Lifetimes of Stark-shifted image states

The inelastic lifetimes of electrons in image-potential states at Cu(100) that are Stark-shifted by the electrostatic tip-sample interaction in the scanning tunneling microscope are calculated using the many-body GW approximation. The results demonstrate that in typical tunneling conditions the image state lifetimes are significantly reduced from their field-free values. The Stark-shift to higher energies increases the number of inelastic scattering channels that are available for decay, with field-induced changes in the image state wave function increasing the efficiency of the inelastic scattering through greater overlap with final state wave functions.Comment: 10 pages, 4 figure

Simulating the nanomechanical response of cyclooctatetraene molecules on a graphene device

We investigate the atomic and electronic structures of cyclooctatetraene (COT) molecules on graphene and analyze their dependence on external gate voltage using first-principles calculations. The external gate voltage is simulated by adding or removing electrons using density functional theory (DFT) calculations. This allows us to investigate how changes in carrier density modify the molecular shape, orientation, adsorption site, diffusion barrier, and diffusion path. For increased hole doping COT molecules gradually change their shape to a more flattened conformation and the distance between the molecules and graphene increases while the diffusion barrier drastically decreases. For increased electron doping an abrupt transition to a planar conformation at a carrier density of -8$\times$10$^{13}$ e/cm$^2$ is observed. These calculations imply that the shape and mobility of adsorbed COT molecules can be controlled by externally gating graphene devices

Mirage phenomena in superconducting quantum corrals

We investigate the local density of states and the order parameter structure inside an elliptic quantum corral on surfaces of isotropic and anisotropic superconductors. The Bogoliubov-de Gennes equations are solved in the presence of non-magnetic and magnetic impurities. We observe and discuss a variety of mirage and anti-mirage phenomena, which specifically reflect the nature of the superconducting pairing state.Comment: 8 pages, 8 figure

Geometry and electronic structure of iridium adsorbed on graphene

We report investigation of the geometry and electronic structure of iridium atoms adsorbed onto graphene through a combined experimental and theoretical study. Ir atoms were deposited onto a flake of graphene on a Pt(111) surface and found to form clusters even at low temperatures. The areal density of the observed clusters on the graphene flake suggests the clusters are most likely pairs of Ir atoms. Theoretical ab initio density functional (DFT) calculations indicate that these Ir dimers are oriented horizontally, near neighboring "bridge" sites of the graphene lattice, as this configuration has the strongest adsorption energy of all high-symmetry configurations for the Ir dimer. A large peak in the local density of states (LDOS) at the Dirac point energy was measured via scanning tunneling spectroscopy, and this result is reproduced by a DFT calculation of the LDOS. The peak at the Dirac point energy is found to be from the Ir s and p states. The LDOS in the monomer case was also calculated, and is found to significantly differ from the experimentally determined data, further supporting the hypothesis of low-temperature clustering

Disorder induced local density of states oscillations on narrow Ag(111) terraces

The local density of states of Ag(111) has been probed in detail on disordered terraces of varying width by dI/dV-mapping with a scanning tunneling microscope at low temperatures. Apparent shifts of the bottom of the surface-state band edge from terrace induced confinement are observed. Disordered terraces show interesting contrast reversals in the dI/dV maps as a function of tip-sample voltage polarity with details that depend on the average width of the terrace and the particular edge profile. In contrast to perfect terraces with straight edges, standing wave patterns are observed parallel to the step edges, i.e. in the non-confined direction. Scattering calculations based on the Ag(111) surface states reproduce these spatial oscillations and all the qualitative features of the standing wave patterns, including the polarity-dependent contrast reversals.Comment: 19 pages, 12 figure

Ab initio study of mirages and magnetic interactions in quantum corrals

The state of the art ab initio calculations of quantum mirages,the spin-polarization of surface-state electrons and the exchange interaction between magnetic adatoms in Cu and Co corrals on Cu(111) are presented. We find that the spin-polarization of the surface-state electrons caused by magnetic adatoms can be projected to a remote location and can be strongly enhanced in corrals compared to an open surface.Our studies give a clear evidence that quantum corrals could permit to tailor the exchange interaction between magnetic adatoms at large separations. The spin-polarization of surface-state electrons at the empty focus in the Co corral used in the experimental setup of Manoharan et al., (Nature 403, 512 (2000)) is revealed.Comment: Submitted to Physical Review Letter

Magnetic properties of Quantum Corrals from first principles calculations

We present calculations for electronic and magnetic properties of surface states confined by a circular quantum corral built of magnetic adatoms (Fe) on a Cu(111) surface. We show the oscillations of charge and magnetization densities within the corral and the possibility of the appearance of spin--polarized states. In order to classify the peaks in the calculated density of states with orbital quantum numbers we analyzed the problem in terms of a simple quantum mechanical circular well model. This model is also used to estimate the behaviour of the magnetization and energy with respect to the radius of the circular corral. The calculations are performed fully relativistically using the embedding technique within the Korringa-Kohn-Rostoker method.Comment: 14 pages, 9 figures, submitted to J. Phys. Cond. Matt. special issue on 'Theory and Simulation of Nanostructures

Reversible writing of high-mobility and high-carrier-density doping patterns in two-dimensional van der Waals heterostructures

A key feature of two-dimensional materials is that the sign and concentration of their carriers can be externally controlled with techniques such as electrostatic gating. However, conventional electrostatic gating has limitations, including a maximum carrier density set by the dielectric breakdown, and ionic liquid gating and direct chemical doping also suffer from drawbacks. Here, we show that an electron-beam-induced doping technique can be used to reversibly write high-resolution doping patterns in hexagonal boron nitride-encapsulated graphene and molybdenum disulfide (MoS2) van der Waals heterostructures. The doped MoS2 device exhibits an order of magnitude decrease of subthreshold swing compared with the device before doping, whereas the doped graphene devices demonstrate a previously inaccessible regime of high carrier concentration and high mobility, even at room temperature. We also show that the approach can be used to write high-quality p–n junctions and nanoscale doping patterns, illustrating that the technique can create nanoscale circuitry in van der Waals heterostructures