Functionality in single-molecule devices: Model calculations and applications of the inelastic electron tunneling signal in molecular junctions

We analyze how functionality could be obtained within single-molecule devices by using a combination of non-equilibrium Green's functions and ab-initio calculations to study the inelastic transport properties of single-molecule junctions. First we apply a full non-equilibrium Green's function technique to a model system with electron-vibration coupling. We show that the features in the inelastic electron tunneling spectra (IETS) of the molecular junctions are virtually independent of the nature of the molecule-lead contacts. Since the contacts are not easily reproducible from one device to another, this is a very useful property. The IETS signal is much more robust versus modifications at the contacts and hence can be used to build functional nanodevices. Second, we consider a realistic model of a organic conjugated molecule. We use ab-initio calculations to study how the vibronic properties of the molecule can be controlled by an external electric field which acts as a gate voltage. The control, through the gate voltage, of the vibron frequencies and (more importantly) of the electron-vibron coupling enables the construction of functionality: non-linear amplification and/or switching is obtained from the IETS signal within a single-molecule device.Comment: Accepted for publication in Journal of Chemical Physic

Non-equilibrium inelastic electronic transport: Polarization effects and vertex corrections to the self-consistent Born approximation

We study the effect of electron-vibron interactions on the inelastic transport properties of single-molecule nanojunctions. We use the non-equilibrium Green's functions technique and a model Hamiltonian to calculate the effects of second-order diagrams (double-exchange DX and dressed-phonon DPH diagrams) on the electron-vibration interaction and consider their effects across the full range of parameter space. The DX diagram, corresponding to a vertex correction, introduces an effective dynamical renormalization of the electron-vibron coupling in both the purely inelastic and the inelastic-resonant features of the IETS. The purely inelastic features correspond to an applied bias around the energy of a vibron, while the inelastic-resonant features correspond to peaks (resonance) in the conductance. The DPH diagram affects only the inelastic resonant features. We also discuss the circumstances in which the second-order diagrams may be approximated in the study of more complex model systems.Comment: To be published in PR

Exploring Botnet Evolution via Multidimensional Models and Visualisation

A botnet is a program designed to perform a specific task using multiple computers connected in a network. In this paper we will focus on botnets being used to distribute malicious programs. In the real world, botnets have been shown to exhibit more aggressive and sophisticated behaviour than traditional malware. Botnets are used to infect computer networks and hence their success depends on the properties of the networks. We observe the behaviour of mathematical models used to describe botnets when botnet parameters are varied to understand if such variation is beneficial to their spread. We also introduce novel models for depicting botnet behaviour using master equations. These models, unlike previous ones, address nodes of distinct categories in a network as a sequence of probability distributions rather than a value at each time interval. We also contribute visualisations for these models. This paper is a substantial expansion of unpublished work the first author performed while on a Nuffield student research placement, with the second author the project supervisor

The ART of IAM: The Winning Strategy for the 2006 Competition

In many dynamic open systems, agents have to interact with one another to achieve their goals. Here, agents may be self-interested, and when trusted to perform an action for others, may betray that trust by not performing the actions as required. In addition, due to the size of such systems, agents will often interact with other agents with which they have little or no past experience. This situation has led to the development of a number of trust and reputation models, which aim to facilitate an agent's decision making in the face of uncertainty regarding the behaviour of its peers. However, these multifarious models employ a variety of different representations of trust between agents, and measure performance in many different ways. This has made it hard to adequately evaluate the relative properties of different models, raising the need for a common platform on which to compare competing mechanisms. To this end, the ART Testbed Competition has been proposed, in which agents using different trust models compete against each other to provide services in an open marketplace. In this paper, we present the winning strategy for this competition in 2006, provide an analysis of the factors that led to this success, and discuss lessons learnt from the competition about issues of trust in multiagent systems in general. Our strategy, IAM, is Intelligent (using statistical models for opponent modelling), Abstemious (spending its money parsimoniously based on its trust model) and Moral (providing fair and honest feedback to those that request it)

Implementing the Affordable Care Act: State Action to Establish SHOP Marketplaces

The Affordable Care Act seeks to help small employers offer coverage by reforming the small-group market and establishing Small Business Health Options Program (SHOP) marketplaces. Seventeen states and the District of Columbia chose to operate their own SHOP marketplaces in 2014, with the federal government operating the SHOP marketplace in 33 states. This brief examines state decisions to enhance the value of SHOP marketplaces for small employers and finds that most have set predictable participation and eligibility requirements and will offer a competitive choice of insurers and plans. States also are seeking to facilitate small employers' shopping experience through online tools and access to personalized assistance. While not all SHOP marketplaces are yet functioning as intended, their establishment offers an opportunity to identify successful strategies for improving the affordability and accessibility of coverage for small employers

Adsorption and Specific-Heat Studies of Monolayer and Submonolayer Films of He3 and He4

A study has been made of the adsorption of He3 and He4 at 4°K on a substrate consisting of a monolayer of argon adsorbed on a sintered copper sponge. The isotherms display distinct steps indicating the completion of first and second adsorbed layers. Comparisons among the adsorption isotherms of helium and of Ar and N2 at 77.4°K yield a self-consistent set of molecular areas. Measurements have been made of the specific heat of five submonolayer coverages of He3 and He4 on Ar-plated Cu sponge. The heat capacities of nearly complete monolayers vary as T2 from 0.3 to 4°K, yielding two-dimensional Debye temperatures Theta (He4)=28±1°K, and Theta (He3)=31±1°K. At lower coverages the molar heat capacities increase and develop contributions linear in T below 1°K. At an intermediate coverage, the heat capacity of He4 exhibits a broad and pronounced maximum near 3°K. Possible mechanisms for the linear terms and the maximum are discussed briefly. Evidence for considerable mobility of He atoms along the surface is adduced from the temperature and coverage dependence of the heat capacity. The T2 behavior for the complete monolayers yields an upper limit of ~10^-11 sec for the lifetime of a He atom in any individual adsorption site, consistent with a theoretical estimate

Non-Collinear Ferromagnetic Luttinger Liquids

The presence of electron-electron interactions in one dimension profoundly changes the properties of a system. The separation of charge and spin degrees of freedom is just one example. We consider what happens when a system consisting of a ferromagnetic region of non-collinearity, i.e. a domain wall, is coupled to interacting electrons in one-dimension (more specifically a Luttinger liquid). The ferromagnetism breaks spin charge separation and the presence of the domain wall introduces a spin dependent scatterer into the problem. The absence of spin charge separation and the effects of the electron correlations results in very different behaviour for the excitations in the system and for spin-transfer-torque effects in this model.Comment: 6 pages, submitted to Journal of Physics: Conference Series for JEMS 201

Which rhizobia nodulate which legumes in New Zealand soils?

Recent work which genotypically characterised rhizobia of native, crop and weed legumes in New Zealand and examined their cross-nodulation ability is reviewed and related to earlier work with focus on New Zealand pasture systems. The New Zealand native legumes were exclusively effectively nodulated by novel strains of Mesorhizobium which did not nodulate crop or weed legumes. Clovers, lucerne, Lotus and grain legumes were effectively nodulated by different genera, species and biovars of rhizobia primarily originating from inoculum. Rhizobial symbionts of white clover have established over wide areas in New Zealand. Weed legumes are effectively nodulated by different genera/species of rhizobia depending on species. Bradyrhizobia that cross-nodulate lupins, gorse, European broom and tagasaste are widespread in New Zealand

Xe films on a decagonal Al-Ni-Co quasicrystal surface

The grand canonical Monte Carlo method is employed to study the adsorption of Xe on a quasicrystalline Al-Ni-Co surface. The calculation uses a semiempirical gas-surface interaction, based on conventional combining rules and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. In this paper we focus on five features not discussed earlier (Phys. Rev. Lett. 95, 136104 (2005)): the range of the average density of the adsorbate, the order of the transition, the orientational degeneracy of the ground state, the isosteric heat of adsorption of the system, and the effect of the vertical cell dimension.Comment: 6 pages, 5 pic