or <i>spinterface</i> resulting from the interaction between a magnetic molecule and a metal surface, or <i>vice versa</i>, has become a key ingredient in creating nanoscale molecular devices with novel functionalities. Here, we present a single-molecule wire that displays large (>10000%) conductance switching by controlling the spin-dependent transport under ambient conditions (room temperature in a liquid cell). The molecular wire is built by trapping individual spin crossover Fe^II complexes between one Au electrode and one ferromagnetic Ni electrode in an organic liquid medium. Large changes in the single-molecule conductance (>100-fold) are measured when the electrons flow from the Au electrode to either an α-up or a β-down spin-polarized Ni electrode. Our calculations show that the current flowing through such an interface appears to be strongly spin-polarized, thus resulting in the observed switching of the single-molecule wire conductance. The observation of such a high spin-dependent conductance switching in a single-molecule wire opens up a new door for the design and control of spin-polarized transport in nanoscale molecular devices at room temperature