表面原子的自旋

原子物理和固体物理作为物理学的两个分支几乎是平行发展的。原子物理始于20 世纪20 年代,主要致力于理解气相中原子的光学和磁学性质;而固体物理是在研究材料纯化的过程中成长起来的,并导致了今天的电子器件。这两个领域实验和理论的概念通常大相径庭。如前者通过量子自旋态来描述磁性,而后者是通过能带的形成来解释固体的磁性。     

Coherent potential approximation for strong-coupling superconducting transition-metal alloys

Within the coherent potential approximation (CPA) a theory for strong-coupling superconducting transition-metal alloys is developed. The configurational dependence of the selfenergy-part is discussed and, in analogy to McMillan, an expression forT _c is derived for a singles band as well as for anΓ′₂₅-band. The electron-phonon coupling constantλ occuring in this expression depends explicitly on: the average component density of states,N _A (0) andN _B (0), the electron-phonon matrix elements of the components, the difference of the atomic energy levels,δ, and the phonon Green's function at zero frequency. In the limit of the pure metal this expression reduces to that developed by Appel and Kohn. It is shown that the change of the phonon density of states in an alloy due to the atomic mass differences — which is very complicated — does not influence the value ofλ.     

Atomic ordering and ferromagnetism in binary alloys

Atomic and magnetic ordering in binary alloys are studied within the mean field approximation. The influence of magnetism on the spatial order-disorder critical temperature and, conversely, the influence of chemical order on the Curie temperature are analyzed in detail. Phase diagrams for various cases are presented. In particular we find that the interplay of the two order parameters yields in some cases unusual sequences of phases as a function of temperature. For example we obtain a possible sequence with increasing temperature of spatially ordered ferromagnet → spatially ordered paramagnet → spatially disordered ferromagnet → disordered paramagnet.     

The single-site approximation for disordered canonical d-band metal alloys

The LCAO theory of substitutionally disordered transition metal alloys, which allows to treat the different bandshapes and naturally leads to a matrix generalization of the concept of the multiplicative off-diagonal randomness, is developed within the single-site approximation. The novel expression for the full averaged resolvent is obtained, and applied to the evaluation of the density of states and the spectral density for the silver-palladium alloys. The results quantitatively agree with the recent KKR-CPA data.     

Atomic assembly of metal surfaces and interfaces

Inert gas ion impacts can be used to manipulate atomic assembly processes during the growth of metallic superlattices but the detailed mechanisms are not well understood. Molecular dynamics simulations are used to investigate the effects of ion incident angle and fluence upon the reassembly and structure of a copper surface partially covered with cobalt asperities. In the low ion energy regime, increasing the ion fluence decreases the cobalt layer surface roughness while gradually leading to an increase in the degree of interfacial mixing. The flattening of asperities occurs by direct (athermal) ion activation of an Ehrlich-Schwoebel mechanism of atom jumping. Intermixing of cobalt surface atoms in an underlying copper layer is found to occur by a knock-on process and the lowest energy barriers for this occur in low-index 〈1 1 0〉 and 〈1 1 2〉 crystal (channeling) directions. The mechanistic insights gained from the study are used to simulate the ion assisted growth of a Cu/Co/Cu multilayer system. Using ion parameters chosen to selectively activate atomic assembly mechanisms that promote flat, unmixed interfacial structures, it is shown that Cu/Co/Cu multilayer structures with high quality, smooth and chemically sharp interfaces can be obtained by using oblique, low energy, moderate fluence ion assistance with an ion mass that is similar to the atomic mass of the metals.     

Non-local small-slope approximation for wave scattering from rough surfaces

A new general analytical approach to solving the problems of wave scattering from rough surfaces, referred to as the non-local small-slope approximation (NLSSA), is suggested. It is formulated in the general form both for vector and scalar waves. This approach is valid for an arbitrary wavelength of radiation provided that the slopes of the undulations are small enough. The NLSSA represents a generalization of the small-slope approximation to situations where double scattering (in the optical sense) appears. It is demonstrated that with appropriate approximations the NLSSA of the lowest order reduces to the small-slope approximation of the second order.     

Atomic short-range order in CuMn alloys

The establishment of atomic short-range order (SRO) has been investigated in CuMn alloys (5, 8,13,16,20 at. % Mn) by measurement of the electrical resistivity during isochronal and isothermal annealing. An increasing degree of SRO is accompanied by a reduction of resistivity; this effect increases with Mn concentration. For 8,13 and 16 at. % Mn SRO kinetics turn out to deviate significantly from single exponential behavior with a maximum at 13 at. % Mn, whereas at 5 and 20 at. % Mn SRO is adjusted in a single exponential process. For data analysis three methods are used: a sum of two and three exponentials as well as a log-normal spectrum of relaxation times. The strong interaction between second-nearest-neighbor atoms in CuMn seems to be essential for SRO kinetics but of minor importance for the value of SRO-induced resistivity change.     

Atomic displacements in bcc dilute alloys

We present here a systematic investigation of the atomic displacements in bcc transition metal (TM) dilute alloys. We have calculated the atomic displacements in bcc (V, Cr, Fe, Nb, Mo, Ta and W) transition metals (TMs) due to 3d, 4d and 5d TMs at the substitutional site using the Kanzaki lattice static method. Wills and Harrison interatomic potential is used to calculate the atomic force constants, the dynamical matrix and the impurity-induced forces. We have thoroughly investigated the atomic displacements using impurities from 3d, 4d and 5d series in the same host metal and the same impurity in different hosts. We have observed a systematic pattern in the atomic displacements for Cr-, Fe-, Nb-, Mo-, Ta-and W-based dilute alloys. The atomic displacements are found to increase with increase in the number of d electrons for all alloys considered except for V dilute alloys. The 3d impurities are found to be more easily dissolved in the 3d host metals than 4d or 5d TMs whereas 4d and 5d impurities show more solubility in 4d and 5d TMs. In general, the relaxation energy calculation suggests that impurities may be easily solvable in 5d TM hosts when compared to 3d or 4d TMs.      

Atomic spin-sensitive dissipation on magnetic surfaces

We identify the mechanism of energy dissipation relevant to spin-sensitive nanomechanics including the recently introduced magnetic exchange force microscopy, where oscillating magnetic tips approach surface atomic spins. The tip-surface exchange couples spin and atom coordinates, leading to a spin-phonon problem with Caldeira-Leggett-type dissipation. In the overdamped regime, that can lead to a hysteretic flip of the local spin with a large spin-dependent dissipation, even down to the very low experimental tip oscillation frequencies, describing recent observations for Fe tips on NiO. A phase transition to an underdamped regime with dramatic drop of magnetic tip dissipation should in principle be possible by tuning tip-surface distance.     

Atomic structure of alkali halide surfaces

The atomic structure of surfaces of alkali halide crystals has been revealed by means of high-resolution dynamic force microscopy. True atomic resolution is demonstrated both on steps surrounding islands or pits, and on a chemically mixed crystal. We have directly observed the enhanced interaction at low-coordinated sites by force microscopy. The growth of NaCl films on metal surfaces and radiation damage in a KBr surface is discussed based on force microscopy results. The damping of the tip oscillation in dynamic force microscopy might provide insight into dissipation processes on the atomic scale. Finally, we present atomically resolved images of wear debris found after scratching a KBr surface. PACS 68.37.-d; 68.37.Ps; 75.55.Fv     

原子反射镜及其应用

原子反射镜是人们从事原子光学实验研究的重要器件之一.本文将简单综述采用冷原子磁、光操控技术发展起来的诸如消逝波原子反射镜、半高斯光束原子反射镜、周期性磁化的磁带反射镜、周期性排列的永久磁铁反射镜和载流导线磁反射镜等各种原子反射镜的基本原理、实验方案及其最新进展,并就原子反射镜在原子光学实验中的应用作一简单介绍.     

Atomic configuration of antiphase boundaries in alloys

Computer simulation methods in the approximation of pairwise interatomic Morse potentials are used to calculate the atomic configurations and the energy of formation of 1/2 <110> {111} antiphase boundaries in ordered Cu₃Au and Ni₃Fe alloys. Atomic displacements result in considerable smearing of the first five planes closest to an antiphase boundary and a slip of atomic planes. These effects substantially reduce the energy of formation of antiphase boundaries. The influence of the size factor on the atomic configuration on the antiphase boundaries is discussed.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 2, pp. 43–47, February, 1985.In conclusion, the authors express their thanks to A. I. Tsaregorodtsev for useful discussion of this work.     

High order numerical approximation of minimal surfaces

We present an algorithm for finding high order numerical approximations of minimal surfaces with a fixed boundary. The algorithm employs parametrization by high order polynomials and a linearization of the weak formulation of the Laplace–Beltrami operator to arrive at an iterative procedure to evolve from a given initial surface to the final minimal surface. For the steady state solution we measure the approximation error in a few cases where the exact solution is known. In the framework of parametric interpolation, the choice of interpolation points (mesh nodes) is directly affecting the approximation error, and we discuss how to best update the mesh on the evolutionary surface such that the parametrization remains smooth. In our test cases we may achieve exponential convergence in the approximation of the minimal surface as the polynomial degree increases, but the rate of convergence greatly differs with different choices of mesh update algorithms. The present work is also of relevance to high order numerical approximation of fluid flow problems involving free surfaces.     

Atomic scale friction between clean graphite surfaces

We investigate atomic scale friction between clean graphite surfaces by using molecular dynamics. The simulation reproduces atomic scale stick-slip motion and a low frictional coefficient, both of which are observed in experiments using frictional force microscope. It is made clear that the microscopic origin of low frictional coefficients of graphite lies on the honeycomb structure of each layer, not only on the weak interlayer interaction as believed so far.     

Coherent potential approximation calculations for electronic spectra of one-dimensional quaternary alloys

The coherent potential approximation is used to investigate the electronic structure of quaternary alloys of the types A_xC_yB_{1 − x − y}D and A_xB_{1 − x}C_yD_{1 − y}. Formalisms are developed for and application is made to the calculation of the electronic state densities for both types of alloys in the one-dimensional, one-site per atom, nearest-neighbor tight-binding approximation.     

A differential method for calculation of aspherical surfaces using polynomial approximation

A simple method is described for the correction of the spherical aberration using one aspherical surface represented by a polynomial approximation. The same principle is useful for the design of aplanatic systems containing two aspherical surfaces.Finally, the author wishes to convey his thanks to Mr. J. Vondich (Institute of Physics, Czechoslovak Academy of Sciences) for performing the calculations.     

Weighted curvature approximation: numerical tests for 2D dielectric surfaces

Abstract

The weighted curvature approximation (WCA) was recently introduced by Elfouhaily et al [7] as a unifying scattering theory that reproduces formally both the tangent-plane and the small-perturbation model in the appropriate limits, and is structurally identical to the former approximation with some different slope-dependent kernel. Due to the simplicity of its formulation, the WCA is interesting from a numerical point of view and the aim of the present paper is to establish its accuracy on some representative test cases. We derive statistical formulae for the coherent field and the cross-section in the case of stationary Gaussian random surfaces. We then specialize to the case of isotropic Gaussian spectra and perform numerical comparisons against rigorous method of moments (MoM)-based results on 2D dielectric surfaces. We show that the WCA remains extremely accurate in a roughness range where other first-order classical approximations (small-slope and Kirchhoff) clearly fail, at the same computational cost.

(Some figures in this article are in colour only in the electronic version)