1. Ballistic conductance calculation of atomic-scale nanowires of Au and Co Peter Bennett, Arizona State University, ECS-0304682 State-of-the-art electron transport calculations have been made for metallic nanowires consisting of few-atom metal chains, in a zig-zag configuration that allows a range of inter-atom spacing while preserving bond-lengths. Cobalt and gold were chosen to investigate the effect of localized states (d and f levels) and to investigate the role of spin, both of which are outstanding issues in the topic of quantum point contacts. It was found that Co conductance is quantized, but only in the majority channel. In contrast, Au is not spin polarized, and shows well-behaved quantization, but only for chains less than 3 atoms wide. In addition, the zigzag Au chain can show a Peierls instability, making it insulating. These results help explain recent measurements for quantum point contacts. Ke, van Schilfgaarde, Kotani & Bennett, Nanotechnology 18 (2007) 095709 Top: Conductance quantization as a function of wire cross section. Bottom: Band structure and ball-and-stick model of the Au chains with different surface roughness: (a) planar geometry; (b) rotates every Au pair by 90 ◦ around the z axis; (c) rotates every Au pair by 180 ◦.

2. Atomic Calculation of Au Nanowire Break-Junction Peter Bennett, Arizona State University, ECS-0304682 Nanoscale break-junctions are widely used to study the electrical properties of single molecules by briefly capturing them between nanoscale electrodes during rupture of the junction. In this project, state-of-the-art calculations have been used to explore the mechanical behavior of a gold nanowire break-junction. Unlike most calculations, lattice motions are included. It is found that atom motions perpendicular to the wire axis are significant at room temperature. The lattice motion causes wire breakage via a thermal-assisted process, which occurs at a lower energy than for the normally assumed linear chain. The magnitude of this “dynamic bond-breaking” process has been calculated and agrees favorably with experimental measurements from the same group (see next Highlight). Ke, van Schilfgaarde, Kotani & Bennett, Nanotechnology (accepted). (a) (b)(c) Figure. (a) Ball-and-stick model of the Au chain between two electrodes for an electrode spacing Z = 20.77 Å, following the constrained energy minimization and deformation path described in the text. The position profile of the central chain atom (labeled B), is plotted. Other atoms plotted are in their initial relaxed condition before the pulling down process. (b) Eigenvector of the lowest phonon mode. (c) Total energy as a function of z coordinate displacement of atom B from its initial position z 0.

3. Current-induced Local Heating in Single Molecule Junctions Peter Bennett, Arizona State University, ECS-0304682 Huang, Chen, D’ Agosta, Bennett, Di Ventra & Tao. Figure. Effective temperature (T eff ) in single alkanedithiol junctions (HS-(CH 2 ) n -SH, or C n, n=6, 8, 10) vs. bias voltage. The experiment was performed in toluene at the room temperature. The stretching rate was fixed at 20 nm/s. The Inset is a schematic illustration of the formation of single alkanedithiol junction (Au/S-(CH 2 ) n -S/Au) by STM break junction approach. Current-induced local-heating in a molecular device is a fundamentally and technologically important problem. We have measured local-heating in single molecules (n-alkanedithiol, n=6, 8 and 10) covalently attached to two gold electrodes as a function of applied bias voltage and molecular length. The effective temperature of the molecular junctions, due to local heating, increases with the applied bias and then decreases after reaching a maximum. At a given bias, the effective temperature decreases with the molecular length. These experimental findings are in agreement with the theoretical prediction based on a hydrodynamic approach including both electron-phonon and electron- electron interactions.