Fluctuational Instabilities of Alkali and Noble Metal Nanowires

We introduce a continuum approach to studying the lifetimes of monovalent metal nanowires. By modelling the thermal fluctuations of cylindrical nanowires through the use of stochastic Ginzburg-Landau classical field theories, we construct a self-consistent approach to the fluctuation-induced `necking' of nanowires. Our theory provides quantitative estimates of the lifetimes for alkali metal nanowires in the conductance range 10 < G/G_0 < 100 (where G_0=2e^2/h is the conductance quantum), and allows us to account for qualitative differences in the conductance histograms of alkali vs. noble metal nanowires

Comment on "Nonlinear current-voltage curves of gold quantum point contacts" [Appl. Phys. Lett. 87, 103104 (2005)]

In a recent Letter [Appl. Phys. Lett. 87, 103104 (2005)], Yoshida et al. report that nonlinearities in current-voltage curves of gold quantum point contacts occur as a result of a shortening of the distance between electrodes at finite bias, presumably due to thermal expansion. For short wires, the electrode displacement induces a thickening of the wire, as well as nonlinearities of the IV curve, while the radius of long wires is left unchanged, thus resulting in a linear IV curve. We argue here that electron shell effects, which favor wires with certain "magic radii," prevent the thickening of long wires under compression, but have little effect on wires below a critical length.Comment: Version accepted for publication in Applied Physics Letter

Theory of metastability in simple metal nanowires

Thermally induced conductance jumps of metal nanowires are modeled using stochastic Ginzburg-Landau field theories. Changes in radius are predicted to occur via the nucleation of surface kinks at the wire ends, consistent with recent electron microscopy studies. The activation rate displays nontrivial dependence on nanowire length, and undergoes first- or second-order-like transitions as a function of length. The activation barriers of the most stable structures are predicted to be universal, i.e., independent of the radius of the wire, and proportional to the square root of the surface tension. The reduction of the activation barrier under strain is also determined.Comment: 5 pages, 3 figure

Frequency over function : raised levels of CD127low/- regulatory T cells in the tumour microenvironment compared with the periphery of head and neck cancer patients

Objective: Regulatory T cells (Tregs) are known to infiltrate the tumour microenvironment of many cancers, including head and neck malignancies, and are thought to contribute to the host's impaired anti-tumour immune response. However, their immunosuppressive function remains poorly understood within the tumour microenvironment and this study aimed to address this. Methods: The frequency and suppressive capacity of two CD4?CD127low/- Treg populations, separated on the basis of different levels of CD25 expression (CD25inter and CD25high), from the tumour/node microenvironment and peripheral circulation of newly-presenting head and neck squamous cell carcinoma patients (n=19), were assessed using multicolour flow cytometry. Results: The proportion of Tregs (CD4?CD25high/?interCD127low/-) in the tumour/node microenvironment was significantly elevated compared with the peripheral circulation (p&lt;0.001) and similar percentages were present in both the primary tumour and metastatic lymph node. The percentage of suppression induced by Tregs isolated from tumour associated nodes on the proliferation of nodal effector T cells was similar to that of peripheral Tregs on peripheral effector T cells. However, when the suppressive activity of both nodal and peripheral Tregs was compared on the same peripheral effectors, peripheral Tregs suppressed proliferation to a greater extent. Conclusion: This work shows that the recruitment and percentages of tumour infiltrating Tregs are key factors in modulating the immune environment of head and neck tumours

Many-body theory of electronic transport in single-molecule heterojunctions

A many-body theory of molecular junction transport based on nonequilibrium Green's functions is developed, which treats coherent quantum effects and Coulomb interactions on an equal footing. The central quantity of the many-body theory is the Coulomb self-energy matrix $\Sigma_{\rm C}$ of the junction. $\Sigma_{\rm C}$ is evaluated exactly in the sequential tunneling limit, and the correction due to finite tunneling width is evaluated self-consistently using a conserving approximation based on diagrammatic perturbation theory on the Keldysh contour. Our approach reproduces the key features of both the Coulomb blockade and coherent transport regimes simultaneously in a single unified transport theory. As a first application of our theory, we have calculated the thermoelectric power and differential conductance spectrum of a benzenedithiol-gold junction using a semi-empirical $\pi$-electron Hamiltonian that accurately describes the full spectrum of electronic excitations of the molecule up to 8--10eV.Comment: 13 pages, 7 figure

Developing a Framework to Identify Local Business and Government Vulnerability to Sea-Level Rise: A Case Study of Coastal Virginia

In this paper we develop methods for identifying local business and government vulnerabilities to sea-level rise and the natural hazards associated with it. Unlike the fairly large literature on measuring social vulnerability to natural hazards, there are very few papers that discuss methods for measuring local business or local government vulnerability even though businesses and governments are also differentially affected natural hazards. Our goal is to create measures that are easily replicable using readily available data and that are easy to explain to local planners, policy makers, and citizens. We implement our measures of local business and government vulnerability for our study area, Coastal Virginia. We then combine those measures with a physical vulnerability measure to identify the areas in Coastal Virginia where planners and policy makers need to more closely examine the potential impacts of sea-level rise on their local businesses and government. While our methods are tailored to Coastal Virginia, they could be easily applied in other areas threatened by sea-level rise

How to measure the spreading width for decay of superdeformed nuclei

A new expression for the branching ratio for the decay via the E1 process in the normal-deformed band of superdeformed nuclei is given within a simple two-level model. Using this expression, the spreading or tunneling width Gamma^downarrow for superdeformed decay can be expressed entirely in terms of experimentally known quantities. We show how to determine the tunneling matrix element V from the measured value of Gamma^downarrow and a statistical model of the energy levels. The accuracy of the two-level approximation is verified by considering the effects of the other normal-deformed states.Comment: 4 pages, 4 figure

The Order of Phase Transitions in Barrier Crossing

A spatially extended classical system with metastable states subject to weak spatiotemporal noise can exhibit a transition in its activation behavior when one or more external parameters are varied. Depending on the potential, the transition can be first or second-order, but there exists no systematic theory of the relation between the order of the transition and the shape of the potential barrier. In this paper, we address that question in detail for a general class of systems whose order parameter is describable by a classical field that can vary both in space and time, and whose zero-noise dynamics are governed by a smooth polynomial potential. We show that a quartic potential barrier can only have second-order transitions, confirming an earlier conjecture [1]. We then derive, through a combination of analytical and numerical arguments, both necessary conditions and sufficient conditions to have a first-order vs. a second-order transition in noise-induced activation behavior, for a large class of systems with smooth polynomial potentials of arbitrary order. We find in particular that the order of the transition is especially sensitive to the potential behavior near the top of the barrier.Comment: 8 pages, 6 figures with extended introduction and discussion; version accepted for publication by Phys. Rev.