Positron scattering and annihilation in hydrogenlike ions

Diagrammatic many-body theory is used to calculate the scattering phase shifts, normalized annihilation rates Zeff, and annihilation ? spectra for positron collisions with the hydrogenlike ions He+, Li2+, B4+, and F8+. Short-range electron-positron correlations and longer-range positron-ion correlations are accounted for by evaluating nonlocal corrections to the annihilation vertex and the exact positron self-energy. The numerical calculation of the many-body theory diagrams is performed using B-spline basis sets. To elucidate the role of the positron-ion repulsion, the annihilation rate is also estimated analytically in the Coulomb-Born approximation. It is found that the energy dependence and magnitude of Zeff are governed by the Gamow factor that characterizes the suppression of the positron wave function near the ion. For all of the H-like ions, the correlation enhancement of the annihilation rate is found to be predominantly due to corrections to the annihilation vertex, while the corrections to the positron wave function play only a minor role. Results of the calculations for s-, p-, and d-wave incident positrons of energies up to the positronium-formation threshold are presented. Where comparison is possible, our values are in excellent agreement with the results obtained using other, e.g., variational, methods. The annihilation-vertex enhancement factors obtained in the present calculations are found to scale approximately as 1+(1.6+0.46l)/Zi, where Zi is the net charge of the ion and l is the positron orbital angular momentum. Our results for positron annihilation in H-like ions provide insights into the problem of positron annihilation with core electrons in atoms and condensed matter systems, which have similar binding energies.

X-ray scattering from warm dense iron

We have carried out X-ray scattering experiments on iron foil samples that have been compressed and heated using laser-driven shocks created with the VULCAN laser system at the Rutherford-Appleton Laboratory. This is the highest Z element studied in such experiments so far and the first time scattering from warm dense iron has been reported. Because of the importance of iron in telluric planets, the work is relevant to studies of warm dense matter in planetary interiors. We report scattering results as well as shock breakout results that, in conjunction with hydrodynamic simulations, suggest the target has been compressed to a molten state at several 100 GPa pressure. Initial comparison with modelling suggests more work is needed to understand the structure factor of warm dense iron. (C) 2013 Published by Elsevier B.V.

Spin-wave modes in Ni nanorod arrays studied by Brillouin light scattering

Dynamic magnetic properties of arrays of Ni nanorods with a low aspect ratio have been investigated. It has been shown that the spectra of spin-wave resonances localized on nanorods with a low aspect ratio typically feature the presence of zones with high density of states resulting in a characteristic two-peak pattern of Stokes and anti-Stokes lines of magneto-optical (MO) Brillouin light scattering with pronounced Stokes–anti-Stokes (S-AS) asymmetry. A simple theoretical model based on the analysis of the elliptic character of the polarization of the optical wave interacting with a dipole magnetostatic wave has been proposed. It has been shown that the S-AS asymmetry is due entirely to the asymmetry of the MO interaction efficiency with respect to time reversal of the magnetic precession in a magnon.

Effect of magnetic bias on Gaussian pulse scattering by stacked nonlinear semiconductor layers

The nonlinear scattering of pulses by periodic stacks of semiconductor layers with magnetic bias has been studied in the self-consistent problem formulation, taking into account mobility of carriers. The three-wave mixing technique has been applied to the analysis of the waveform evolution in the stacks illuminated by two Gaussian pulses with different central frequencies and lengths. The effects of external magnetic bias, and stack physical and geometrical parameters on the properties of the scattered waveforms are discussed. © 2013 IEEE.

Low-energy scattering of electrons by caesium atoms

The relativistic R-matrix method is used to calculate elastic and inelastic cross sections for electrons incident on caesium atoms with energies from 0-3 eV. These cross sections reveal a wealth of resonance structure in this energy range. The differential cross sections as well as the spin polarisation function S( theta ) and the left-right asymmetry function S( theta ) are calculated and enable conclusions to be drawn on the importance of spin-dependent interactions.

An investigation of the ((3p)(4s) ) resonances in the elastic scattering of electrons by argon atoms

Initial results are presented on the elastic scattering of electrons by argon atoms in the neighbourhood of the well known ((3p) (4s)) resonances. An R-matrix calculation is employed to investigate how the resonance characteristics change as the spin-orbit interaction is switched on.

Electron scattering by atoms and ions using the Breit-Pauli Hamiltonian:An R-matrix approach

The R-matrix method describing the scattering of low-energy electrons by complex atoms and ions is extended to include terms of the Breit-Pauli Hamiltonian. An application is made to the astrophysically important 1s 2s S-1s 2s2p P transition in Fe XXIII, where in the most accurate calculations carried out all terms of the 1s 2s, 1s2s2p and 1s2p configurations are included in the expansion describing the collision. This gives up to 28 coupled channels for each total angular momentum and parity which are solved on a CRAY-1. The collision strengths are increased by more than a factor of two from their non-relativistic values at all energies considered.

Low-energy scattering of electrons by caesium atoms. II

The relativistic R-matrix method is used to calculate elastic and inelastic cross sections for electrons incident on caesium atoms with energies from 0-3 eV. In addition to the total cross sections, results are presented on the differential cross sections, sigma , and the spin polarisation, P, of the scattered electrons as a function of energy at the scattering angles 10 degrees , 50 degrees , 90 degrees and 150 degrees . The calculation reveals a wealth of resonances around the P and P thresholds. The resonances are analysed in detail and their role in the scattering process is discussed.

The scattering of low-energy electrons by argon atoms

Phaseshifts, differential, total and momentum transfer cross sections are calculated using an R-matrix approach for the elastic scattering of electrons by argon atoms in the impact energy range 0-19 eV. The coupled-state calculation is based upon a single-configuration atomic ground-state wavefunction coupled to a P pseudostate. A critical assessment of earlier theoretical and experimental data is made and the conclusion is reached that the present results are the most satisfactory over the entire energy range considered.

Nonreciprocal scattering by stacked nonlinear magneto-active semiconductor layers

The combinatorial frequency generation by the periodic stacks of magnetically biased semiconductor layers has been modelled in a self-consistent problem formulation, taking into account the nonlinear dynamics of carriers. It is shown that magnetic bias not only renders nonreciprocity of the three-wave mixing process but also significantly enhances the nonlinear interactions in the stacks, especially at the frequencies close to the intrinsic magneto-plasma resonances of the constituent layers. The main mechanisms and properties of the combinatorial frequency generation and emission from the stacks are illustrated by the simulation results, and the effects of the individual layer parameters and the structure arrangement on the stack nonlinear and nonreciprocal response are discussed. © 2014 Elsevier B.V. All rights reserved.

Application of AAO Matrix in Aligned Gold Nanorod Array Substrates for Surface-Enhanced Fluorescence and Raman Scattering

In this paper, we probed surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) from probe molecule Rhodamine 6G (R6G) on self-standing Au nanorod array substrates made using a combination of anodization and potentiostatic electrodeposition. The initial substrates were embedded within a porous alumina template (AAO). By controlling the thickness of the AAO matrix, SEF and SERS were observed exhibiting an inverse relationship. SERS and SEF showed a non-linear response to the removal of AAO matrix due to an inhomogeneous plasmon activity across the nanorod which was supported by FDTD calculations. We showed that by optimizing the level of AAO thickness, we could obtain either maximized SERS, SEF or simultaneously observe both SERS and SEF together.

Brillouin light scattering by spin waves in magnetic metamaterials based on Co nanorods

We report on the investigations of spin wave modes in arrays of densely packed Co nanorods using Brillouin light scattering. We have observed a significant role of spin wave modes along the nanorod axis in contrast to infinite magnetic nanowires. Unusual optical properties featuring an inverted Stokes/anti-Stokes asymmetry of the Brillouin scattering spectra have been observed. The spectrum of spin wave modes in the nanorod array has been calculated and compared with the experiment. Experimental observations are explained in terms of a combined numerical-analytical approach taking into account both the low aspect ratio of individual magnetic nanorods and dipolar magnetic coupling between the nanorods in the array. The optical studies of spin-wave modes in the metamaterials with low aspect ratio nanorods have revealed new magnetic and magneto-optical properties compared to continuous magnetic films or infinite magnetic nanowires. Such magnetic metamaterials are important class of active metamaterials needed for prospective data storage and signal processing applications. (c) 2012 Optical Society of America

Brillouin scattering of light by spin waves in ferromagnetic nanorods

We report the investigations of spin wave modes of arrays of Ni and Co nanorods using Brillouin light scattering. We have revealed the significant influence of spin wave modes along the nanorod axis in contrast to infinite magnetic nanowires. Unusual optical properties featuring an inverted Stokes/anti-Stokes asymmetry of the Brillouin scattering spectra have been observed. The spectrum of spin wave modes in the nanorod array has been calculated and compared with the experiment. Experimental observations are explained in terms of a combined numerical-analytical approach taking into account both the low aspect ratio of individual magnetic nanorods and dipolar magnetic coupling between the nanorods in the array. The optical studies of spin-wave modes in nanorod metamaterials with low aspect ratio nanorods have revealed new magnetic and magneto-optical properties compared to continuous magnetic films or infinite magnetic nanowires. Such magnetic artificial materials are important class of active metamaterials needed for prospective data storage and signal processing applications. © 2012 Elsevier B.V.

INTERPLAY OF CAF2 SUBSTRATE ROUGHENING AND FILM DEPOSITION RATE ON RADIATIVE SCATTERING OF SURFACE-PLASMON POLARITONS FROM AG FILMS

The radiative decay of surface plasmon polaritons has been investigated in an attempt to characterize the surface roughness of Ag films prepared under different conditions. The polaritons were excited by the method of attenuated total reflection of light. The films were deposited on the face of a 60-degrees BK-7 glass prism at a rate that was deliberately fixed in two different ranges (centred on 0.1 and 10 nm s-1) and in some cases a CaF2 underlayer was used to roughen the film surfaces. The intensity of the scattered light emitted from the opposite face of the films was measured as a function of direction for each using the same sensitivity scale and was correlated with the preparation of the film. It was found that on nominally smooth substrates fast-deposited thinner films give out more light and are deduced to have greater short wavelength (300-600 nm) roughness amplitude. There is also evidence for long wavelenth (7 mum) periodic roughness due to the prism substrate itself. On CaF2 roughened surfaces the light output from the films is further increased and the peak intensity is backward directed with respect to the exciting laser beam direction. Here roughness on a lateral scale of 350 nm is responsible. Also, elastic scattering of surface plasmon polaritons at grain boundaries reduces the light output from fast deposited, small grain, films on CaF2 roughened surfaces. Overall, a consistent picture of roughness induced radiative polariton decay emerges for all cases studied.