Positronium collisions with rare-gas atoms

We calculate elastic scattering of positronium (Ps) by the Xe atom using the recently developed pseudopotential method [I. I. Fabrikant and G. F. Gribakin, Phys. Rev. A 90, 052717 (2014)] and review general features of Ps scattering from heavier rare-gas atoms: Ar, Kr, and Xe. The total scattering cross section is dominated by two contributions: elastic scattering and Ps ionization (breakup). To calculate the Ps ionization cross sections we use the binary-encounter method for Ps collisions with an atomic target. Our results for the ionization cross section agree well with previous calculations carried out in the impulse approximation. Our total Ps-Xe cross section, when plotted as a function of the projectile velocity, exhibits similarity with the electron-Xe cross section for the collision velocities higher than 0.8 a.u., and agrees very well with the measurements at Ps velocities above 0.5 a.u.Comment: 7 pages, 7 figures, submitted to J. Phys.

Positronium collisions with rare-gas atoms: Free-electron gas plus orthogonalizing pseudopotential model

Positronium collisions with rare-gas atoms are treated using the free-electron-gas approximation for exchange and correlation potential. The results confirm the absence of the Ramsauer-Townsend minimum in elastic scattering cross sections, but show lower cross sections in the lower-energy region when compared to previous pseudopotential calculations. This is explained by a more attractive ab initio correlation potential as compared to the previously used empirical potential. The results in the thermal-energy region agree very well with most swarm measurements for all rare-gas atoms. At higher energies, the results are compared with beam experiments and agreement for heavier rare-gas atoms Ar, Kr, and Xe is found to be very good. For He and Ne, some discrepancies with beam measurements are observed. This is explained by a poorer performance of the free-electron-gas potentials, based on the statistical Thomas-Fermi model, for systems with fewer electrons

Empirical study of cartograms

We report on an empirical study investigating the effectiveness and efficiency of spatial inference making with contiguous (value-by-area) cartograms, compared to informational equivalent choropleth maps, combined with graduated circles. We find significant differences in people's inference-making performance dependent on the map type. Overall, results suggest that the choropleth map with graduated circles is more effective and more efficient than the cartogram for the analysis of population census data. However, map effectiveness and efficiency also significantly depends on the inference task complexity, and more surprisingly, on the shape characteristics of the depicted enumeration units. For simple tasks, cartograms seem as effective and efficient as the more traditional mapping method. For complex inference questions, inference performance with cartograms is significantly dependent on whether regular or irregular zones are distorted. As we know still very little about the perception and cognition of cartograms, we hope to shed new light for this intriguing mapping method with this empirical study

Nanoelectromechanics of Piezoresponse Force Microscopy

To achieve quantitative interpretation of Piezoresponse Force Microscopy (PFM), including resolution limits, tip bias- and strain-induced phenomena and spectroscopy, analytical representations for tip-induced electroelastic fields inside the material are derived for the cases of weak and strong indentation. In the weak indentation case, electrostatic field distribution is calculated using image charge model. In the strong indentation case, the solution of the coupled electroelastic problem for piezoelectric indentation is used to obtain the electric field and strain distribution in the ferroelectric material. This establishes a complete continuum mechanics description of the PFM contact mechanics and imaging mechanism. The electroelastic field distribution allows signal generation volume in PFM to be determined. These rigorous solutions are compared with the electrostatic point charge and sphere-plane models, and the applicability limits for asymptotic point charge and point force models are established. The implications of these results for ferroelectric polarization switching processes are analyzed.Comment: 81 pages, 19 figures, to be published in Phys. Rev.

Scale Invariance in Road Networks

We study the topological and geographic structure of the national road networks of the United States, England and Denmark. By transforming these networks into their dual representation, where roads are vertices and an edge connects two vertices if the corresponding roads ever intersect, we show that they exhibit both topological and geographic scale invariance. That is, we show that for sufficiently large geographic areas, the dual degree distribution follows a power law with exponent 2.2 < alpha < 2.4, and that journeys, regardless of their length, have a largely identical structure. To explain these properties, we introduce and analyze a simple fractal model of road placement that reproduces the observed structure, and suggests a testable connection between the scaling exponent alpha and the fractal dimensions governing the placement of roads and intersections.Comment: 6 pages, 10 figures; revision incorporates more rigorous statistical analyses; matches journal versio

Semiempirical \u3ci\u3eR\u3c/i\u3e-matrix theory of low energy electron–CF\u3csub\u3e3\u3c/sub\u3eCl inelastic scattering

We apply a semiempirical R-matrix theory to calculations of vibrational excitation and dissociative attachment in the CF3Cl molecule for electron energies below about 3 eV. We employ two sets of model parameters corresponding to two different forms of the CF3Cl− potential curve. We find that our present, ab initio calculated anion curve gives vibrational excitation and dissociative attachment cross sections in good agreement with experimental measurements. We also compare the results of our theory with those of a recently published classical theory

Positronium collisions with polar molecules

We calculate elastic and positronium (Ps) break-up cross sections for collisions of Ps with the polar molecules CO, HCl, and LiF in the fixed-nuclei approximation. We incorporate electron exchange and correlation for these processes by using the free-electron-gas model developed earlier for Ps scattering by rare-gas atoms, N2, O2, and CO2 molecules. The present target molecules provide a range of dipole moments from the weakly polar CO to the strongly polar LiF. We find that Ps scattering is similar to electron scattering when the cross sections are plotted as a function of projectile velocity for the targets with smaller dipole moments (CO, HCl). However, we do not see such a similarity for LiF which has a large dipole moment. Below the Ps break-up threshold we observe resonance structures similar to those obtained earlier for the other molecular targets that we have studied

Greedy Selfish Network Creation

We introduce and analyze greedy equilibria (GE) for the well-known model of selfish network creation by Fabrikant et al.[PODC'03]. GE are interesting for two reasons: (1) they model outcomes found by agents which prefer smooth adaptations over radical strategy-changes, (2) GE are outcomes found by agents which do not have enough computational resources to play optimally. In the model of Fabrikant et al. agents correspond to Internet Service Providers which buy network links to improve their quality of network usage. It is known that computing a best response in this model is NP-hard. Hence, poly-time agents are likely not to play optimally. But how good are networks created by such agents? We answer this question for very simple agents. Quite surprisingly, naive greedy play suffices to create remarkably stable networks. Specifically, we show that in the SUM version, where agents attempt to minimize their average distance to all other agents, GE capture Nash equilibria (NE) on trees and that any GE is in 3-approximate NE on general networks. For the latter we also provide a lower bound of 3/2 on the approximation ratio. For the MAX version, where agents attempt to minimize their maximum distance, we show that any GE-star is in 2-approximate NE and any GE-tree having larger diameter is in 6/5-approximate NE. Both bounds are tight. We contrast these positive results by providing a linear lower bound on the approximation ratio for the MAX version on general networks in GE. This result implies a locality gap of $\Omega(n)$ for the metric min-max facility location problem, where n is the number of clients.Comment: 28 pages, 8 figures. An extended abstract of this work was accepted at WINE'1

Modifying the photodetachment near a metal surface by a weak electric field

We show the photodetachment cross sections of H near a metal surface can be modified using a weak static electric field. The modification is possible because the oscillatory part of the cross section near a metal surface is directly connected with the transit-time and the action of the detached-electron closed-orbit which can be changed systematically by varying the static electric field strength. Photodetachment cross sections for various photon energies and electric field values are calculated and displayed.Comment: 16 pages, 7 figure