Production of Millisecond Dips in Sco X-1 Count Rates by Dead Time Effects

Chang et al. (2006) reported millisecond duration dips in the X-ray intensity of Sco X-1 and attributed them to occultations of the source by small trans-Neptunian objects (TNOs). We have found multiple lines of evidence that these dips are not astronomical in origin, but rather the result of high-energy charged particle events in the RXTE PCA detectors. Our analysis of the RXTE data indicates that at most 10% of the observed dips in Sco X-1 could be due to occultations by TNOs, and, furthermore, we find no positive or supporting evidence for any of them being due to TNOs. We therefore believe that it is a mistake to conclude that any TNOs have been detected via occultation of Sco X-1.Comment: Submitted to ApJ; uses emulateapj.cls, 8 pages with 8 figure

Local quantum critical point in the pseudogap Anderson model: finite-T dynamics and omega/T scaling

The pseudogap Anderson impurity model is a paradigm for locally critical quantum phase transitions. Within the framework of the local moment approach we study its finite-T dynamics, as embodied in the single-particle spectrum, in the vicinity of the symmetric quantum critical point (QCP) separating generalized Fermi-liquid (Kondo screened) and local moment phases. The scaling spectra in both phases, and at the QCP itself, are obtained analytically. A key result is that pure omega/T-scaling obtains at the QCP, where the Kondo resonance has just collapsed. The connection between the scaling spectra in either phase and that at the QCP is explored in detail.Comment: 12 pages, 7 figure

A bioeconomic model for determining the optimal response to a new weed incursion in Australian cropping systems

Invasions by non-indigenous plant species pose serious economic threats to Australian agricultural industries. When an invasion is discovered a decision has to be made as to whether to attempt to eradicate it, contain it or do nothing. These decisions should be based on long term benefits and costs. This paper describes a bioeconomic simulation framework with a mathematical model representing weed spread linked to a dynamic programming model to provide a means of determining the economically optimal weed management strategies over time. The modelling framework is used to evaluate case study invasive weed control problems in the Australian grains industry.weeds, incursion, bioeconomic model, Crop Production/Industries, Research Methods/ Statistical Methods,

Macroscopic and microscopic studies of electrical properties of very thin silicon dioxide subject to electrical stress

The electrical characteristics of various size tunnel switch diode devices, composed of Al/SiO2/n-Si/p+-Si layers, which operate with a range of parameters (such as current densities in excess of 104 A/cm2) that stress the oxide layer far beyond the levels used in typical thin oxide metal-oxide semiconductor research have been examined. It is found that the first time a large current and electric field are applied to the device, a "forming" process enhances transport through the oxide in the vicinity of the edges of the gate electrode, but the oxide still retains its integrity as a tunnel barrier. The device operation is relatively stable to stresses of greater than 107 C/cm2 areally averaged, time-integrated charge injection. Duplication and characterization of these modified oxide tunneling properties was attempted using scanning tunneling microscopy (STM) to stress and probe the oxide. Electrical stressing with the STM tip creates regions of reduced conductivity, possibly resulting from trapped charge in the oxide. Lateral variations in the conductivity of the unstressed oxide over regions roughly 20–50 nm across were also found

Epitaxial silicon grown on CeO2/Si(111) structure by molecular beam epitaxy

Using electron beam evaporation, a Si/CeO2/Si(111) structure has been grown in a molecular beam epitaxy machine. In situ low energy electron diffraction, cross sectional transmission electron microscopy, selected area diffraction, and atomic force microscopy have been used to structurally characterize the overlying silicon layer and show it to be single crystalline and epitaxially oriented. Rutherford backscattering and energy dispersive x-ray analysis have been used to confirm the presence of a continuous 23 Å CeO2 layer at the interface. Rutherford backscattering and x-ray photoemission spectroscopy show an additional presence of cerium both at the exposed silicon surface and incorporated in low levels (~ 1%) within the silicon film, suggesting a growth mechanism with cerium riding atop the silicon growth front leaving behind small amounts of cerium incorporated in the growing silicon crystal

PT-Symmetric Sinusoidal Optical Lattices at the Symmetry-Breaking Threshold

The $PT$ symmetric potential $V_0[\cos(2\pi x/a)+i\lambda\sin(2\pi x/a)]$ has a completely real spectrum for $\lambda\le 1$, and begins to develop complex eigenvalues for $\lambda>1$. At the symmetry-breaking threshold $\lambda=1$ some of the eigenvectors become degenerate, giving rise to a Jordan-block structure for each degenerate eigenvector. In general this is expected to result in a secular growth in the amplitude of the wave. However, it has been shown in a recent paper by Longhi, by numerical simulation and by the use of perturbation theory, that for a broad initial wave packet this growth is suppressed, and instead a saturation leading to a constant maximum amplitude is observed. We revisit this problem by explicitly constructing the Bloch wave-functions and the associated Jordan functions and using the method of stationary states to find the dependence on the longitudinal distance $z$ for a variety of different initial wave packets. This allows us to show in detail how the saturation of the linear growth arises from the close connection between the contributions of the Jordan functions and those of the neighbouring Bloch waves.Comment: 15 pages, 7 figures Minor corrections, additional reference