Pb induced charge accumulation on InAs(1 1 1)B

The Pb/InAs(1 1 1)B interface has been studied by synchrotron radiation photoelectron spectroscopy (SR-PES) of valence band and In4d, As3d and Pb5d core levels. Room temperature deposition of ∼1 ML of Pb on InAs(1 1 1)B leads to an ordered overlayer that induces a metallic channel at the surface, as seen through a weak emission in the vicinity of the Fermi level. Its narrow localization in reciprocal space supports the formation of a two-dimensional free electron gas (2DEG) in the surface region. It is proposed that the adsorbed metal layer swaps the initial polarisation of the surface and thus pulls electrons back to the surface. This charge re-arrangement increases the charge density in the accumulation layer and reduces the screening length and thus the depth of the potential well at the surface.     

Surface diffusion of Pb clusters on Cu(1 1 1) influenced by size dependent cluster-substrate misfit: Monte Carlo tight binding simulation model

The migration of two-dimensional Pb clusters on Cu(1 1 1) surface is studied in the framework of three-dimensional (3D) continuum space tight binding (TB) computational model. Monte Carlo simulations based on many-body TB interactions reveal significant influence of cluster-substrate misfit on diffusion behavior of two-dimensional (2D) clusters. The analysis of pair distribution function (PDF) demonstrates that cluster lattice constant depends on the number of atoms N for 1 < N < 10. The observed effect is more pronounced for heteroepitaxial than homoepitaxial systems. It can be explained in the framework of model accounting for enhanced charge transfer in heteroepitaxy and strong influence of surface potential on cluster atomic arrangement. The variation of lattice constant leads to variable misfit which affects the island migration behavior. The results are discussed in a physical model that implies cluster diffusion with size dependent cluster-substrate misfit.     

Two-dimensional plasmons in hole space charge layers on silicon

We have observed two-dimensional plasmons in hole space charge layers of silicon. On Si(110) surfaces the plasmon mass depends on the charge density and shows a significant anisotropy for different directions of the plasmon wavevector in the surface. The determination of the plasmon mass allows detailed informations on the anisotropic and nonparabolic 2-D bandstructure of hole space charge layers.     

Adsorption of Cs on InAs(1 1 1) surfaces

Caesiated InAs(1 1 1)B (1 × 1) and InAs(1 1 1)A (2 × 2) surfaces have been studied by photoelectron spectroscopy. On the InAs(1 1 1)B a new (√3 × √3)R30° reconstruction was observed. During Cs evaporation remarkably small changes are observed in the lone pair states, and no sign of an accumulation layer at the surface can be observed. Instead, the additional charge provided by Cs is rapidly transported towards the bulk. On the InAs(1 1 1)A cesium behaves as a typical electropositive alkali metal donator that enhances the already existing accumulation layer.     

Landau-level splitting in graphene in high magnetic fields

The quantum Hall (QH) effect in two-dimensional electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors nu = 0, +/-1, +/-4 are discovered at magnetic fields B > 20 T, indicating the lifting of the fourfold degeneracy of the previously observed QH states at nu = +/-4(absolute value(n) + 1/2), where n is the Landau-level index. In particular, the presence of the nu = 0, +/-1 QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at nu = +/-4 is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the n = 1 Landau level.     

表层氟化聚乙烯中的空间电荷

本文试图简要地汇集近年来与近期我们所取得的关于表面氟化对聚乙烯(PE) 空间电荷行为影响的研究结果, 总结与探讨PE中的空间电荷积累与其氟化层特性和特征间的关联. 这些结果显示在氟化反应气中没有氧存在时一个非常薄的氟化层能产生有效的电荷抑制, 而当氧存在时为达到有效的电荷阻挡、需要一个具有高氟化度的非常厚的氟化层. 在影响空间电荷的诸电学因素中, 氟化层的电荷传导特性比其电荷俘获特性和介电常数或极性对阻止电荷注入材料内部更为重要, 尽管氟化层的高介电常数和被俘获的电荷会降低界面电场、因此减少电荷的电极注入. 氟化层的电荷传导特性密切关联于其自由体积, 反应混合气中存在的氧对减小自由体积、因此对电荷的抑制具有强的负面影响.     

Pockels effect of water in the electric double layer at the interface between water and transparent electrode

We have obtained the first experimental evidence for the Pockels effect of water, which is induced by a high electric field in the electric double layer (EDL) on the water-transparent electrode interface. The electric-field induced energy shift of the visible interference fringes of a 300 nm indium-tin-oxide (ITO) electrode layer is observed, indicating a negative refractive index change at the interface. Numerical calculation reproduces well the experimental observation, showing that the signal mainly originates from water in the EDL. The Pockels constants of water are estimated to be r₃₃ = 5.1 × 100 pm/V and r₁₃ = 1.7 × 100 pm/V. The large anisotropy of the Pockels effect of water is deduced from the incidence angle dependence of the p-polarization signal. At the same time, the ITO shows a blue shift of the band gap in the UV due to the band population effect in the space charge layer. The plasma frequency in the near IR is also expected to increase due to the band population effect, since the ITO has a high doped carrier population close to metal. A negative refractive index change in the ITO space charge layer is induced from both effects, but its effect on the signal is estimated to be much smaller than that of the negative refractive index change of water in the EDL.     

Rotating skyrmions of the (2 + 1)-dimensional Skyrme gauge model with a Chern-Simons term

The (2 + 1)-dimensional Skyrme gauge model with a Chern-Simons term is considered. The presence of the Chern-Simons term makes possible the existence of two-dimensional skyrmions in this model, which carry magnetic flux and have an electric charge and a nonzero angular momentum. It is shown that the model also admits the existence of two-dimensional skyrmions with a nonzero phase frequency of rotation. Due to the nontrivial topological properties of the model, the magnetic flux, the electric charge, and the angular momentum of a two-dimensional rotating skyrmion turn out to be interrelated. Analytic and numerical investigations of the properties of rotating two-dimensional skyrmions are carried out.     

Ionic conductivity in epitaxial AgCl films

The space charge conductance and bulk conductivity of epitaxial {111} and {200} films of AgCl with thickness in the 1-μ range has been measured. Interstitial silver ions predominate in concentration within the space charge layer and in the bulk, but are present in concentration roughly $\text{1}\text{10}$ the concentration present in similar AgBr films. Values of the free energy change required for interstitial formation (0·56 eV) and vacancy formation (0·69 eV) were obtained for generation at the surface sites on a {111} film; similar values are reported for {200} films, although the value depends somewhat on the substrate. The concentration of ionic carriers present in the AgCl thin films is 100–1000 times greater than in bulk single crystals.     

A Discussion of Deuteron Transverse Charge Densities

The deuteron transverse charge density ρ_C(b) is the two-dimensional Fourier transform of its charge form factor in the impact space. We show that different parameterizations of the charge form factors provide different ρ_C(b), in particular at the central value of impact parameter (b=0), although all the parameterizations can well reproduce the form factors in the region of small Q². In addition, we also check the explicit contributions from the different coordinate intervals of the deuteron wave function to its root-mean-square radius.     

Effect of the space charge of emitted electrons on field electron emission

It is shown that the conventional technique of substituting the field strength at the space charge-emitter interface that is calculated with the Poisson equation into the Fowler-Nordheim formula considerably overestimates the effect of space charge on field electron emission. In this work, the space-charge-induced field attenuation as a function of the emission current density and radius of curvature of the emitter surface is derived using the model of a planar space-charge layer. It is argued that field electron emission cannot be studied in terms of the spherical diode model, since it assumes the presence of a space charge on the back (nonemitting) emitter surface, which is in fact absent. It is stated that one should consider the discrete character of the charges when investigating the space charge in field electron emission, because the mean spacing between the electrons emitted far exceeds the emission barrier width.     

Surface state lifetime in quasi-one-dimensional oxygen stripes on Cu(1 1 0)

Scanning tunneling spectroscopy data on the lifetime and bottom energy of a surface state band in quasi-one-dimensional stripes are presented as a function of stripe width. The adsorbate system Cu(1 1 0)-p(2 × 1)O has been used to prepare a two-dimensional adsorbate layer and almost defect free quasi-one-dimensional stripes with a width of 3-15 CuO rows, respectively. The onset of an unoccupied surface state band (at 0.56 eV above Fermi-energy on the fully covered surface) has been analyzed with respect to the quasi-particle lifetime at the crossover from two to one dimension. A drastic increase of the onset-width with decreasing stripe width is observed. It can be explained by a decrease of quasi-particle lifetime using a Fabry-Pérot-like model. We obtain a reduction of lifetime from 37 ± 8 fs on the two-dimensional adsorbate layer to 5 ± 1 fs on a three CuO rows wide stripe.     

Generating high-current monoenergetic proton beams by a circularly polarized laser pulse in the phase-stable acceleration regime

A new ion acceleration method, namely, phase-stable acceleration, using circularly-polarized laser pulses is proposed. When the initial target density n(0) and thickness D satisfy a(L) approximately (n(0)/n(c))D/lambda(L) and D>l(s) with a(L), lambda(L), l(s), and n(c) the normalized laser amplitude, the laser wavelength in vacuum, the plasma skin depth, and the critical density of the incident laser pulse, respectively, a quasiequilibrium for the electrons is established by the light pressure and the space charge electrostatic field at the interacting front of the laser pulse. The ions within the skin depth of the laser pulse are synchronously accelerated and bunched by the electrostatic field, and thereby a high-intensity monoenergetic proton beam can be generated. The proton dynamics is investigated analytically and the results are verified by one- and two-dimensional particle-in-cell simulations.     

Spin and charge transport in the presence of spin-orbit interaction

We present the study of spin and charge transport in nanostructures in the presence of spin-orbit (SO) interaction. Single band tight binding Hamiltonians for Elliot-Yafet and Rashba SO interaction are derived. Using these tight binding Hamiltonians and spin resolved Landauer-Büttiker formula, spin and charge transport is studied. Specifically numerical results are presented for a new method to perform magnetic scanning tunneling microscopy with non-magnetic tip but in the presence of Elliot-Yafet SO interaction. The spin relaxation phenomena in two-dimensional electron gas in the presence of Rashba SO interaction are studied and contrary to naive expectation, it is shown that disorder helps to reduce spin relaxation.     

Modelling of diffusion and conductivity relaxation of oxide ceramics

A two-dimensional square grain model has been applied to simulate simultaneously the diffusion process and relaxation of the dc conduction of polycrystalline oxide materials due to a sudden change of the oxygen partial pressure of the surrounding gas phase. The numerical calculations are performed by employing the finite element approach. The grains are squares of equal side length (average grain size) and the grain boundaries may consist of thin slabs of uniform thickness. An additional (space charge) layer adjacent to the grain boundary cores (thin slabs) either blocking (depletion layer) or highly conductive for electronic charge carriers may surround the grains. The electronic transport number of the mixed ionic-electronic conducting oxide ceramics may be close to unity (predominant electronic conduction). If the chemical diffusion coefficient of the neutral mobile component (oxygen) of the grain boundary core regions is assumed to be higher by many orders of magnitude than that in the bulk, the simulated relaxation curves for mass transport (diffusion) and dc conduction can deviate remarkably from each other. Deviations between the relaxation of mass transport and dc conduction are found in the case of considerably different electronic conductivities of grain boundary core regions, space charge layers, and bulk. On the contrary, the relaxation curves of mass transport and electronic conductivity are in perfect coincidence, when either effective medium diffusion occurs or the effective conductivity is unaffected by the individual conductivities of core regions and possible space charge layers, i.e. the grain boundary resistivity is negligible.     

Interfacial behavior of alkyltrichlorosilane monolayers on silicon: Control of flat-band potential and surface state distribution using chain length variation

The passivating behavior of octadecyltrichlorosilane (C₁₈), dodecyltrichlorosilane (C₁₂) and octyltrichlorosilane (C₈) self-assembled monolayers (SAMs) on Si(1 0 0), has been quantitatively compared using cyclic voltammetry and impedance analysis in the presence and absence of an external redox probes like ferrocene. In all these cases, Fourier transform infra red (FTIR) spectroscopy and contact angle measurements give clear evidence for the presence of a closely packed, oriented and hydrophobic monolayer. The electron transfer behavior of ferrocene is found to be drastically affected by the presence of monolayer and the reasons for such significant variation are analyzed in terms of the change in resistance, dielectric thickness and coverage of the monolayer. Double layer capacitance is found to decrease systematically with increasing the chain length of the monolayer suggesting a smooth variation in the “plane of closest approach” with the thickness of the monolayer, despite the presence of a space charge layer on Si electrode. Comparison of the electrochemical properties of the SAM-derivatized Si electrodes with that of a bare Si electrode using impedance analysis exhibits a four order of magnitude decrease in the apparent rate constant of ferrocene oxidation due to the barrier provided by various monolayers (C₈, C₁₂, C₁₈). A peak in the capacitance-potential curve presumably, due to surface states, is suppressed with an increase in the chain length of the monolayer. In addition, a positive shift in flat-band potential (E_fb) with the monolayer chain length, suggests the covalent coupling of the silane monolayer.     

Alloy formation in the Au{1 1 1}/Ni system - An investigation with scanning tunnelling microscopy and medium energy ion scattering

Using the techniques of scanning tunnelling microscopy (STM) and medium energy ion scattering (MEIS), we examine the growth and annealing behaviour of ultrathin Ni films on Au{1 1 1} at 300 K. As has been shown previously, submonolayer growth of Ni on Au{1 1 1} is strongly influenced by the presence of the herringbone reconstruction with two-dimensional clusters nucleating at herringbone elbows. Second layer growth commences prior to the completion of the monolayer. After multiple layers have been deposited, the surface morphology retains a similar cluster-like appearance. Annealing produces surfaces exhibiting long range Moiré structures and, at higher temperature, triangular misfit dislocations. We use MEIS to examine the composition and structure of these surface alloy phases and conclude that in each case, they consist of an essentially pure Au surface layer on a bimetallic second layer.     

Betatron resonance crossing during the intense beam bunching

In the process of bunching an intense beam, the betatron oscillation frequency of particles changes due to a growth in their momentum rejection (in the presence of chromaticity) and changes in the Coulomb shift. These changes lead to the intersection of nonlinear betatron resonances. A mathematical model of the dynamics of ions in the process of bunching an intense beam has been built in order to assess the influence of transverse betatron resonances on the motion of particles. This model takes into account the influence of space charge on the longitudinal and transverse oscillations of particles in the presence of one- and two-dimensional betatron resonances. An analysis of the motion of particles near the one- and two-dimensional third-order resonances is given. Results of the numerical simulation of particle losses and emittance growth in the process of bunching an intense beam in the storage ring at the Institute for Theoretical and Experimental Physics (ITEP) are presented.     

Dissolution behaviour of the barrier layer of porous oxide films on aluminum formed in phosphoric acid studied by a re-anodizing technique

Chemical dissolution of the barrier layer of porous oxide films formed on aluminum foil (99.5% purity) in the 4% phosphoric acid after immersion in 2 mol dm⁻³ sulphuric acid at 50 °C has been studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. A digital voltmeter with a computer system was used to record the change in the anode potential with re-anodizing time. It has been found that the barrier layer material may consist of two or three regions according to the dissolution rate. The barrier oxide contains two layers at 35 V: the outer layer with the highest dissolution rate and the inner layer with low dissolution rate. The barrier oxide contains three layers at 40 V and above it: the outer layer with high dissolution rate, the middle layer with the highest dissolution rate and the inner layer with low dissolution rate. It has been shown that there is a dependence of the dissolution rate on the surface charge of anodic oxide film. Annealing of porous alumina films for 1 h at 200 °C leads to disappearance of layers with different dissolution rates in the barrier oxide. We explained this phenomenon by the absence of the space charge in the barrier oxide of such films.     

Diffusive spin dynamics in ferromagnetic thin films with a Rashba interaction

In a ferromagnetic metal layer, the coupled charge and spin diffusion equations are obtained in the presence of both Rashba spin-orbit interaction and magnetism. The misalignment between the magnetization and the nonequilibrium spin density induced by the Rashba field gives rise to Rashba spin torque acting on the ferromagnetic order parameter. In a general form, we find that the Rashba torque consists of both in-plane and out-of-plane components, i.e., T=T(perpendicular)y^×m^+T(parallel)m^×(y^×m^). Numerical simulations on a two-dimensional nanowire consider the impact of diffusion on the Rashba torque and reveal a large enhancement to the ratio T(parallel)/T(perpendicular) for thin wires. Our theory provides an explanation for the mechanism driving the magnetization switching in a single ferromagnet as observed in the recent experiments.