348 research outputs found
Two-probe theory of scanning tunneling microscopy of single molecules: Zn(II)-etioporphyrin on alumina
We explore theoretically the scanning tunneling microscopy of single
molecules on substrates using a framework of two local probes. This framework
is appropriate for studying electron flow in tip/molecule/substrate systems
where a thin insulating layer between the molecule and a conducting substrate
transmits electrons non-uniformly and thus confines electron transmission
between the molecule and substrate laterally to a nanoscale region
significantly smaller in size than the molecule. The tip-molecule coupling and
molecule-substrate coupling are treated on the same footing, as local probes to
the molecule, with electron flow modelled using the Lippmann-Schwinger Green
function scattering technique. STM images are simulated for various positions
of the stationary (substrate) probe below a Zn(II)-etioporphyrin I molecule. We
find that these images have a strong dependence on the substrate probe
position, indicating that electron flow can depend strongly on both tip
position and the location of the dominant molecule-substrate coupling.
Differences in the STM images are explained in terms of the molecular orbitals
that mediate electron flow in each case. Recent experimental results, showing
STM topographs of Zn(II)-etioporphyrin I on alumina/NiAl(110) to be strongly
dependent on which individual molecule on the substrate is being probed, are
explained using this model. A further experimental test of the model is also
proposed.Comment: Physical Review B, in pres
Quantum master equation for electron transport through quantum dots and single molecules
A quantum master equation (QME) is derived for the many-body density matrix
of an open current-carrying system weakly coupled to two metal leads. The
dynamics and the steady-state properties of the system for arbitrary bias are
studied using projection operator techniques, which keep track of number of
electrons in the system. We show that coherences between system states with
different number of electrons, n, (Fock space coherences) do not contribute to
the transport to second order in system-lead coupling.
However, coherences between states with the same n may effect transport
properties when the damping rate is of the order or faster then the system Bohr
frequencies.
For large bias, when all the system many-body states lie between the chemical
potentials of the two leads, we recover previous results. In the rotating wave
approximation (when the damping is slow compared to the Bohr frequencies of the
system), the dynamics of populations and the coherences in the system
eigenbasis are decoupled. The QME then reduces to a birth and death master
equation for populations.Comment: 22 pages, 8 figures, paper accepted in Phys. Rev.
Theory of a Scanning Tunneling Microscope with a Two-Protrusion Tip
We consider a scanning tunneling microscope (STM) such that tunneling occurs
through two atomically sharp protrusions on its tip. When the two protrusions
are separated by at least several atomic spacings, the differential conductance
of this STM depends on the electronic transport in the sample between the
protrusions. Furthermore two-protrusion tips commonly occur during STM tip
preparation. We explore possible applications to probing dynamical impurity
potentials on a metallic surface and local transport in an anisotropic
superconductor.Comment: revtex, 11 pages, 6 figures upon reques
NO structures adsorbed on Rh(111) : theoretical approach to high-coverage STM images
Theoretical modeling of scanning tunneling microscopy (STM) measurements is used for the interpretation of images of nitrogen monoxide on Rh(111) surfaces in order to gain insight into the factors which control the contrast of an STM image, especially in the case of high coverage overlayers. Topographic images of NO/Rh(111) for different coverages and adsorption positions were calculated. These results were used to analyze the experimental images obtained for the p(2×2)-3NO and p(3×3)-7NO high coverage structures. The theoretical calculations confirm that not all NO molecules present on the surface can be observed experimentally, the image being dominated by the contribution of top NO molecules in the adlayer. In addition, the calculations reveal that destructive interference effects between molecular contributions in the tunnel current play a decisive role for the different contrast of the two high coverage structures. A general discussion of why and how the differences in the adsorbate surface configuration reflect the experimental STM images is given
Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water
International audienceThe kinetic stability of any material in water relies on the presence of surface weak spots responsible for chemical weathering by hydrolysis. Being able to identify the atomistic nature of these sites and the first steps of transformation is therefore critical to master the decomposition processes. This is the challenge that we tackle here: combining experimental and modeling studies we investigate the stability of alumina in water. Exploring the reactivity of shape-controlled crystals, we identify experimentally a specific facet as the location of the weak spots. Using biased ab initio molecular dynamics, we recognize this weak spot as a surface exposed tetra-coordinated Al atom and further provide a detailed mechanism of the first steps of hydrolysis. This understanding is of great importance to heterogeneous catalysis where alumina is a major support. Furthermore, it paves the way to atomistic understanding of interfacial reactions, at the crossroad of a variety of fields of research
An Automated Approach for Developing Graph-Theoretical Cluster Expansions of the Total Energy of Adsorbed Layers
Analytical study of non-linear transport across a semiconductor-metal junction
In this paper we study analytically a one-dimensional model for a
semiconductor-metal junction. We study the formation of Tamm states and how
they evolve when the semi-infinite semiconductor and metal are coupled
together. The non-linear current, as a function of the bias voltage, is studied
using the non-equilibrium Green's function method and the density matrix of the
interface is given. The electronic occupation of the sites defining the
interface has strong non-linearities as function of the bias voltage due to
strong resonances present in the Green's functions of the junction sites. The
surface Green's function is computed analytically by solving a quadratic matrix
equation, which does not require adding a small imaginary constant to the
energy. The wave function for the surface states is given
Microscopic origin of the conducting channels in metallic atomic-size contacts
We present a theoretical approach which allows to determine the number and
orbital character of the conducting channels in metallic atomic contacts. We
show how the conducting channels arise from the atomic orbitals having a
significant contribution to the bands around the Fermi level. Our theory
predicts that the number of conducting channels with non negligible
transmission is 3 for Al and 5 for Nb one-atom contacts, in agreement with
recent experiments. These results are shown to be robust with respect to
disorder. The experimental values of the channels transmissions lie within the
calculated distributions.Comment: 11 pages, 4 ps-figures. Submitted to Phys. Rev. Let
Coherent electron-phonon coupling and polaron-like transport in molecular wires
We present a technique to calculate the transport properties through
one-dimensional models of molecular wires. The calculations include inelastic
electron scattering due to electron-lattice interaction. The coupling between
the electron and the lattice is crucial to determine the transport properties
in one-dimensional systems subject to Peierls transition since it drives the
transition itself. The electron-phonon coupling is treated as a quantum
coherent process, in the sense that no random dephasing due to electron-phonon
interactions is introduced in the scattering wave functions. We show that
charge carrier injection, even in the tunneling regime, induces lattice
distortions localized around the tunneling electron. The transport in the
molecular wire is due to polaron-like propagation. We show typical examples of
the lattice distortions induced by charge injection into the wire. In the
tunneling regime, the electron transmission is strongly enhanced in comparison
with the case of elastic scattering through the undistorted molecular wire. We
also show that although lattice fluctuations modify the electron transmission
through the wire, the modifications are qualitatively different from those
obtained by the quantum electron-phonon inelastic scattering technique. Our
results should hold in principle for other one-dimensional atomic-scale wires
subject to Peierls transitions.Comment: 21 pages, 8 figures, accepted for publication in Phys. Rev. B (to
appear march 2001
- …