http://myweb.rz.uni-augsburg.de/~eckern/WEH-287/ABSTRACTS/ruitenbeek.pdf

Quantum properties of atomic-sized conductors

Carbon nanotubes1,2 are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge3. Defects are known to modify the electrical resistance of carbon nanotubes4; they can be present in as-grown carbon nanotubes, but controlling their density externally opens a path towards the tuning of the electronic characteristics of the nanotube. In this work, consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room-temperature resistance versus SWNT length (R(L)) along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation, and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a SWNT of 400 nm length.

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Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime

Carbon nanotubes are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge. Defects are known to modify the electrical resistance of carbon nanotubes. They can be present in as-grown carbon nanotubes, but controlling externally their density opens a path towards the tuning of the nanotube electronic characteristics. In this work consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room temperature resistance versus SWNT-length [R(L)] along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a 400 nm SWNT length.

Electrical currents flowing in ferromagnetic materials are spin-polarised as a result of the spin-dependent band structure. When the spatial direction of the polarisation changes, in a domain structure, the electrons must somehow accommodate the necessary change in direction of their spin angular momentum as they pass through the wall. Reflection, scattering, or a transfer of angular momentum onto the lattice are all possible outcomes, depending on the circumstances. This gives rise to a variety of different physical effects, most importantly a contribution to the electrical resistance caused by the wall, and a motion of the wall driven by the spin-polarised current. Historical and recent research on these topics is reviewed. Contents PAGE 1. Introduction5862. Spin-polarised current587 2.1. Tunnelling current spin polarisation589 2.2. Ballistic current spin polarisation592 2.3. Diffusive current spin polarisation5933. Magnetic Domain Walls598 3.1. Basics of domain walls598 3.1.1. Domain wall thickness and...

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Spin-polarised currents and magnetic domain walls

One of the outstanding advancements in electronic-structure density-functional methods is the Sankey–Niklewski (SN) approach [Sankey and Niklewski, Phys. Rev. B 40, 3979 (1989)]; a method for computing total energies and forces, within an ab initio tight-binding formalism. Over the past two decades, several improvements to the method have been proposed and utilized to calculate materials ranging from biomolecules to semiconductors. In particular, the improved method (called FIREBALL) uses separable pseudopotentials and goes beyond the minimal sp3 basis set of the SN method, allowing for double numerical (DN) basis sets with the addition of polarization orbitals and d-orbitals to the basis set. Herein, we report a review of the method, some improved theoretical developments, and some recent application to a variety of systems.

Advances and applications in the FIREBALL ab initio tight-binding molecular-dynamics formalism

We present a theory for the scanning tunneling microscope (STM) current based on a Keldysh Green function formalism. In our formalism, we solve self-consistently an ab initio linear combination of atomic orbitals Hamiltonian within a local density formalism. Total energy calculations for xenon deposited on metal surfaces are performed to obtain the equilibrium position, and the Green functions needed to compute the current are obtained at the same time. Structural and nonstructural effects that can influence the correct interpretation of experimental STM results are studied. We find good agreement between our calculations and experimental images taken under highly controlled conditions, and we conclude that STM images should be analyzed by comparing iteratively the theory and the experiment, much in the same way as it is usually done for other surface sensitive techniques like low-energy electron diffraction, photoelectron diffraction, surface-extended x-ray-absorption fine structure spectroscopy, etc. \textcopyright{} 1996 The American Physical Society.

Theory of the scanning tunneling microscope: Xe on Ni and Al.

Conductance steps for atomic point contacts of Au, Ni, and Pt have been measured. Jump-to-contact and jump-to-tunnel processes have been identified and their conductances measured. Differences between conductance steps for noble and transition metals are interpreted as being due to the d orbitals that, in transition metals, provide new channels to the electron conductance. This interpretation is supported by a theoretical analysis, which shows good agreement with the experimental data. @S0163-1829~96!03323-1#

Conductance step for a single-atom contact in the scanning tunneling microscope: Noble and transition metals.

The influence of the shape and electronic structure of the tip in scanning tunneling spectroscopy is studied by a combined theoretical and experimental approach. Tunneling conductance spectra of a Cu(111) surface are used to detect and characterize the electronic states of W tips. The characteristic tunneling conductance of the Cu(111) surface state is the only feature present in spectra recorded with a blunt tip, while spectra obtained with sharp tips display also tip resonances. By means of a self-consistent tight-binding model of realistic tip structures we show that a sharp W tip exhibits, in contrast with blunt tips, narrow resonant states around the Fermi level.

N. Mingo

Papers

Theory of the scanning tunneling microscope: Xe on Ni and Al.

Conductance step for a single-atom contact in the scanning tunneling microscope: Noble and transition metals.

Electron Resonances in Sharp Tips and Their Role in Tunneling Spectroscopy

Adsorption of noble gases on metal surfaces and the scanning tunneling microscope

Lateral forces and atomic desorption induced by the electric field created by STM tips on metal surfaces