Low-Temperature Orientation Dependence of Step Stiffness on {111} Surfaces

For hexagonal nets, descriptive of {111} fcc surfaces, we derive from combinatoric arguments a simple, low-temperature formula for the orientation dependence of the surface step line tension and stiffness, as well as the leading correction, based on the Ising model with nearest-neighbor (NN) interactions. Our formula agrees well with experimental data for both Ag and Cu{111} surfaces, indicating that NN-interactions alone can account for the data in these cases (in contrast to results for Cu{001}). Experimentally significant corollaries of the low-temperature derivation show that the step line tension cannot be extracted from the stiffness and that with plausible assumptions the low-temperature stiffness should have 6-fold symmetry, in contrast to the 3-fold symmetry of the crystal shape. We examine Zia's exact implicit solution in detail, using numerical methods for general orientations and deriving many analytic results including explicit solutions in the two high-symmetry directions. From these exact results we rederive our simple result and explore subtle behavior near close-packed directions. To account for the 3-fold symmetry in a lattice gas model, we invoke a novel orientation-dependent trio interaction and examine its consequences.Comment: 11 pages, 8 figure

Microscopically-based energy density functionals for nuclei using the density matrix expansion: Implementation and pre-optimization

In a recent series of papers, Gebremariam, Bogner, and Duguet derived a microscopically based nuclear energy density functional by applying the Density Matrix Expansion (DME) to the Hartree-Fock energy obtained from chiral effective field theory (EFT) two- and three-nucleon interactions. Due to the structure of the chiral interactions, each coupling in the DME functional is given as the sum of a coupling constant arising from zero-range contact interactions and a coupling function of the density arising from the finite-range pion exchanges. Since the contact contributions have essentially the same structure as those entering empirical Skyrme functionals, a microscopically guided Skyrme phenomenology has been suggested in which the contact terms in the DME functional are released for optimization to finite-density observables to capture short-range correlation energy contributions from beyond Hartree-Fock. The present paper is the first attempt to assess the ability of the newly suggested DME functional, which has a much richer set of density dependencies than traditional Skyrme functionals, to generate sensible and stable results for nuclear applications. The results of the first proof-of-principle calculations are given, and numerous practical issues related to the implementation of the new functional in existing Skyrme codes are discussed. Using a restricted singular value decomposition (SVD) optimization procedure, it is found that the new DME functional gives numerically stable results and exhibits a small but systematic reduction of our test $\chi^2$ function compared to standard Skyrme functionals, thus justifying its suitability for future global optimizations and large-scale calculations.Comment: 17 pages, 6 figure

Instabilities in the Nuclear Energy Density Functional

In the field of Energy Density Functionals (EDF) used in nuclear structure and dynamics, one of the unsolved issues is the stability of the functional. Numerical issues aside, some EDFs are unstable with respect to particular perturbations of the nuclear ground-state density. The aim of this contribution is to raise questions about the origin and nature of these instabilities, the techniques used to diagnose and prevent them, and the domain of density functions in which one should expect a nuclear EDF to be stable.Comment: Special issue "Open Problems in Nuclear Structure Theory" of Jour.Phys.G - accepted. 7 pages, 2 figure

Nuclear energy density functional from chiral two- and three-nucleon interactions

An improved density-matrix expansion is used to calculate the nuclear energy density functional from chiral two- and three-nucleon interactions. The two-body interaction comprises long-range one- and two-pion exchange contributions and a set of contact terms contributing up to fourth power in momenta. In addition we employ the leading order chiral three-nucleon interaction with its parameters $c_E, c_D$ and $c_{1,3,4}$ fixed in calculations of nuclear few-body systems. With this input the nuclear energy density functional is derived to first order in the two- and three-nucleon interaction. We find that the strength functions $F_\nabla(\rho)$ and $F_{so}(\rho)$ of the surface and spin-orbit terms compare in the relevant density range reasonably with results of phenomenological Skyrme forces. However, an improved description requires (at least) the treatment of the two-body interaction to second order. This observation is in line with the deficiencies in the nuclear matter equation of state $\bar E(\rho)$ that remain in the Hartree-Fock approximation with low-momentum two- and three-nucleon interactions.Comment: 16 pages, 12 figures, submitted to Eur. Phys. J.

Polio Outbreak Response in Ethiopia

Background: Ethiopia had been polio-free for almost four years until December 2004. However, between December 2004 and February 2006, 24 children were paralysed as a result of infection with wild poliovirus imported from the neighbouring country of Sudan. In response, the country has attempted to document the impact of various response measures on the containment of wild poliovirus transmission. Objectives: This study aims at systematic and epidemiological assessment of the extent of the outbreak, its determinants, and the lessons learned as well as the implications for future control strategies to interrupt wild poliovirus transmission. Design: A cross-sectional study design with qualitative and quantitative data collection approaches was used to conduct the epidemiologic assessment. Subjects: All confirmed wild poliovirus cases, and reported acute flaccid paralysis cases in close proximity to the confirmed polio cases were the study subjects. Child caretakers and health service providers were interviewed as part of the investigation. Results: Between December 2004 and February 2006, eight children from Tigray Regional State, nine children from Amhara Regional State and seven children from Oromia Regional State were paralysed as a result of infection with wild poliovirus type 1. Genetic sequencing demonstrated two separate importations to Ethiopia. Risk factors that may have facilitated spread of the outbreak within the country included gaps in vaccination coverage and interruption of the cold chain system, gaps in acute flaccid paralysis surveillance performance, high population mobility, poor environmental sanitation, crowded living conditions and unsafe drinking water. In response to the outbreak, Ethiopia conducted detailed outbreak investigations within two days of confirmation of the index cases. Large-scale, house-to-house vaccination campaigns were also implemented. As a result, the three regions interrupted the wild poliovirus transmission within the regions within one year of confirmation of the index case. Conclusion: Outbreak response activities were successful in interrupting the imported wild poliovirus transmission in Tigray, Amhara and Oromia Regional States of Ethiopia within a oneyear period of time. In Ethiopia, programme strategies should be intensified to contain further spread and prevent future importation of wild poliovirus. Large-scale immunisation campaigns should reach every child, including those isolated by geography, poverty and security. East African Medical Journla Vol. 85 (5) 2008: pp. 222-23

Using the Wigner-Ibach Surmise to Analyze Terrace-Width Distributions: History, User's Guide, and Advances

A history is given of the applications of the simple expression generalized from the surmise by Wigner and also by Ibach to extract the strength of the interaction between steps on a vicinal surface, via the terrace width distribution (TWD). A concise guide for use with experiments and a summary of some recent extensions are provided.Comment: 11 pages, 4 figures, reformatted (with revtex) version of refereed paper for special issue of Applied Physics A entitled "From Surface Science to Device Physics", in honor of the retirements of Prof. H. Ibach and Prof. H. L\"ut