Potential, core-level and d band shifts at transition metal surfaces

We have extended the validity of the correlation between the surface 3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d transition metal series and to the neighboring elements Sr and Ag via accurate first-principles calculations. We find that the correlation is quasilinear and robust with respect to the differencies both between initial and final-state calculations of the SCLS's and two distinct measures of the SDBS's. We show that despite the complex spatial dependence of the surface potential shift (SPS) and the location of the 3d and 4d orbitals in different regions of space, the correlation exists because the sampling of the SPS by the 3d and 4d orbitals remains similar. We show further that the sign change of the SCLS's across the transition series does indeed arise from the d band-narrowing mechanism previously proposed. However, while in the heavier transition metals the predicted increase of d electrons in the surface layer relative to the bulk arises primarily from transfers from s and p states to d states within the surface layer, in the lighter transition metals the predicted decrease of surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.

The prevention of intrapartum HIV/Aids transmission from mother to child

The transmission of HIV/AIDS from mother to child is the main cause of HIV/AIDS in children and child mortality. Two-thirds of children with HIV/AIDS are infected in the intrapartum period (Farley, 2000:1 -2). Midwives, through effective practices, can lower the transmission of HIV/AIDS from mother to child in the intrapartum period. The aim of the study was to determine which preventive practices registered midwives in provincial labour wards in Bloemfontein, implement to lower the risk of mother to child transmission of HIV/AIDS and in doing so to lower child mortality. A descriptive research design was used. Fifty-one midwives completed questionnaires to determine what preventive practices are used. The study showed that most of the midwives did not implement the interventions that could lower the chances of HI V/Aids transmission from mother to child during the intrapartum period. Midwives therefore may not have a positive impact on child mortality. The researchers concluded that midwives do not implement the interventions mentioned in the study because of a lack of knowledge and skills. Of the midwives who completed questionnaires, 67% completed their training between 1990 and 2000. This lack of knowledge may be due to a lack in the curriculum of training institutions and of an up-to-date policy concerning the transmission o f HIV/Aids in p ro v in c ia l in s titu tio n s in Bloemfontein. It is recommended that all affordable interventions should be incorporated in the policy.Health Studie

Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride

Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes. Full C incorporation required only 30-60 min of milling. Grain size of sintered samples decreased with increased milling time to less than 30 nm for 20-50 hrs of milling. Milling had a weak detrimental effect on connectivity. Strong irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to increased milling time was observed and correlated linearly with inverse grain size (1/d). As a result, high field Jc benefited greatly from lengthy powder milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nano-dispersions.Comment: 12 pages, 11 figure

Local Isoelectronic Reactivity of Solid Surfaces

The quantity w^N(r) = ( 1/ k^2 T_el)[partial n(r, T_el) / partial T_el]_(v(r),N) is introduced as a convenient measure of the local isoelectronic reactivity of surfaces. It characterizes the local polarizability of the surface and it can be calculated easily. The quantity w^N(r) supplements the charge transfer reactivity measured e.g. by the local softness to which it is closely related. We demonstrate the applicability and virtues of the function w^N(r) for the example of hydrogen dissociation and adsorption on Pd(100).Comment: RevTeX, 13 pages, 3 figures, to appear in Phys. Rev. Let

Comment on "Antilocalization in a 2D Electron Gas in a Random Magnetic Field"

In a recent Letter, Taras-Semchuk and Efetov reconsider the problem of electron localization in a random magnetic field in two dimensions. They claim that due to the long-range nature of the vector potential correlations an additional term appears in the effective field theory ($\sigma$-model) of the problem, leading to delocalization at the one-loop level. This calls into question the results of earlier analytical studies, where the random magnetic field problem was mapped onto the conventional unitary-class $\sigma$-model, implying that the leading quantum correction is of two-loop order and of a localizing nature. We show in this Comment, however, that the new term in fact does not exist and was erroneously obtained by Taras-Semchuk and Efetov because of an inconsistent treatment violating gauge invariance.Comment: 1 page, 2 figure

On the Sets of Real Numbers Recognized by Finite Automata in Multiple Bases

This article studies the expressive power of finite automata recognizing sets of real numbers encoded in positional notation. We consider Muller automata as well as the restricted class of weak deterministic automata, used as symbolic set representations in actual applications. In previous work, it has been established that the sets of numbers that are recognizable by weak deterministic automata in two bases that do not share the same set of prime factors are exactly those that are definable in the first order additive theory of real and integer numbers. This result extends Cobham's theorem, which characterizes the sets of integer numbers that are recognizable by finite automata in multiple bases. In this article, we first generalize this result to multiplicatively independent bases, which brings it closer to the original statement of Cobham's theorem. Then, we study the sets of reals recognizable by Muller automata in two bases. We show with a counterexample that, in this setting, Cobham's theorem does not generalize to multiplicatively independent bases. Finally, we prove that the sets of reals that are recognizable by Muller automata in two bases that do not share the same set of prime factors are exactly those definable in the first order additive theory of real and integer numbers. These sets are thus also recognizable by weak deterministic automata. This result leads to a precise characterization of the sets of real numbers that are recognizable in multiple bases, and provides a theoretical justification to the use of weak automata as symbolic representations of sets.Comment: 17 page

Force calculation and atomic-structure optimization for the full-potential linearized augmented plane-wave code WIEN

Following the approach of Yu, Singh, and Krakauer [Phys. Rev. B 43 (1991) 6411] we extended the linearized augmented plane wave code WIEN of Blaha, Schwarz, and coworkers by the evaluation of forces. In this paper we describe the approach, demonstrate the high accuracy of the force calculation, and use them for an efficient geometry optimization of poly-atomic systems.Comment: submitted to Comp. Phys. Commun., 27 pages, 3 figure

Potential Energy Surface for H_2 Dissociation over Pd(100)

The potential energy surface (PES) of dissociative adsorption of H_2 on Pd(100) is investigated using density functional theory and the full-potential linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are identified which have a vanishing energy barrier. A pronounced dependence of the potential energy on ``cartwheel'' rotations of the molecular axis is found. The calculated PES shows no indication of the presence of a precursor state in front of the surface. Both results indicate that steering effects determine the observed decrease of the sticking coefficient at low energies of the H_2 molecules. We show that the topology of the PES is related to the dependence of the covalent H(s)-Pd(d) interactions on the orientation of the H_2 molecule.Comment: RevTeX, 8 pages, 5 figures in uufiles forma

Improved risk-based strategies for disease management in the pig production chain

To minimize financial losses in times of crisis, it is necessary to prove methods of maintaining trade in a case of disease. This work shows that the identification of modules and clusters could be of high relevance if these clusters impede disease spread or if so-called Ad hoc-connector-points serving as routes of transmission between clusters could be identified. Furthermore the advantages of a risk-based selection of critical control points for surveillance or monitoring can be shown. This work provides new approaches to review and possibly optimize existing disease prevention and control strategies

Co-Evolution of quasispecies: B-cell mutation rates maximize viral error catastrophes

Co-evolution of two coupled quasispecies is studied, motivated by the competition between viral evolution and adapting immune response. In this co-adaptive model, besides the classical error catastrophe for high virus mutation rates, a second ``adaptation-'' catastrophe occurs, when virus mutation rates are too small to escape immune attack. Maximizing both regimes of viral error catastrophes is a possible strategy for an optimal immune response, reducing the range of allowed viral mutation rates to a minimum. From this requirement one obtains constraints on B-cell mutation rates and receptor lengths, yielding an estimate of somatic hypermutation rates in the germinal center in accordance with observation.Comment: 4 pages RevTeX including 2 figure