Aiding the design of radiation resistant materials with multiphysics simulations of damage processes

The design of metals and alloys resistant to radiation damage involves the physics of electronic excitations and the creation of defects and microstructure. During irradiation damage of metals by high energy particles, energy is exchanged between ions and electrons. Such non-adiabatic processes violate the Born-Oppenheimer approximation, on which all conservative classical interatomic potentials rest. By treating the electrons of a metal explicitly and quantum mechanically we are able to explore the influence of electronic excitations on the ionic motion during irradiation damage. Simple theories suggest that moving ions should feel a damping force proportional to their velocity and directly opposed to it. In contrast, our simulations of a forced oscillating ion have revealed the full complexity of this force: in reality it is anisotropic and dependent on the ion velocity and local atomic environment. A large set of collision cascade simulations has allowed us to explore the form of the damping force further. We have a means of testing various schemes in the literature for incorporating such a force within molecular dynamics (MD) against our semi-classical evolution with explicitly modelled electrons. We find that a model in which the damping force is dependent upon the local electron density is superior to a simple fixed damping model. We also find that applying a lower kinetic energy cut-off for the damping force results in a worse model. A detailed examination of the nature of the forces reveals that there is much scope for further improving the electronic force models within MD. © 2010 Materials Research Society.Accepted versio

PSR B0809+74: Understanding Its Perplexing Subpulse-separation (P2) Variations

The longitude separation between adjacent drifting subpulses, $P_2$, is roughly constant for many pulsars. It was then perplexing when pulsar B0809+74 was found to exhibit substantial variations in this measure, both with wavelength and with longitude position within the pulse window. We analyze these variations between 40 and 1400 MHz, and we show that they stem primarily from the incoherent superposition of the two orthogonal modes of polarization.Comment: Submitted for publication Astronomy and Astrophysic

Vacuum polarization in two-dimensional static spacetimes and dimensional reduction

We obtain an analytic approximation for the effective action of a quantum scalar field in a general static two-dimensional spacetime. We apply this to the dilaton gravity model resulting from the spherical reduction of a massive, non-minimally coupled scalar field in the four-dimensional Schwarzschild geometry. Careful analysis near the event horizon shows the resulting two-dimensional system to be regular in the Hartle-Hawking state for general values of the field mass, coupling, and angular momentum, while at spatial infinity it reduces to a thermal gas at the black-hole temperature.Comment: REVTeX 4, 23 pages. Accepted by PRD. Minor modifications from original versio

Impact of modulation on CMB B-mode polarization experiments

We investigate the impact of both slow and fast polarization modulation strategies on the science return of upcoming ground-based experiments aimed at measuring the B-mode polarization of the CMB. Using simulations of the Clover experiment, we compare the ability of modulated and un-modulated observations to recover the signature of gravitational waves in the polarized CMB sky in the presence of a number of anticipated systematic effects. The general expectations that fast modulation is helpful in mitigating low-frequency detector noise, and that the additional redundancy in the projection of the instrument's polarization sensitivity directions onto the sky when modulating reduces the impact of instrumental polarization, are borne out by our simulations. Neither low-frequency polarized atmospheric fluctuations nor systematic errors in the polarization sensitivity directions are mitigated by modulation. Additionally, we find no significant reduction in the effect of pointing errors by modulation. For a Clover-like experiment, pointing jitter should be negligible but any systematic mis-calibration of the polarization coordinate reference system results in significant E-B mixing on all angular scales and will require careful control. We also stress the importance of combining data from multiple detectors in order to remove the effects of common-mode systematics (such as 1/f atmospheric noise) on the measured polarization signal. Finally we compare the performance of our simulated experiment with the predicted performance from a Fisher analysis. We find good agreement between the Fisher predictions and the simulations except for the very largest scales where the power spectrum estimator we have used introduces additional variance to the B-mode signal recovered from our simulations.Comment: Replaced with version accepted by MNRAS. Analysis of half-wave plate systematic (differential transmittance) adde

New York: the animated city

The urban landscape of New York City is one that is familiar to many, but, through the medium of animation, this familiarity has been consistently challenged. Often metamorphic, and always meticulously constructed, animated imagery encourages reflective thinking. Focusing on the themes of construction, destruction, and interactivity, this article seeks to cast critical light upon the animated double life that New York City has lived through the following moving image texts: Disney’s Fantasia 2000 (1999), Patrick Jean’s computer-generated short Pixels (2009), and Rockstar Games’ open-world blockbuster Grand Theft Auto IV (2008)

Internet banking acceptance model: Cross-market examination

This article proposes a revised technology acceptance model to measure consumers’ acceptance of Internet banking, the Internet Banking Acceptance Model (IBAM). Data was collected from 618 university students in the United Kingdom and Saudi Arabia. The results suggest the importance of attitude, such that attitude and behavioral intentions emerge as a single factor, denoted as “attitudinal intentions” (AI). Structural equation modeling confirms the fit of the model, in which perceived usefulness and trust fully mediate the impact of subjective norms and perceived manageability on AI. The invariance analysis demonstrates the psychometric equivalence of the IBAM measurements between the two country groups. At the structural level, the influence of trust and system usefulness on AI vary between the two countries, emphasizing the potential role of cultures in IS adoption. The IBAM is robust and parsimonious, explaining over 80% of AI