Unoccupied Band Structure of NbSe2 by Very-Low-Energy Electron Diffraction: Experiment and Theory

A combined experimental and theoretical study of very-low-energy electron diffraction at the (0001) surface of 2H-NbSe2 is presented. Electron transmission spectra have been measured for energies up to 50 eV above the Fermi level with k|| varying along the GammaK line of the Brillouin zone. Ab initio calculations of the spectra have been performed with the extended linear augmented plane wave k-p method. The experimental spectra are interpreted in terms of three-dimensional one-electron band structure. Special attention is paid to the quasi-particle lifetimes: by comparing the broadening of the spectral structures in the experimental and calculated spectra the energy dependence of the optical potential Vi is determined. A sharp increase of Vi at 20 eV is detected, which is associated with a plasmon peak in the Im(-1/epsilon) function. Furthermore, the electron energy loss spectrum and the reflectivity of NbSe2 are calculated ab initio and compared with optical experiments. The obtained information on the dispersions and lifetimes of the unoccupied states is important for photoemission studies of the 3D band structure of the valence band.Comment: 17 pages, 11 Postscript figures, submitted to Phys. Rev.

Adiabatic nonlinear waves with trapped particles: II. Wave dispersion

A general nonlinear dispersion relation is derived in a nondifferential form for an adiabatic sinusoidal Langmuir wave in collisionless plasma, allowing for an arbitrary distribution of trapped electrons. The linear dielectric function is generalized, and the nonlinear kinetic frequency shift $\omega_{\rm NL}$ is found analytically as a function of the wave amplitude $a$. Smooth distributions yield $\omega_{\rm NL} \propto \sqrt{a}$, as usual. However, beam-like distributions of trapped electrons result in different power laws, or even a logarithmic nonlinearity, which are derived as asymptotic limits of the same dispersion relation. Such beams are formed whenever the phase velocity changes, because the trapped distribution is in autoresonance and thus evolves differently from the passing distribution. Hence, even adiabatic $\omega_{\rm NL}(a)$ is generally nonlocal.Comment: submitted together with Papers I and II

A Theory of Magnets with Competing Double Exchange and Superexchange Interactions

We study the competition between ferromagnetic double exchange (DE) and nearest-neighbour antiferromagnetic exchange in CMR materials. Towards this end, a single site mean field theory is proposed which emphasizes the hopping-mediated nature of the DE contribution. We find that the competition between these two exchange interactions leads to ferro- or antiferromagnetic order with incomplete saturation of the (sub)lattice magnetization. This conclusion is in contrast to previous results in the literature which find a canted spin arrangement under similar circumstances. We attribute this difference to the highly anisotropic exchange interactions used elsewhere. The associated experimental implications are discussed.Comment: 4 pages, Latex-Revtex, 3 PostScript figures. Please see report cond-mat/980523

Origin of inverse Rashba-Edelstein effect detected at the Cu/Bi interface using lateral spin valves

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).The spin transport and spin-to-charge current conversion properties of bismuth are investigated using permalloy/copper/bismuth (Py/Cu/Bi) lateral spin valve structures. The spin current is strongly absorbed at the surface of Bi, leading to ultrashort spin-diffusion lengths. A spin-to-charge current conversion is measured, which is attributed to the inverse Rashba-Edelstein effect at the Cu/Bi interface. The spin-current-induced charge current is found to change direction with increasing temperature. A theoretical analysis relates this behavior to the complex spin structure and dispersion of the surface states at the Fermi energy. The understanding of this phenomenon opens novel possibilities to exploit spin-orbit coupling to create, manipulate, and detect spin currents in two-dimensional systems.This work was supported by the European Commission under the Marie Curie Actions (Grant No. 256470-ITAMOSCINOM), the SUDOE (Grant No. TRAIN2-SOE2/P1/E-280), and the European Research Council (Grant No. 257654-SPINTROS), by the Spanish MINECO under Projects No. MAT2012-37638, No. MAT2011-13099-E, No. MAT2011-27553-C02, No. MAT2014-51982-C2-1-R, No. MAT2015-69725-REDT, and No. FIS2013-48286-C2-1-P, including FEDER funds, by the Basque Government under Project No. PI2011-1, and by the Aragon Regional Government under Project No. E26. M.I. and E.V. thank the Basque Government for Ph.D. fellowships (Grants No. BFI-2011-106 and No. BFI-2010-163).Peer Reviewe

Core reconstruction in pseudopotential calculations

A new method is presented for obtaining all-electron results from a pseudopotential calculation. This is achieved by carrying out a localised calculation in the region of an atomic nucleus using the embedding potential method of Inglesfield [J.Phys. C {\bf 14}, 3795 (1981)]. In this method the core region is \emph{reconstructed}, and none of the simplifying approximations (such as spherical symmetry of the charge density/potential or frozen core electrons) that previous solutions to this problem have required are made. The embedding method requires an accurate real space Green function, and an analysis of the errors introduced in constructing this from a set of numerical eigenstates is given. Results are presented for an all-electron reconstruction of bulk aluminium, for both the charge density and the density of states.Comment: 14 pages, 5 figure

Dispelling urban myths about default uncertainty factors in chemical risk assessment - Sufficient protection against mixture effects?

© 2013 Martin et al.; licensee BioMed Central LtdThis article has been made available through the Brunel Open Access Publishing Fund.Assessing the detrimental health effects of chemicals requires the extrapolation of experimental data in animals to human populations. This is achieved by applying a default uncertainty factor of 100 to doses not found to be associated with observable effects in laboratory animals. It is commonly assumed that the toxicokinetic and toxicodynamic sub-components of this default uncertainty factor represent worst-case scenarios and that the multiplication of those components yields conservative estimates of safe levels for humans. It is sometimes claimed that this conservatism also offers adequate protection from mixture effects. By analysing the evolution of uncertainty factors from a historical perspective, we expose that the default factor and its sub-components are intended to represent adequate rather than worst-case scenarios. The intention of using assessment factors for mixture effects was abandoned thirty years ago. It is also often ignored that the conservatism (or otherwise) of uncertainty factors can only be considered in relation to a defined level of protection. A protection equivalent to an effect magnitude of 0.001-0.0001% over background incidence is generally considered acceptable. However, it is impossible to say whether this level of protection is in fact realised with the tolerable doses that are derived by employing uncertainty factors. Accordingly, it is difficult to assess whether uncertainty factors overestimate or underestimate the sensitivity differences in human populations. It is also often not appreciated that the outcome of probabilistic approaches to the multiplication of sub-factors is dependent on the choice of probability distributions. Therefore, the idea that default uncertainty factors are overly conservative worst-case scenarios which can account both for the lack of statistical power in animal experiments and protect against potential mixture effects is ill-founded. We contend that precautionary regulation should provide an incentive to generate better data and recommend adopting a pragmatic, but scientifically better founded approach to mixture risk assessment. © 2013 Martin et al.; licensee BioMed Central Ltd.Oak Foundatio