682 research outputs found
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 -810 e/cm 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
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
A simple interpretation of quantum mirages
In an interesting new experiment the electronic structure of a magnetic atom
adsorbed on the surface of Cu(111), observed by STM, was projected into a
remote location on the same surface. The purpose of the present paper is to
interpret this experiment with a model Hamiltonian, using ellipses of the size
of the experimental ones, containing about 2300 atoms. The charge distribution
for the different wavefunctions is analyzed, in particular, for those with
energy close to the Fermi energy of copper Ef. Some of them show two symmetric
maxima located on the principal axis of the ellipse but not necessarily at the
foci. If a Co atom is adsorbed at the site where the wavefunction with energy
has a maximum and the interaction is small, the main effect of the
adsorbed atom will be to split this particular wavefunction in two. The total
charge density will remain the same but the local density of states will
present a dip at Ef at any site where the charge density is large enough. We
relate the presence of this dip to the observation of quantum mirages. Our
interpretation suggests that other sites, apart from the foci of the ellipses,
can be used for projecting atomic images and also indicates the conditions for
other non magnetic adsorbates to produce mirages.Comment: 3 pages, 3 Fig
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
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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
Spin-polarized surface states close to adatoms on Cu(111)
We present a theoretical study of surface states close to 3d transition metal
adatoms (Cr, Mn, Fe, Co, Ni and Cu) on a Cu(111) surface in terms of an
embedding technique using the fully relativistic Korringa-Kohn-Rostoker method.
For each of the adatoms we found resonances in the s-like states to be
attributed to a localization of the surface states in the presence of an
impurity. We studied the change of the s-like densities of states in the
vicinity of the surface state band-edge due to scattering effects mediated via
the adatom's d-orbitals. The obtained results show that a magnetic impurity
causes spin-polarization of the surface states. In particular, the long-range
oscillations of the spin-polarized s-like density of states around an Fe adatom
are demonstrated.Comment: 5 pages, 5 figures, submitted to PR
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