Crystallographic determination of metal surfaces with helium scattering

Helium-surface diffraction data has been used to construct the interaction potential between a Helium atom and a metal surface. The constructed potential fits experimental data for all beam energies and angles of incidence and more importantly is independent of beam energy. Such a potential essentially consists of two parts: a short range repulsive corrugated potential due to electronic overlap, and a long range attractive potential due to dispersion forces. We have generated potentials for He scattering for Ni, Cu and Au and in all cases found excellent agreement with experiments. We also report that the repulsive part of the He scattering potential can be obtained rather simply from first principles without adjustable parameters. This suggests that one can determine surface crystallography from first principles without adjustable parameters.     

An exact iterative solution of the atom-surface scattering problem for realistic potentials

A general method, based on high order distorted wave perturbation theory, is developed to obtain exact reflection intensities for low energy atoms scattered by solid surfaces. Resonance shapes are calculated using projection techniques to obtain uniform convergence. Important differences with the hard corrugated wall model arise when the calculations for a corrugated Morse potential are compared with experiment for helium scattered by a Cu(110) surface.     

Manipulation of cold atoms by an adaptable magnetic reflector

Adaptive optics for cold atoms has been experimentally realized by applying a bias magnetic field to a static magnetic mirror. The mirror consist of a 12-mm-diameter piece of commercial videotape, having a sine wave of wavelength 25.4 μm recorded in a single track across its width, curved to form a concave reflector with radius of curvature R=54 mm. We have studied the performance of the mirror by monitoring the evolution of a 24 μK cloud of ⁸⁵Rb atoms bouncing on it. A uniform static external magnetic field was added to the mirror field causing a corrugated potential from which the atoms bounce with increased angular spread. The characteristic angular distribution of the surface normal is mapped at the peak of the bounce for atoms dropped from a height of R/2 and at the peak of the second bounce for a drop height of R/4. In a second experiment a time-dependent magnetic field was applied and the angular distribution of the cloud was measured as a function of field frequency. In this scheme we demonstrate a corrugated potential whose time-dependent magnitude behaves like a diffraction grating of variable depth. Finally a rotating field was added to generate a corrugated potential that moves with a velocity given by the product of the external field rotation frequency and the videotape wavelength. This travelling grating provides a new method of manipulation as cold atoms are transported across the surface by surfing along the moving wave. Two theoretical methods have been developed to predict the behaviour of atoms reflecting from these stationary, variable magnitude and moving corrugated potentials. A simple analytic theory provides excellent agreement for reflection from a stationary corrugated potential and gives good agreement when extended to the case of a travelling grating. A Monte Carlo simulation was also performed by brute force numeric integration of the equations of motion for atoms reflecting from all three corrugated potential cases. Received: 1 December 1999 / Revised version: 3 February 2000 / Published online: 5 April 2000     

Multiphonon contributions to diffracted intensities and resonance profiles in atom-surface scattering

In this Letter, multiphonon contributions to the elastic diffracted intensities and resonance profiles are considered in atom-surface scattering within the close-coupling formalism. Application to the scattering of He atoms from the corrugated Cu(110) and Cu(113) surfaces is shown.     

The scattering of atoms from a corrugated wall model with the GR method in the system He/LiF(001)

In this communication we present a new method, called GR, to calculate the diffraction probabilities of atomic beams from a hard corrugated surface model. The method is applied to the system He/LiF(001). We show that the method gives a convergent solution, within 10^?4 in unitarity, when β₀ ? 0.12, where β₀ is the dimensionless total corrugation amplitude. This limit suggests that the method converges for “any” crystallographic surface of interest. The agreement between the experimental data and the calculated curves is good. From this agreement we conclude that the surface corrugation for He/LiF(001) is cosine-like and the “best” corrugated surface is given.     

Corrugating and anticorrugating static interactions in helium-atom scattering from metal surfaces

We perform a density-functional-theory calculation of the static repulsive potential of He scattering off a noble and a simple metal surface. The classical turning point of He on Cu(111) is found to be closer to the metal when the adatom is at top than at bridge site (anticorrugating effect). The potential of He on Al(111) is instead corrugated. By comparing the results of the two systems, we conclude that the He-metal anticorrugating effect occurs when the kinetic energy difference for He at top and bridge sites is larger than the electrostatic one, and an induced localized dipole on He is formed.     

First-principles calculation of the single impurity surface Kondo resonance

We perform first-principles calculations of the surface and bulk wave functions of the Cu(111) surface and their hybridization energies to a Co adatom, including the potential scattering from the Co. By analyzing the calculated hybridization energies, we find the bulk states dominate the contribution to the Kondo temperature, in agreement with recent experiments. Furthermore, we also calculate the tunneling conductance of a scanning tunneling microscope and compare our results with recent experiments of Co impurities in the Cu(111) surface. Good quantitative agreement is found at short parallel impurity-tip distances (<6 A). Our results indicate the need for a new formulation of the problem at larger distances.     

Electronic structure of the corrugated Cu3N network on Cu(110): Tunneling spectroscopy investigations

The local electronic structure of a submonolayer-thick corrugated Cu₃N–Cu(110) network is investigated by low temperature scanning tunneling spectroscopy (LT-STS). The corrugation present in the Cu₃N network plays a vital role by causing the surface electronic features to vary locally due to different N–Cu bonding. Our studies indicate a work function larger by 0.9 eV for Cu₃N compared to bare Cu(110), suggesting the formation of a significant surface dipole. Theoretically predicted various N 2p, Cu 3d hybridized states have been shown and explicitly verified by a combination of constant height and constant current tunneling spectroscopy measurements, thereby providing information about the chemical composition of the N–Cu(110) network.     

Helium scattering from metal surfaces: potential determination and crystallography

We describe a method for determining the interaction potential between a helium atom and a metal surface from diffraction data. Results are presented for He/Ni(110) and He/Cu(110). For He/Cu(110) the potential 3s in good agreement with a first principles calculation but this is not true for He/Ni(110). We also show that a surface relaxation of 10% produces measurable changes in intensities.     

Femtosecond electron dynamics at adsorbate–metal interfaces and the dielectric continuum model

2 overlayers adsorbed on Cu(111). With increasing number of adsorbate layers the binding energies of the image potential states are found to decrease while their lifetimes increase (except for the second image potential state on 2 to 3 ML Xe/Cu(111)). These trends are most pronounced for nitrogen, where the binding energy of the first image potential state decreases by a factor of 3.5 from 0 to 2 ML N₂/Cu(111); at the same time the lifetime increases from 22 to 700 fs. The results are discussed in the framework of the dielectric continuum model, which approximates the adsorbate layers by a dielectric slab in front of the metal surface. For Xe, the agreement between measured and calculated lifetimes improves significantly if the full dispersion curve of the Xe 6s conduction band is taken into account. Received: 2 November 1998     

Electronic structure of the Cu(111) surface

Self-consistent electronic structure calculations are reported on bulk Cu, and 3- and 5-layer Cu films. These yield a size insensitive work function, φ = 5.0±.1 eV, and a surface energy of 0.75 eV, in agreement with experiment. Good size convergence of the film potential permits the construction of a self-consistent potential for an 11-layer Cu(111) film, whose spectral properties we studied. A prominent p-like surface band was found within 0.1 eV of experiment, serving as a check on the surface potential.     

Classical trajectory study of the dissociation of hydrogen on copper single crystals: II. Cu(100) and Cu(110)

We report a second series of classical trajectory calculations for the activated dissociation of hydrogen on copper single crystals. We have modified the empirical interaction potential surface used previously by raising the energy requirements for dissociation, thereby lowering the dissociation probabilities and bringing their magnitudes in line with experiment. Dissociation probabilities on both a Cu(100) and Cu(110) surface for H₂ and D₂ are computed. The results, while in reasonable magnitude agreement with experiment, still do not fit the experimental translational energy dependence well and do not predict the correct isotope effect for Cu(110). A detailed comparison of the differences between the ground and first vibrationally excited states for the (100) and (110) surfaces suggest that within the approximations of this model calculation, the potential surfaces employed have underestimated the effects of surface roughness in determining the incident energy and angle dependence of the dissociation probabilities.     

Photoacoustic investigation of resonance absorption on corrugated surfaces

The PhotoAcoustic (PA) method has been used to study the optical resonance absorptions on corrugated surfaces. The resonance absorptions due to the excitations of surface plasmons and guide-waves of corrugated surfaces have been measured experimentally, and the effects of the groove depth and period of the corrugated surface on the absorption curve have been observed. Moreover, the differential method based on the rigorous use of electromagnetic theory is used to calculate the parameters of the grooves and the optical absorptivities of the samples. The theoretical results on the resonance absorption peaks are in good agreement with those of the PA experiments. Then, the parameters of the grooves can be estimated by the PA investigations.     

Aeroacoustic interaction in a corrugated duct

The sound generation by an air flow in a corrugated tube is studied experimentally for different values of the corrugation pitch and different tube lengths. The Strouhal numbers of sound generated in different tubes with different flow velocities lie within 0.4–0.6. As the flow velocity increases, the Strouhal number decreases. The effect of sound absorption by an air flow in a corrugated duct is described: in a corrugated tube with a flow, at frequencies below the generation frequency, the absorption of sound produced by an external source is observed. A semiempirical model of aeroacoustic interaction in a corrugated tube is proposed. The model provides a qualitative agreement with the experiment.     

Corrugated Horn Antenna for Low-Power Testing of the Quasioptical Transmission Lines at TJ-II Stellerator

This paper presents the design and characterisation of a millimetre-wave corrugated horn antenna to perform low power measurements of the main parameters of the quasioptical transmission lines of the Electron Cyclotron Resonance Heating (ECRH) at TJ-II stellerator experiment working in Madrid (E). The antenna generates a practically pure Gaussian Beam at 53.2 GHz that simulates the stellerator gyrotron output beam. Simulations of the designed corrugated horn, by using a specially developed computer code, are presented. The far field radiation pattern of the constructed prototype was measured at the ESTEC Compact Antenna Test Range (CATR). The measurement results are in excellent agreement with those obtained from simulations.     

Sound attenuation in metals: Reformulation of Pippard theory to include microscopic band structure and scattering

Pippard's theory is written in Boltzmann equation form which allows both band effects and microscopic scattering to be included. A new formulation of deformation potential theory is an essential ingredient. Numerical calculations are given for Cu, using realistic electron and phonon eigenstates and the rigid-muffin-tin model for electron-phonon coupling. The calculations cover the crossover from hydrodynamic to quantum regimes, and show satisfactory agreement with experiment.     

Monte Carlo simulations of copper clustering in Fe–Cu alloys under irradiation

We present the computational approach for studying the microstructures of Cu clusters in Fe–Cu alloys by combining the molecular dynamics (MD) simulation and Monte Carlo methods. The MD simulation is used to characterize the primary damage resulting from the displacement cascade in Fe. Then, using the Metropolis Monte Carlo methods, the microstructure of the Cu clusters is predicted under the assumption that the system will evolve towards the equilibrium state. The formation of the Cu clusters is apparent for Fe–Cu alloys of a higher Cu content (1.0 w/o), whereas the degree of Cu clustering is not significant for the lower Cu content (0.1 w/o) alloys. The atomic configuration of the Cu–vacancy complex under irradiation, produced by this simulation, is in a fair agreement with the experiments. The simulation is expected to provide important information on the Cu-cluster morphology, which is useful for experimental data analysis.     

A general method for analyzing em wave scattering from arbitrarily shaped two-dimensional periodic surfaces

In this paper the Method of Lines (MoL) is successfully extended to solve the EM wave scattering problems of periodic surfaces with arbitrary profile. As examples, the scattering coefficients of space harmonics of corrugated and sinusoidal surfaces are calculated. The results are in good agreement with available data from Wirgin and from A.K.Jordan et al. In addition, the results of comb structure are also calculated. The flexibility and less computation of this method make it eligible for analyzing various two-dimensional periodic structures.     

用嵌入原子方法研究Cu(100)面声子色散

本文采用基于密度泛函理论发展的嵌入原子方法,推导了由半经验函数嵌入能和排斥势所表达的表面晶格动力学矩阵公式,以CU(100)面为例,得到了该面三个主要对称方向的色散关系.其中,沿ΓX方向的瑞利声子色散曲线与新近的电子能量损失谱所得实验结果相比,符合较好.文中还讨论了一些表面模的局域程度以及振动方式. 关键词：