Ab initio study of a mechanically gated molecule: From weak to strong correlation

The electronic spectrum of a chemically contacted molecule in the junction of a scanning tunneling microscope can be modified by tip retraction. We analyze this effect by a combination of density functional, many-body perturbation and numerical renormalization group theory, taking into account both the non-locality and the dynamics of electronic correlation. Our findings, in particular the evolution from a broad quasiparticle resonance below to a narrow Kondo resonance at the Fermi energy, correspond to the experimental observations.Comment: 4 pages, 3 figure

Imaging Pauli repulsion in scanning tunneling microscopy

A scanning tunneling microscope (STM) has been equipped with a nanoscale force sensor and signal transducer composed of a single D2 molecule that is confined in the STM junction. The uncalibrated sensor is used to obtain ultra-high geometric image resolution of a complex organic molecule adsorbed on a noble metal surface. By means of conductance-distance spectroscopy and corresponding density functional calculations the mechanism of the sensor/transducer is identified. It probes the short-range Pauli repulsion and converts this signal into variations of the junction conductance.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let

Electrical transport through a mechanically gated molecular wire

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green's functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment.Comment: 31 pages, 10 figure

Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an Unconventional Orientation on SiC

We report the use of a surfactant molecule during the epitaxy of graphene on SiC(0001) that leads to the growth in an unconventional orientation, namely $R0^\circ$ rotation with respect to the SiC lattice. It yields a very high-quality single-layer graphene with a uniform orientation with respect to the substrate, on the wafer scale. We find an increased quality and homogeneity compared to the approach based on the use of a pre-oriented template to induce the unconventional orientation. Using spot profile analysis low energy electron diffraction, angle-resolved photoelectron spectroscopy, and the normal incidence x-ray standing wave technique, we assess the crystalline quality and coverage of the graphene layer. Combined with the presence of a covalently-bound graphene layer in the conventional orientation underneath, our surfactant-mediated growth offers an ideal platform to prepare epitaxial twisted bilayer graphene via intercalation.Comment: 7 pages, 3 figure

Site-selective adsorption of naphthalene-tetracarboxylic-dianhydride on Ag(110): First-principles calculations

The mechanism of adsorption of the 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) molecule on the Ag(110) surface is elucidated on the basis of extensive density functional theory calculations. This molecule, together with its perylene counterpart, PTCDA, are archetype organic semiconductors investigated experimentally over the past 20 years. We find that the bonding of the molecule to the substrate is highly site-selective, being determined by electron transfer to the LUMO of the molecule and local electrostatic attraction between negatively charged carboxyl oxygens and positively charged silver atoms in [1-10] atomic rows. The adsorption energy in the most stable site is 0.9eV. A similar mechanism is expected to govern the adsorption of PTCDA on Ag(110) as well.Comment: 8 pages, 4 figures, high-quality figures available upon reques

Kondo effect by controlled cleavage of a single molecule contact

Conductance measurements of a molecular wire, contacted between an epitaxial molecule-metal bond and the tip of a scanning tunneling microscope, are reported. Controlled retraction of the tip gradually de-hybridizes the molecule from the metal substrate. This tunes the wire into the Kondo regime in which the renormalized molecular transport orbital serves as spin impurity at half filling and the Kondo resonance opens up an additional transport channel. Numerical renormalization group simulations suggest this type of behavior to be generic for a common class of metal molecule bonds. The results demonstrate a new approach to single-molecule experiments with atomic-scale contact control and prepare the way for the ab initio simulation of many-body transport through single-molecule junctions.Comment: Main text: 41 pages including references and captions, 9 figures. Supplementary information: 5 pages including 2 figures New experimental and theoretical data supporting initial claims are added. The paper has been reworked from the letter format into a longer versio

Quantification of finite-temperature effects on adsorption geometries of π\pi-conjugated molecules

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of non-local collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial.Comment: 12 pages, 3 figure

A novel high-current, high-resolution, low-kinetic-energy electron source for inverse photoemission spectroscopy

A high-current electron source for inverse photoemission spectroscopy (IPES) is described. The source comprises a thermal cathode electron emission system, an electrostatic deflector-monochromator, and a lens system for variable kinetic energy (1.6 - 20 eV) at the target. When scaled to the energy resolution, the electron current is an order of magnitude higher than that of previously described electron sources developed in the context of electron energy loss spectroscopy. Surprisingly, the experimentally measured energy resolution turned out to be significantly better than calculated by standard programs, which include the electron-electron repulsion in the continuum approximation. The achieved currents are also significantly higher than predicted. We attribute this "inverse Boersch-effect" to a mechanism of velocity selection in the forward direction by binary electron-electron collisions

Dynamical bi-stability of single-molecule junctions: A combined experimental/theoretical study of PTCDA on Ag(111)

The dynamics of a molecular junction consisting of a PTCDA molecule between the tip of a scanning tunneling microscope and a Ag(111) surface have been investigated experimentally and theoretically. Repeated switching of a PTCDA molecule between two conductance states is studied by low-temperature scanning tunneling microscopy for the first time, and is found to be dependent on the tip-substrate distance and the applied bias. Using a minimal model Hamiltonian approach combined with density-functional calculations, the switching is shown to be related to the scattering of electrons tunneling through the junction, which progressively excite the relevant chemical bond. Depending on the direction in which the molecule switches, different molecular orbitals are shown to dominate the transport and thus the vibrational heating process. This in turn can dramatically affect the switching rate, leading to non-monotonic behavior with respect to bias under certain conditions. In this work, rather than simply assuming a constant density of states as in previous works, it was modeled by Lorentzians. This allows for the successful description of this non-monotonic behavior of the switching rate, thus demonstrating the importance of modeling the density of states realistically.Comment: 20 pages, 6 figures, 1 tabl

A Generalized Diffusion Tensor for Fully Anisotropic Diffusion of Energetic Particles in the Heliospheric Magnetic Field

The spatial diffusion of cosmic rays in turbulent magnetic fields can, in the most general case, be fully anisotropic, i.e. one has to distinguish three diffusion axes in a local, field-aligned frame. We reexamine the transformation for the diffusion tensor from this local to a global frame, in which the Parker transport equation for energetic particles is usually formulated and solved. Particularly, we generalize the transformation formulas to allow for an explicit choice of two principal local perpendicular diffusion axes. This generalization includes the 'traditional' diffusion tensor in the special case of isotropic perpendicular diffusion. For the local frame, we motivate the choice of the Frenet-Serret trihedron which is related to the intrinsic magnetic field geometry. We directly compare the old and the new tensor elements for two heliospheric magnetic field configurations, namely the hybrid Fisk and the Parker field. Subsequently, we examine the significance of the different formulations for the diffusion tensor in a standard 3D model for the modulation of galactic protons. For this we utilize a numerical code to evaluate a system of stochastic differential equations equivalent to the Parker transport equation and present the resulting modulated spectra. The computed differential fluxes based on the new tensor formulation deviate from those obtained with the 'traditional' one (only valid for isotropic perpendicular diffusion) by up to 60% for energies below a few hundred MeV depending on heliocentric distance.Comment: 8 pages, 6 figures, accepted in Ap