High-frequency asymptotic expansions of a backscattering cross-section and an HH/VV polarization ratio for smooth two-dimensional surfaces

High-frequency asymptotic expansions of electrical and magnetic fields are obtained at a perfectly conducting smooth 2D surface illuminated by a plane incident wave in two cases of TE and TM polarization. Corrections to the geometrical optics backscattering HH and VV cross-sections are derived and the polarization ratio HH/VV is estimated for the specular points of a general form. It is shown that the coefficient of the first term in this series ∼ 1/k² depends not only on the local surface curvature radius at the specular reflecting points,but also on higher derivatives up to the sixth order.

For backscattering from a statistically rough surface, averaging over surface random derivatives at the specular points was performed. It was shown that for statistically uniform (stationary on space variables) random surfaces, the polarization ratio HH/VV (dB) is positive on average and proportional to ∼ 1/k²     

Radar backscattering from Gerstner's sea surface wave

In the framework of a two-scale scattering model, radar backscattering from the rough sea surface was considered. The sea surface was modelled as a superposition of a nonlinear, large-scale Gerstner's wave and small-scale resonant Bragg scattering ripples. The zero-order diffracted field was found by a geometrical optics approach, with shadowing taken into account, and by an 'exact' solution of the diffraction problem obtained numerically. For vertical and horizontal polarizations, the spatial distribution of specific scattering cross sections along the large-scale wave was obtained. The spatially averaged specific backscattering cross sections, as well as the mean Doppler frequency shifts at both polarizations, obtained by the geometrical optics approach are compared with those obtained by using the 'exact' solution of the large-scale diffraction problem. The roles of shadowing and multiple wave scattering processes are discussed, and qualitative explanations of the difference between these two approaches are given.     

In situ, time-resolved normal incidence reflectance spectroscopy of polycrystalline platinum microelectrodes in aqueous electrolytes

In situ normal incidence reflectance spectra of polycrystalline Pt microelectrodes have been monitored as a function of the applied potential in aqueous 0.5 M H(2)SO(4) using a He-Ne laser source (633 nm) and a beam splitter/microscope objective arrangement. Data recorded during voltammetric cycles in Ar-purged solutions revealed a linear correlation between the normalized change in reflectance, DeltaR/R = (R(s) - R(ref))/R(ref) (where R(s) and R(ref) are the light intensities measured by the detector at the sampling, s, and reference potentials, ref, respectively), and the extent of oxidation of the Pt surface over a wide coverage range. Reflectance spectra were also collected in CO-saturated 0.5 M H(2)SO(4) during chronocoulometric measurements involving judiciously selected limits for both the potential step and duty cycle parameters. Analysis of these results made it possible to extract contributions to the current derived from oxide formation during oxidation of adsorbed and bulk CO, based strictly on the optical response.     

Gamma-Ray Transitions Induced in Nuclear Spin Isomers by X-Rays

Because of the high density of energy storage and the large cross section for its release, nuclear spin isomers have attracted considerable recent interest. The triggering of induced gamma emission from them has encouraged efforts to develop intense sources of short-wavelength radiation. One of the more interesting examples is the 16⁺ 4-qp isomer of ¹⁷⁸Hf which stores 2.445 MeV for a half-life of 31 years meaning that as a material, such isomeric ¹⁷⁸Hf would store 1.3 GJ/g. Recently, a sample containing 6.3×10¹⁴ nuclei of the isomer of ¹⁷⁸Hf was irradiated with X-ray pulses derived from a device operated at 15 mA to produce bremsstrahlung radiation with end point energies set to values between 60 and 90 keV. Emission of gamma radiation from the sample was increased by 1–2% above the quiescent value of spontaneous emission. Such an accelerated decay of the ¹⁷⁸Hf isomer is consistent with an integrated cross section of 2.2×10⁻²² cm² keV if the resonant absorption of the X-rays takes place below 20 keV as indicated by the use of selective absorbing filters in the irradiating beam. The work reported here describes the current experimental focus and results recently obtained with the use of coincident detection of emitted gamma photons by several detectors. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlation‐induced magnetic order and magnetic phenomena in transition‐metal δ‐doped cubic ZnO

The properties of transition‐metal (V, Cr, Mn, Fe, Co, Ni) δ‐doped ZnO are reported based on ab‐initio electronic structure calculations where the on‐site electronic correlations are included using the Hubbard parameters. The calculated electronic and magnetic properties are considerably altered with respect to usual band‐structure calculations. Most of the studied systems are found to be either half‐metals or ferromagnetic/antiferromagnetic semiconductors and thus can be employed in a variety of spintronic applications as spin‐filter materials.

     

Poisson brackets and symplectic invariants

We introduce new invariants associated with collections of compact subsets of a symplectic manifold. They are defined through an elementary-looking variational problem involving Poisson brackets. The proof of the non-triviality of these invariants involves various flavors of Floer theory, including the μ ³-operation in Donaldson-Fukaya category. We present applications to approximation theory on symplectic manifolds and to Hamiltonian dynamics.     

Bioaccumulative and conchological assessment of heavy metal transfer in a soil-plant-snail food chain

ABSTRACT: BACKGROUND: Copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) can pose serious threats to environmental health because they tend to bioaccumulate in terrestrial ecosystems. We investigated under field conditions the transfer of these heavy metals in a soil-plant-snail food chain in Banat area, Romania. The main goal of this paper was to assess the Roman snail (Helix pomatia) usefulness in environmental monitoring as bioindicator of heavy metal accumulation. Eight sampling sites, selected by different history of heavy metal (HM) exposure, were chosen to be sampled for soil, nettle leaves, and newly matured snails. This study also aimed to identify the putative effects of HM accumulation in the environment on phenotypic variability in selected shell features, which included shell height (SH), relative shell height (RSH), and whorl number (WN). RESULTS: Significantly higher amounts of HMs were accumulated in snail hepatopancreas and not in foot. Cu, Zn, and Cd have biomagnified in the snail body, particularly in the hepatopancreas. In contrast, Pb decreased when going up into the food chain. Zn, Cd, and Pb correlated highly with each other at all levels of the investigated food chain. Zn and Pb exhibited an effective soil-plant transfer, whereas in the snail body only foot Cu concentration was correlated with that in soil. There were significant differences among sampling sites for WN, SH, and RSH when compared with reference snails. WN was strongly correlated with Cd and Pb concentrations in nettle leaves but not with Cu and Zn. SH was independent of HM concentrations in soil, snail hepatopancreas, and foot. However, SH correlated negatively with nettle leaves concentrations for each HM except Cu. In contrast, RSH correlated significantly only with Pb concentration in hepatopancreas. CONCLUSIONS: The snail hepatopancreas accumulates high amounts of HMs, and therefore, this organ can function as a reliable biomarker for tracking HM bioavailability in soil. Long-term exposure to HMs via contaminated food might influence the variability of shell traits in snail populations. Therefore, our results highlight the Roman snail (Helix pomatia) potential to be used in environmental monitoring studies as bioindicator of HM pollution.