Impulsive radiation from a horizontal electric dipole above an imperfectly conducting surface

Solutions for the impulsive wave fields generated by a horizontal electric dipole situated above an imperfectly conducting surface are derived. The space-time expressions for the reflected wave fields open the door to analysis of their properties in the far-, intermediate-, and near-field regions, and can serve as benchmark for numerical methods employed to wave simulation with applications in antenna design and radio communication. The EM properties of the conductive material are represented by a surface impedance and translated to the wave motion via employing the local plane wave relation as the boundary condition. At the core of tackling the impedance boundary value problem is the derivation of three space-time reflected-wave Green's functions. In contrast to the vertical electric dipole problem, a coupling term is present in the transform-domain wave solutions, and hinders direct application of the extended Cagniard-De Hoop method. A partial-fraction decomposition of this coupling term is the key to furnishing the transformation back to the time domain. Numerical results illustrate time traces and spectra of the measurable reflected electric field strength

Behavior of Complex Knots in Single DNA Molecules

We used optical tweezers to tie individual DNA molecules in knots. Although these knots become highly localized under tension, they remain surprisingly mobile and undergo thermal diffusion with classical random walk statistics. The diffusion constants of knots with different complexities correlate with theoretical calculations of knot sizes. We show that this correlation can be explained by a simple hydrodynamical model of "self-reptation" of the knot along a polymer

Early warning systems of financial crises: implementation of a currency crisis model for Uganda

The objective of this paper is to implement a prototype of a currency crisis model as part of an early warning system framework for Uganda. The financial systems of developing countries like Uganda are especially vulnerable and therefore robust instruments to predict crises are needed. Our model is based on the signals approach developed by Kaminsky, Lizondo and Reinhart (1998) and Kaminsky and Reinhart (1999). The basic idea of the signals approach is to monitor several indicators that tend to exhibit an unusual behaviour in the periods preceding a crisis. When an indicator crosses a threshold, then it is said to issue a signal that a currency crisis may occur within a given period. We implemented this signals approach for Uganda. One of the main challenges in this connection is that Uganda during the analyzed periods had no currency crisis. Therefore, we modified the model in a way that it estimates some of the performance measures based on empirical studies to obtain usable results. The outcomes of our calculations performed well and were economically validated. --Currency crises,Uganda,early warning systems,balance of payment crises,crisis prediction,vulnerability indicators,signals approach

Tuning hydrogen adsorption on graphene by gate voltage

In order to realize applications of hydrogen-adsorbed graphene, a main issue is how to control hydrogen adsorption/desorption at room temperature. In this study, we demonstrate the possibility to tune hydrogen adsorption on graphene by applying a gate voltage. The influence of the gate voltage on graphene and its hydrogen adsorption properties was investigated by electrical transport measurements, scanning tunneling microscopy, and density functional theory calculations. We show that more hydrogen adsorbs on graphene with negative gate voltage (p-type doping), compared to that without gate voltage or positive gate voltage (n-type doping). Theoretical calculations explain the gate voltage dependence of hydrogen adsorption as modifications of the adsorption energy and diffusion barrier of hydrogen on graphene by charge doping