Initial stages of oxidation of Cu(1 1 1)

Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O₂ at low pressure (<5 × 10⁻⁶ mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu-Cu distance (∼0.3 nm), consistent with local Cu₂O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu₂O(1 1 1)-like structure.     

A Novel Implementation Technique of Conical Scan Radar Using A Programmable Phased Array

In radar, planar phased array antenna plays vital role in electronic scanning in the azimuth and elevation direction to the horizon. In most operations using planar phased array both the coordinates of azimuth and elevation, are steered electronically. In this paper a conceptual schematic of a phased array antenna with programmable time delay units has been presented. It is shown that by suitably exploiting the time delay matrix one can have electronic beam rotation around the target axis as required in conical scan. Thus both the elevation and azimuth motors in conical scan system are replaced by electronic scanning. Heuristically, we have selected eight consecutive points for beam rotation in a polygon shape and can also be extended almost circular shape by increasing number of array elements and phase shifter (delays) in the delay matrix. The array requires dual control of phase gradient and individual phase values. The whole array is controlled by micro-controller. This presents exciting possibilities in radar operation.     

Forward scattering of pulses from a rough sea surface by Fourier synthesis of parabolic equation solutions

A variable depth step implementation of the range-dependent acoustic model (RAM) is applied to the modeling of forward scattering from a rough sea surface. The sea surface is treated within RAM simply as an internal interface between a water layer and an air upper halfspace. A comparison with a numerically exact integral equation is undertaken for the scattering of single frequencies from Pierson-Moskowitz sea surfaces. The method is extended to model the variability of linear frequency modulated pulses from a series of frozen sea surfaces in a shallow water waveguide. The subsequent effect of rough boundary scattering on the replica correlation process is investigated.     

Low-temperature fate of the 0.7 structure in a point contact: a Kondo-like correlated state in an open system

Besides the usual conductance plateaus at multiples of 2e(2)/h, quantum point contacts typically show an extra plateau at approximately 0.7(2e(2)/h), believed to arise from electron-electron interactions that prohibit the two spin channels from being simultaneously occupied. We present evidence that the disappearance of the 0.7 structure at very low temperature signals the formation of a Kondo-like correlated spin state. Evidence includes a zero-bias conductance peak that splits in a parallel field, scaling of conductance to a modified Kondo form, and consistency between peak width and the Kondo temperature.     

Characteristic modes and the transition to chaos of a resonant Josephson circuit

The periodic modes of a voltage-driven resonant small-junction Josephson circuit are studied by accurate numerical methods starting from large dissipation. As dissipation decreases, sections of the average current vs. voltage characteristic become unstable and new branches develop on those sections, corresponding to new modes which are exact subharmonics of the old mode. For low enough dissipation chaotic ranges of voltage occur, i.e., ranges with no stable periodic modes. This circuit is a component of many experimental circuits, e.g., finite junctions, DC and RF squids, etc., and so the behavior found here should occur widely.     

Molecular orbital study of co chemisorption and oxidation on a Pt(111) surface

A molecular orbital study was made, using an atom superposition and electron delocalization (ASED) technique, of the structures and energy levels of CO on Pt(111) surface. CO is predicted to be preferentially adsorbed at a height of 2.05 Å from the surface at a 1-fold position with the carbon end down. The calculated binding energy (1.7 eV) is in good agreement with the recent experimental result (1.5 eV) of Campbell et al. Calculated binding energies for bridging (1.3 eV) and high coordinate (1.1 eV) sites are predicted to be smaller in magnitude. Calculated results are used to discuss the ordering of energy levels of adsorbed CO. The interaction between CO (adsorbed) and O (adsorbed) has been studied to estimate the energy of activation for the oxidation of CO on Pt(111) surface. The calculated activation energy (1.6 eV) is in reasonable agreement with the recent experimental result (1 eV) of Campbell et al. The Langmuir-Hinshelwood mechanism is found to be favored. We predict CO₂ bonds vertically.     

Small-scale fracture toughness of ceramic thin films: the effects of specimen geometry,ion beam notching and high temperature on chromium nitride toughness evaluation

For the implementation of thin ceramic hard coatings into intensive application environments, the fracture toughness is a particularly important material design parameter. Characterisation of the fracture toughness of small-scale specimens has been a topic of great debate, due to size effects, plasticity, residual stress effects and the influence of ion penetration from the sample fabrication process. In this work, several different small-scale fracture toughness geometries (single-beam cantilever, double-beam cantilever and micro-pillar splitting) were compared, fabricated from a thin physical vapour-deposited ceramic film using a focused ion beam source, and then the effect of the gallium-milled notch on mode I toughness quantification investigated. It was found that notching using a focused gallium source influences small-scale toughness measurements and can lead to an overestimation of the fracture toughness values for chromium nitride (CrN) thin films. The effects of gallium ion irradiation were further studied by performing the first small-scale high-temperature toughness measurements within the scanning electron microscope, with the consequence that annealing at high temperatures allows for diffusion of the gallium to grain boundaries promoting embrittlement in small-scale CrN samples. This work highlights the sensitivity of some materials to gallium ion penetration effects, and the profound effect that it can have on fracture toughness evaluation.