Surface currents on a perfect conductor, induced by a magnetic dipole

An oscillating magnetic dipole located near a perfect conductor induces a current density on the surface of the metal. We have derived an expression for this current density, and studied its field line patterns for various orientations of the dipole moment. When the dipole moment is perpendicular to the surface, the field lines are circles which run clockwise and counterclockwise. For a linear dipole oriented parallel to the surface, the field line pattern is much more complex, and it contains singular points. When the dipole moment rotates in a plane parallel to the surface, the field lines are spirals. A field line spirals inward from infinity to some given point, after which it spirals outward back to infinity. We have also considered the Poynting vector of the electromagnetic field near the surface, and we found that its field lines can have singular points or exhibit a vortex.     

Investigation on transient electromagnetic scattering from a randomly rough surface and the perfect electric conductor target with an arbitrary cross section above it

The Time-Domain-Integral-Equation (TDIE) method is proposed to analyze transient scattering interaction between a two-dimensional infinitely long conducting target with an arbitrary cross section and a one-dimensional rough surface. Based on the electric-field-integral-equation in time domain, the explicit and implicit solutions of MOT (Marching-on-time) are derived and presented. The current response at the center of the rough surface and the far electric field response with time in the composite model are calculated and analyzed. The numerical results are compared and verified with those obtained by conventional MOM-IDFT (Method of Moment-inverse discrete Fourier transform). Finally, the influence of the size, the location of the target and the incident angle on the current response and the far electric fields response are discussed in detail. Supported by the National Natural Science Foundation of China (Grant No. 60571058) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070701010)     

Behavior of zirconium in the ZrOW(100) system at high temperature, studied by ISS, AES and work-function measurements

The behavior of zirconium atoms at the W(100) surface associated with oxygen adsorption at different sample temperatures has been studied by Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and the relative change of the work function (Δф) measured by the onset of the secondary electron energy distribution. The results have revealed: (i) adsorption of zirconium onto the W(100) surface followed by the elevation of the sample temperature up to 1710 K in an oxygen partial pressure of 2.7 × 10⁻⁴ induces complete diffusion of zirconium atoms into the W(100) substrate; (ii) further exposure of oxygen induces co-existence of oxygen and tungsten on the surface at 1710 K, resulting in a work function of 4.37 eV; (iii) keeping the sample temperature at 1710 K, simple evacuation of the system has resulted in surface segregation of zirconium atoms to the surface to form a zirconium atomic layer on the top-most surface, reducing the work function to 2.7 eV. The results have revealed that this specific behavior of zirconium atoms at high temperature assures, with very good reproducibility, the highly stable performance and long service life of Zr---O/W(100)-emitters in practical use, even in a low vacuum of 10⁻⁶ Pa.     

Point sources and rays in the phase-space representation of random electromagnetic fields

Random stationary electromagnetic fields can be modeled in terms of point sources disposed onto two distinct layers, named the radiant and the virtual layers. It allows describing the propagation of the power and the states of spatial coherence and polarization of the field by means of separate phase-space ray-maps. Spatial coherence of the field, correlation of the polarization angles and achievement of the Fresnel-Arago laws within the structured spatial coherence supports are the necessary conditions that determine the electromagnetic point sources in the virtual layer. The contributions of these sources produce two types of modulations on the electromagnetic field at the observation plane, one of them of scalar type is the interferometric power modulation, and the other one of vector type is de modulation of the polarization state provided by the radiant point sources, according to the Stokes' parameters. Some examples are presented.     

Photon-stimulated desorption of methanol on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds

Photostimulated desorption experiments have been performed on deuterated methanol adsorbed on Si(111)7 × 7 and Si(100)2 × 1 at the C 1s and O 1s thresholds. D⁺ and the masses of the series CD⁺_x are produced in the photofragmentation process in both energy ranges. A comparison has been made with the photofragmentation spectra of methanol in the gas phase and two different desorption mechanisms have been hypothesized for the desorption of D⁺ and higher masses from the silicon surfaces at the C 1s threshold.     

Experiments on the reflection of cold atoms from magnetic thin films

We report the results from a series of experiments in which ferromagnetic thin films were used as atom mirrors for laser-cooled rubidium atoms released from a magneto-optical trap. The thin films were made of cobalt and lanthanum calcium manganite (LCMO) with thicknesses between 20 and 300 nm. The magnetic domains in these thin films have a periodic structure where the spatial period is of the order of the thickness of the film, and the field decays exponentially above the film over a length scale comparable to the domain size. Thus, the neutral atoms reflect off these films from distances comparable to the thickness of the film, resulting in modification of the reflectivity due to the competition between the repulsive magnetic force and the attractive short-range forces such as van der Waals and Casimir forces. The smoothness of the atom mirror is also modified due to the proximity of the magnetic domains. The reflectivity is sensitive to the domain structure and size, which can be modified in LCMO by applying a modest external magnetic field. In this paper, we discuss the evaluation of the thin films as magnetic mirrors for atom optics, and the measurement of the van der Waals force with an accuracy of about 15%, using cobalt thin films. We also discuss some preliminary results on the temperature-dependent reflectivity for atoms near the ferromagnetic transition at 250 K in the LCMO film, and on the domain dynamics and relaxation.     

Comparison of the interaction of Pd with positively and negatively poled LiNbO3(0 0 0 1)

To investigate the possibility of manipulating the surface chemical properties of finely dispersed metal films through ferroelectric polarization, the interaction of palladium with oppositely poled LiNbO₃(0 0 0 1) substrates was characterized. Low energy ion scattering indicated that the Pd tended to form three-dimensional clusters on both positively and negatively poled substrates even at the lowest coverages. X-ray photoelectron spectroscopy (XPS) showed an upward shift in the binding energy of the Pd 3d core levels of 0.9 eV at the lowest Pd coverages, which slowly decayed toward the bulk value with increasing Pd coverage. These shifts were independent of the poling direction of the substrate and similar to those attributed to cluster size effects on inert supports. Thus, the spectroscopic data suggested that Pd does not interact strongly with LiNbO₃ surfaces. The surface chemical properties of the Pd clusters were investigated using CO temperature programmed desorption. On both positively and negatively poled substrates, CO desorption from freshly deposited Pd showed a splitting of the broad 460 K desorption peak characteristic of bulk Pd into distinct peaks at 270 and 490 K as the Pd coverage was decreased below 1.0 ML; behavior that also resembles that seen on inert supports. It was found that a small fraction of the adsorbed CO may dissociate (<2%) for Pd on both positively and negatively poled substrates. The thermal response of the smaller Pd clusters on the LiNbO₃ surfaces, however, was different from that of inert substrates. In a manner similar to Nb₂O₅, when CO desorption experiments were carried out a second time, the adsorption capacity decreased and the higher temperature desorption peak shifted from 490 K to below 450 K. This behavior was independent of the substrate poling direction. Thus, while there was evidence that LiNbO₃ does not behave as a completely inert support, no significant differences between positively and negatively poled surfaces were observed. This lack of sensitivity of the surface properties of the Pd to the poling direction of the substrate is attributed to the three-dimensional Pd clusters being too thick for their surfaces to be influenced by the polarization of the underlying substrate.     

Study on electromagnetic backscattering and Doppler spectrum of a moving spherical target above time-varying sea surface

The surface electric current of the sea surface and the first-order scattering field from the spherical target were obtained by employing Physical Optics method and Mie theory, respectively. The backscattering field of the time-evolving sea surface was calculated by using Kirchhoff Approximation. Meanwhile, by taking the advantage of a newly developed technique that utilizes the reciprocity theorem, the difficulty in formulating the secondary coupling scattering fields from the spherical target above the sea surface was reduced. The dependence of the secondary coupling backscattering field on the size and the position of the spherical target was discussed, and the characteristic of the Doppler spectrum of the composite backscattering field with different incident angles was analyzed in detail. Supported by the National Natural Science Foundation of China (Grant No. 60571058) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070701010)     

Theoretical and experimental study of N(D) formation in nitrogen collisions with a metal surface

The dynamics of nitrogen collisions with metals partially covered by alkali atoms is studied both experimentally and theoretically. Our attention focuses on the formation of N⁻(¹D) metastable ions and their interaction with the surface. We present the electron energy spectra induced by slow collisions of N⁺ ions with partially cesiated Pd(111) surfaces under grazing incidence. These spectra display, as a function of Cs coverage, a sharp feature which is due to the autodetachment of N⁻(2p⁴, ¹D) to the N(2p³, ⁴S) ground state. Our calculations, performed with the coupled angular mode (CAM) method on the basis of the resonant electron exchange between the nitrogen atom in states of the 2p³ configuration and the metal surface, consistently explain how negative ions formed close to the surface can survive against electron loss to the metal during the outgoing trajectory and can later decay as free ions. In order to understand the alkali coverage dependence of the N⁻(¹D)-N(⁴S) peak intensity, the local character of the nitrogen interaction with the surface partially covered by adsorbate atoms has been taken into account.     

Local spectroscopy of carrier reflection at the interface between a normal metal and the Peierls conductor K0.3MoO3

The nature of carrier reflection from a normal metal-Peierls conductor interface is clarified by studying the characteristics of point contacts formed with an intermediate metallic layer sputtered onto the (010) face of K_0.3MoO₃ crystals (injection along the chains) or the face (injection perpendicular to the chains). In the Peierls state, for bias voltages eV smaller than the Peierls gap Δ_P, an excess differential resistance is observed with a local minimum at V=0. The magnitude of the excess resistance is proportional to a ²/d ², where a is the contact diameter and d is the thickness of the metal film. The excess resistance is much higher for injection along the chains than for injection perpendicular to them. A comparative analysis of the data for different injection directions indicates that the dominant contribution to the excess resistance for injection along the chains is from normal reflection of carriers without changes in the sign of their charge and with a momentum transfer 2p _F to the condensate of electron-hole pairs carried away from the interface. Zh. éksp. Teor. Fiz. 113, 1830–1842 (May 1998)     

Probing the interactions of Pt, Rh and bimetallic Pt-Rh clusters with the TiO2(1 1 0) support

Pt, Rh, and Pt-Rh clusters on TiO₂(1 1 0) have been investigated by scanning tunneling microscopy (STM), soft X-ray photoelectron spectroscopy (sXPS), and low energy ion scattering (LEIS). The surface compositions of Pt-Rh clusters are Pt-rich (66-80% Pt) for room temperature deposition of both 2 ML of Pt on 2 ML of Rh (Rh + Pt) and 2 ML of Rh on 2 ML of Pt (Pt + Rh). Pt and Rh atoms readily diffuse within the clusters at room temperature, and although diffusion is slower at 240 K, intermixing of Pt and Rh still occurs. The binding energies of surface and bulk states for Rh(3d_5/2) and Pt(4f_7/2) can be distinguished in sXPS studies, and an analysis of these spectra indicates that the surface compositions of the Pt + Rh and Rh + Pt clusters are similar at room temperature but not identical. In addition to sintering, the pure Pt, pure Rh and Pt-Rh clusters become completely encapsulated by titania upon heating to 700 K. sXPS investigations show that annealing the clusters to 850 K induces reduction of titania support to Ti⁺² and Ti⁺³, with the extent of reduction being the greatest for Pt, the least for Rh and intermediate for Pt-Rh. We propose that TiO₂ is reduced at the metal-titania interface on top of the clusters, not at the base of the clusters. Furthermore, the extent of titania reduction is greater for metal clusters with weaker metal-oxygen bonds because oxygen atoms are less likely to migrate to the top of the clusters, and therefore the encapsulating titania is oxygen-deficient.     

Visible light scattering study at 470, 550, and 660 nm of components of mineral dust aerosol: Hematite and goethite

Iron oxides, usually in the form of hematite or goethite, comprise an important component of atmospheric mineral dust aerosol. Because these minerals are strong visible absorbers they play a critical role in determining the overall impact of dust aerosol on climate forcing. In this work, results from light scattering measurements from hematite and goethite dust aerosol are presented for three visible wavelengths, λ=470, 550, and 660 nm. We observe important systematic differences in the scattering between these different iron oxide samples, as well as significant wavelength dependence across the visible region of the spectrum. Aerosol size distributions are measured simultaneously with the light scattering, enabling a rigorous comparison between theoretical light scattering models and experimental data. Theoretical simulations of the scattering are carried out using both Mie and T-Matrix theories. Simulations are in reasonably good agreement with experimental data for hematite; thus, our data offer a useful check on tabulated optical constants for hematite. However, simulations show very poor agreement for goethite. The poor agreement in the goethite case is likely the result of particle shape effects related to the rod-like morphology of the goethite particles. This study demonstrates how particle mineralogy and morphology play an important role in dictating the optical properties of mineral dust aerosol, a major component of tropospheric dust.     

Adsorption of hydrophobic polyelectrolytes at the air/water interface: Conformational effect and history dependence

We present an experimental study of the adsorption of hydrophobic highly charged polyelectrolytes on a neutral and hydrophobic surface, the air/water interface. The polymer was a randomly sulphonated polystyrene with charge fractions between 0.3 and 0.9 and the adsorbed layers were characterised by Langmuir through measurements, ellipsometry and X-ray reflectivity. The adsorption rate is always very slow and the resulting layers are very thin (< 3 nm). A maximum of adsorption with the charge fraction is observed which we relate to the conformation of the chains in solution. We show that adsorption is partially irreversible, strongly hysteretic and that the state of an adsorbed layer depends on its history. Received 16 June 2000     

Magnetic properties of deposited gadolinium atoms,dimers and their monoxides

Magnetic properties of gadolinium clusters deposited on magnetic ultrathin Fe/Cu(100) films have been measured using X-ray Magnetic Circular Dichroism spectroscopy (XMCD). 3d↦4f as well as 4d↦4f absorption spectra are presented and discussed. Changes in the relative peak intensities between the monomer, dimer and the corresponding monoxides are observed for the multiplets. The comparison with bulk measurements and calculations exhibit a transition from atomic like to bulk like properties.     

Extension of the radiation spectrum method for the reflection calculation at the end of a strongly guiding optical waveguide

In this work we extend the radiation spectrum method (RSM) with evanescent modes to use it for the calculation of the reflection coefficient at the end of a strongly guiding dielectric waveguide. The extension is made by considering the coupling between the radiation (or evanescent) modes at both sides of the optical discontinuity. To insure the convergence of the method, this extension is achieved analytically. Using this technique, we show that for small guide width, the generation of evanescent modes results in a complex reflection coefficient. The magnitude and the phase of the reflection coefficient are calculated and compared with the simple theory of the effective index as well as the finite difference time domain (FDTD) technique. The spectra of the transmitted and reflected fields are also obtained.     

Structure of the neopentane monolayer adsorbed on MgO(0 0 1): experiments and calculations

The monolayer of C(CH₃)₄ molecules adsorbed upon a highly homogeneous MgO powder surface has been studied by means of neutron diffraction techniques at the temperature 210 K. The structure of the monolayer presents a strong commensurability c(2 × 4) with the MgO(0 0 1) surface. The results of potential calculations support the proposed structure.