Theory of infra-red and optical spectra of antiferromagnets

The contributions of magnons to the optical properties of antiferromagnets having the rutile structure are discussed. The properties considered are electric-dipole active two-magnon absorption in the infra-red, and magnon sidebands on sharp-line exciton transitions in the visible. The discussion is based on a thorough treatment of the properties of excitons and magnons in the antiferromagnetically ordered state. The site-group and space-group symmetries of the magnetic excitations are derived and the selection rules for electric and magnetic dipole transitions are determined. The occurrence of magnetic Davydov splittings of the excitons is investigated, and their symmetry properties throughout the Brillouin zone are derived. The functional dependences of exciton energy on wave vector are calculated. Applications of the theory are made to experimental results on excitons and magnons in MnF₂, FeF₂ and CoF₂.

The possible mechanisms for two-magnon and magnon-sideband absorption are discussed, and the influence of crystal symmetry on these mechanisms is described. The two-magnon state responsible for electric-dipole absorption is identified and selection rules for electric-dipole activity are presented. A spin Hamiltonian for the two-magnon process is set up and used to derive expressions for absorption coefficients for electric vector parallel and perpendicular to the crystal c-axis. Comparison with experiment for MnF₂ yields numerical values for the parameters of the basic coupling mechanism. The exciton-magnon states which give rise to magnon-sideband absorption are explicitly constructed and electric-dipole selection rules are derived for all possible types of sideband. Spin Hamiltonians for the various magnonsideband absorption processes are presented and used to derive expressions for sideband shapes. The results are applied to the experimental spectra for MnF₂ and FeF₂ and the sideband shapes in MnF₂ are calculated numerically. The sideband shapes observable in emission spectra are also briefly discussed.     

Dependence on substrate topography of growth of nanosized dendritic structures in an electron-beam-induced deposition process

Nanometer-sized W-dendrites are fabricated on Al₂O₃ substrates with an electron-beam-induced deposition process. Dependence of growth of nanodendrite on surface topography is investigated with transmission electron microscopy. It is confirmed that the nanodendrite grows on convex surfaces but not around a hole on a substrate. These are attributed to different distribution of charges on surfaces with different topographies during electron beam irradiation when charges are produced on the surface due to emission of second electrons. The charges accumulate on convex surface and do not distribute around a hole. Therefore, the nanodendrite grows on the former and not on the latter.     

Catalysis of dispersed silver particles on directional etching of silicon

Silver particles are dispersed on silicon by magnetron sputtering and post-annealing to investigate the catalytic effects of individual silver particles on wet etching of silicon surface. According to scanning electron microscopy, dispersed deep holes are present and the major etching direction is vertical to the surface of a Si(1 0 0) wafer or inclined to that on a Si(1 1 1) wafer. Our experiments indicate that the effect of the anisotropy of Si on directional etching is fundamental and the wafer resistivity and experimental process have important influence on the etching results. In addition, aggregation of silver particles and random horizontal etching on the surface of the wafer are caused by the local imbalance between the oxidant and HF. Our results enable better understanding of the catalytic effects of metal particles on silicon and are helpful to the preparation new silicon nanostructures.     

A density-functional theory investigation on desorption of O2 on Sn（111） and its comparison with initial oxidation on the X（111） （X=Si,Ge, Sn,Pb） surfaces

<正>The first-principles calculations are performed to investigate the adsorption of O₂ molecules on an Sn（111） 2×2 surface.The chemisorbed adsorption precursor states for O₂ are identified to be along the parallel and vertical channels, and the surface reconstructions of Sn（lll） induced by oxygen adsorption are studied.Based on this,the adsorption behaviours of O₂ on X（111）（X=Si,Ge,Sn,Pb） surfaces are analysed,and the most stable adsorption channels of O₂ on X（111）（X=Si,Ge,Sn,Pb） are identified.The surface reconstructions and electron distributions along the most stable adsorption channels are discussed and compared.The results show that the O₂ adsorption ability declines gradually and the amount of charge transferred decreases with the enhancement of metallicity.     

Field emission properties of carbon nanotubes grown on silicon nanowire arrays

Carbon nanotubes are synthesized on the silicon nanowire arrays which are fabricated on silicon substrate by chemical vapor depositing SiCl₄ and H₂ gases in the presence of Au catalysts. The silicon nanowires are single-crystal with lengths up to 100 μm and diameters ranging from 50 to 500 nm. The tangled carbon nanotubes are grown directly from the surface of Si nanowires. The field emission properties of the carbon nanotubes are investigated at the gap of 200 μm. The low turn on and threshold fields are obtained. The stabilization of the emission currents is also presented.     

SERS and XPS observation of the adsorption behavior of NO and NO2 on a silver powder surface

The influence of H₂O on the adsorption behavior of NO or NO₂ on a silver powder surface was studied by SERS and XPS at room temperature. Water vapor was found to be responsible for the adsorption of NO on the silver powder surface. When surface species such as Ag₂O are present on the surface, some of the NO₂ molecules are adsorbed on the surface species to produce NO^-₃, whereas NO molecules are adsorbed on a different site to produce NO^-₂.     

Li ion neutralization on Ag layers grown on Cu(1 1 1)

Electron transfer processes in the neutralization of Li⁺ ions on Ag layers grown on Cu(1 1 1) are investigated in quest of quantum confinement effects. Neutralization probabilities in the scattering of Li⁺ for incident ion energies in the 300 eV to 2 keV range are reported for Ag coverages ranging from 0.15 ML to 5 ML. Results are compared to those for Ag(1 1 1) and Cu(1 1 1) surfaces of bulk crystals. Although existing studies of the characteristics of Ag layers on Cu(1 1 1) indicate significant differences in electronic structure as a function of film thickness, the electron transfer probabilities we measure are found to be very close to those for bulk Ag(1 1 1). These results are commented on the basis of existing models and earlier studies of Li ion neutralization on various metals.     

Growth and shapes of a cubic lyotropic liquid crystal

Monocrystals of the cubic lyotropic liquid crystal phase V₁ are studied in droplets of the mixture C ₁₂ EO ₆/water surrounded by water vapor of controlled pressure p. Shapes of monocrystals are found to depend on the conditions of growth from the lamellar phase and on the nature of the substrate. After the growth, when the lamellar phase is exhausted and crystals are in equilibrium with water vapors, their shapes are shown to depend on the pressure p. Thermodynamic aspects of these phenomena are discussed. Received 22 November 2000     

Numerical study of homogeneous dynamo based on experimental von Kármán type flows

A numerical study of the magnetic induction equation has been performed on von Kármán type flows. These flows are generated by two co-axial counter-rotating propellers in cylindrical containers. Such devices are currently used in the von Kármán sodium (VKS) experiment designed to study dynamo action in an unconstrained flow. The mean velocity fields have been measured for different configurations and are introduced in a periodic cylindrical kinematic dynamo code. Depending on the driving configuration, on the poloidal to toroidal flow ratio and on the conductivity of boundaries, some flows are observed to sustain growing magnetic fields for magnetic Reynolds numbers accessible to a sodium experiment. The response of the flow to an external magnetic field has also been studied: The results are in excellent agreement with experimental results in the single propeller case but can differ in the two propellers case.     

On the fractal characterization of Paretian Poisson processes

Paretian Poisson processes are Poisson processes which are defined on the positive half-line, have maximal points, and are quantified by power-law intensities. Paretian Poisson processes are elemental in statistical physics, and are the bedrock of a host of power-law statistics ranging from Pareto’s law to anomalous diffusion. In this paper we establish evenness-based fractal characterizations of Paretian Poisson processes. Considering an array of socioeconomic evenness-based measures of statistical heterogeneity, we show that: amongst the realm of Poisson processes which are defined on the positive half-line, and have maximal points, Paretian Poisson processes are the unique class of ‘fractal processes’ exhibiting scale-invariance. The results established in this paper are diametric to previous results asserting that the scale-invariance of Poisson processes–with respect to physical randomness-based measures of statistical heterogeneity–is characterized by exponential Poissonian intensities.     

Micro-structuring of TiO2 thin films by laser-assisted diffraction processing

Thin films of TiO₂ are deposited by magnetron sputtering on glass substrate and are irradiated by UV radiation using a KrF excimer laser (248 nm). These thin films are patterned with a razor blade placed on the way of the radiation just in front of the TiO₂ thin film. Just near the edge of the razor blade on the thin film, diffraction lines are observed, resulting in the ablation of the film. These patterns are characterized by optical microscopy, mechanical profilometry. Diffraction up to the 35th order is observed. The results are shown to be compatible with a model in which electronic excitation plays the major role.     

Temperature programmed desorption studies of deuterium passivated silicon nanocrystals

As grown silicon (Si) surfaces are known to reconstruct in order to reduce the number of dangling bonds. Surface reconstructions of hydride-terminated Si(1 0 0) and Si(1 1 1) surfaces have already been extensively studied using temperature programmed desorption (TPD). The surfaces of nanocrystals, are yet to be probed using TPD. Si nanocrystals less than 8 nm and ranging from 50 to 200 nm in diameter are grown on SiO₂ surfaces in an ultra high vacuum chamber and the as grown surfaces are exposed to atomic deuterium. Desorption spectra are interpreted using analogies to Si(1 0 0). TPD spectra show that that the nanocrystals surfaces are covered by a mix of monodeuteride, dideuteride and trideuteride species. Monodeuteride species can be isolated by selectively annealing away the dideuteride and trideuteride, monodeuteride and dideuteride species can be isolated by annealing away the trideuteride. The relative populations of the deuterides depend on particle size, and their manner of filling on nanoparticles differs from that for extended surfaces. Etching of the nanocrystal surface is observed during TPD, which is a confirmation of the presence of trideuteride species on the nanocrystal surface.     

Evaluation for the configurational and electronic state of SO3 adsorbed on Pt surface

We evaluate the adsorption of SO₃ molecule on the Pt (1 1 1) surface using the first-principles calculations by a slab model with a periodic boundary condition. We find that there are four stable adsorption configurations on the Pt surface, where SO₃ molecules are adsorbed above the three-fold fcc and hcp sites. In two of these configurations, S and two O atoms are bound to the Pt atoms, and in two other of them, all the three O atoms are bound to Pt surface atoms. Besides, it is found that molecular orbitals of SO₃ and those of Pt surface are hybridized in the active metal d-bands region, that the localized molecular orbitals in SO₃ are stabilized, and that the charge is transferred from Pt to S 3p by SO₃ adsorption on Pt surface though the other interaction of S and O (bound to Pt) component with Pt is little. In addition, the bond between S and O bound to Pt become weak by SO₃ adsorption on Pt surface because the charge polarization to O-Pt bond weakens the bond between S and O bound to Pt. This interaction is assumed to encourage the breakage of S-O bond.     

Proton Spin and Chiral Dynamics

 Recent successful chiral models for the spin fractions of the proton rely on parametrizations that are inconsistent with deep inelastic lepton scattering unless the quark masses are neglected and inconsistent with chiral quark models based on constituent quarks, because only chiral spinflip transitions are considered. Non-spinflip transitions are important for constituent quarks as they depend on the quark masses. Therefore, the models are valid at a scale of about 0.63 GeV to Λ_χ, where dynamical quark masses are expected to be close to current quark masses, rather than Λ_QCD. When constituent quark masses are properly included, chiral quark models generate spin fractions that disagree with the proton spin data. Received July 1, 1998; accepted for publication January 30, 1999     

Practical improvements to real and imaginary spectral based modal parameter measurements of SDOF systems

For a single degree of freedom system, especially with non-light damping, the use of the real spectral part of either the displacement or velocity responses (or the transfer functions based on them) has the advantages of determining the natural frequency (f_n) directly, independent of the response parameter, and providing an accurate measurement for damping (ζ). However, this method is sensitive to spectral phase errors due to temporal misalignment of the signal or due to net inter-channel delay differences caused by signal filtering. Two techniques are presented to correct for these errors; one based on the correct temporal alignment of the impulsive part, and the other on the infimum of the imaginary spectral part. These are first demonstrated using a numerical model, and are shown to facilitate the correct measurement of f_n and bring ζ within the expected error limits due to quantisation. Secondly, they are applied to an experimental system and are seen to greatly improve the consistency between measurements using different methods.     

Morphological evidence for surface pre-melting on Si(1 1 1)

We present what we believe to be the first morphological evidence for the occurrence of surface pre-melting on the Si(1 1 1) surface. Our results complement the extensive previous evidence from diffraction and ion scattering techniques for the presence of pre-melted (liquid-like) layers on Si(1 1 1) below the bulk melting temperature and also suggest how atomic steps are involved in the initiation of such layers. Our results are based on atomic force microscopy studies of morphologies that are preserved when surfaces are annealed in a range of high temperatures and then rapidly cooled to room temperature for observation. A unique feature of the experiments is the use of specially prepared atomically flat or very low step density surfaces; this allows us to see how the liquid-like morphologies are associated with the steps and also allows the high temperature structures to survive the cooling process without being absorbed into the steps which normally would exist on a surface vicinal to (1 1 1). Quenched-in structures ascribed to pre-melting also act as sinks for diffusing ‘excess’ adatoms generated by the (1 × 1) to (7 × 7) transition and this leads to the formation of dendritic islands.     

Adsorption of H2 and CO on clean and oxidized (110) Pt

Binding states and sticking coefficients of CO and H₂ on clean and oxide covered (110) planes of Pt are examined using flash desorption mass spectrometry to characterize binding states and Auger electron spectroscopy (AES) to characterize oxide densities. It is found that on the oxide both adsorbates have new binding states with significantly higher binding energies than on the clean surface. For H₂ the binding states associated with the clean surface are also shifted to higher energies as the oxide coverage increases. The oxide state for H₂ desorbs with first order kinetics, and isotope exchange experiments are used to examine exchange between isotopes and between states. The initial sticking coefficients for CO are 1.0 and 0.85 on clean and oxidized surfaces, and the initial sticking coefficient for H₂ increases from 0.15 on the clean surface to 0.28 on the oxidized surface. Enhanced bonding on the oxide is interpreted in terms of models involving microfacets, electronic structure alteration, and compound formation.     

Electronic band structures of low-dimensional adsorbate systems: Rare-gas adsorption on transition metals

Angle-resolved photoemission data are dis-cussed for five different Xe adlayers which exhibit electronic structures of different dimensionalities. Xe adsorption on Ni (110)-(1 × 2)-3Hand the (×) R30^° Xe layer on Ru (001) reveal two-dimensional (2D) Xe-derived band structures that are characteristic for hexagonal rare-gas layers. Different Xe 5p dispersion widths on Ni and on Ru are found due to the difference in the Xe-Xe nearest-neighbor distance. For three rare-gas systems (two different Xe coverages on hydrogen-modified Pt (110)-(1 × 2)-H and Kr step decoration on a Pt (997) surface) true one-dimensional (1D) band structures are found. For Xe step adsorption on Pt (997), electronic localized (0D) behavior is observed due to an enlarged Xe-Xe separation. The qualitative differences of the band structures in the case of 2D, 1D and 0D rare-gas systems are demonstrated and are explained by the different dimensionalities of the various structures. Received: 3 August 2000 / Accepted: 4 August 2000 / Published online: 7 March 2001     

Structure of the (0001) surface of α-Al2O3 from first principles calculations

Fully self-consistent ab initio calculations based on pseudopotentials are used to study the structure and energetics of the basal-plane surface of α-Al₂O₃. The calculated forces on the atoms are used to relax the atomic positions to equilibrium. It is shown that surface relaxations are very large and lead to a reduction of the surface energy by over a factor of two. The results support the validity of earlier work based on pair-interaction models.     

Thermo-physical parameters and characteristics of self-organized nanogratings induced by femtosecond laser

Self-organized nanoripples are induced on bulk metal Cu and Ag by femtosecond laser, and the influence of number of shots on nanostructure formation has been investigated. The AFM images show that obtained grooves on Cu are about 50 nm deep, and have an average spacing of 481.41 nm, which is smaller compared to the incident radiation wavelength (800 nm). Arrays of ablated craters are machined on Cu and Ag surfaces by femtosecond laser in order to determine the optical characteristics of laser irradiated surface. Compared with that of untreated sample, the locations of maximum absorption wavelength of laser treated samples are not shifted, while average absorbance intensities are enhanced both for modified Ag and Cu surfaces. Finally, the effects of thermal conductivity, dielectric function as well as electron–phonon coupling coefficient on nanograting morphology induced by femtosecond laser are discussed qualitatively.