Calculation of anisotropic strain-engineered patterns of monolayer islands

The self-organization of monolayer epitaxy islands in presence of anisotropies in surface stress, applied stress, and lattice mismatch between the film and substrate materials is investigated. The fundamental nature of island interactions is addressed in the context of a model wherein the system free energy consists of the excess energy and strain energy of atomic surface steps. It is shown the anisotropy can change the character of island interactions. An energy-reducing kinetic relation is adopted to evolve an initially random morphology towards a generally metastable minimum energy state. It is found the self-organization of islands into a regular array requires both the repulsion between islands and tendency for islands to aligned in a particular direction. Small anisotropies provide the required repulsion but not the tendency for islands to align and large anisotropies provide the necessary alignment but cause islands to attract. Modest levels of anisotropy provide the most favorable conditions of self-organization.     

Synthesis of graphene from naphthalene molecules on the surface of a Langmuir monolayer

This paper considers some approaches to the technology of the synthesis of a graphene monolayer at a phase interface. A surfactant monolayer on an aqueous subphase is proposed as the substrate for graphene synthesis. A monolayer is formed by the Langmuir–Blodgett method. Simple polyaromatic molecules, in particular, naphthalene, are considered as the basic substance for the synthesis of graphene. Arachidic acid is used as the basic surfactant molecule. To confirm the possibility of synthesizing graphene by the mentioned method, both experimental and theoretical studies are performed. In the course of experiemnts, it is shown that naphthalene molecules are pressed into the space above arachidic acid molecules upon the compression of monolayer of arachidic acid–naphthalene mixtures (such an assumption is made due to the characteristic value of the surface areas attributed to different phases of the monolayer and also to its characteristic parameters). The formation of a layer of naphthalene molecules on the surface of a monolayer is modeled by the molecular dynamics method (Amber potential). Different variants of the initial distribution of molecules are considered.     

Microdistributions in a diatomic monolayer on the surface

The regularities of the arrangement of atoms of two types on a surface are considered within the framework of a one-dimensional model taking only the interaction between the nearest neighbors into account. Exact relations determining the probabilities of the appearance of all possible atomic configurations are introduced to simplify the form of the interaction on the basis of the notion of detailed equilibrium. A considerable variety of microdistribution regularities in the structure is established; it is determined by the main characteristics of the components and by the temperature. The conditions for the appearance of dense atomic chains, including those with regular alternation of atoms of the two types, are determined. The possibilities of the appearance of small clusters randomly distributed in a relatively homogeneous medium are indicated. The results of the constructions are discussed.     

Chemisorption of a monolayer of atoms on a bcc metal surface

The change in the density of states due to the adsorption of a monolayer of atoms on the (001) surface of a bcc metal is presented. The substrate is described by the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight-Binding (TB) approximation, and both the Green's function formalism and the phase shift technique are employed. Each adatom is represented by a single nondegenerate energy level. Two binding sites for the commensurate monolayer are considered: the on-site and the centered fourfold-site. By assuming that screening of the charges on the adatoms is complete within the surface layer of atoms, the selfconsistency condition of satisfying Friedel's sum rule can be met by varying the orbital energies of the adatoms and the surface plane of atoms of the substrate. The changes in the density of states show strongly skewed bonding and antibonding resonances which occur at different energies for the two binding sites even though equal binding strengths are assumed. A comparison with previous single adatom results shows that the shape and position of the bonding resonance are dependent upon adatom coverage.     

Viscoelasticity evaluation of rubber by surface reflection of supersonic wave

The main characteristic of rubber is a viscoelasticity. So it is important to research the characteristic of the viscoelasticity of the high frequency band for the friction between a rubber material and the hard one with roughness, for instance, the tire and the road. As for the measurement of the viscoelasticity of rubber, DMA (dynamic mechanical analysis) is general. However, some problems are pointed out to the measurement of the high frequency band by DMA. Then, we evaluated the viscoelasticity characteristic by the supersonic wave measurement. However, attenuation of rubber is large, and when the viscoelasticity is measured by the supersonic wave therefore, it is inconvenient and limited in a past method by means of bottom reflection. In this report, we tried the viscoelasticity evaluation by the method of using complex surface reflection coefficient and we compared with the friction coefficient under wide-range friction velocity. As a result, some relationships had been found for two properties. We report the result that character of viscoelasticity of rubber was comparable to friction coefficient.     

Direct measurements of anisotropic energy transfers in a rotating turbulence experiment

We investigate experimentally the influence of a background rotation on the energy transfers in decaying grid turbulence. The anisotropic energy flux density F(r) = <δu(δu)2>, where δu is the vector velocity increment over separation r, is determined for the first time by using particle image velocimetry. We show that rotation induces an anisotropy of the energy flux ?·F, which leads to an anisotropy growth of the energy distribution E(r) = <(δu)2>, in agreement with the von Kármán-Howarth-Monin equation. Surprisingly, our results prove that this anisotropy growth is essentially driven by a nearly radial, but orientation-dependent, energy flux density F(r).     

Bound states of two surface phonons at a crystal surface with an adsorbed monolayer

We obtain the Bethe-Salpeter equation for the Green's function describing the propagation of two surface phonons interacting via cubic and quartic anharmonicity. We solve that equation by keeping only the latter contribution to its kernel. The two-dimensional nature of the problem is reflected in the existence of a bound state of two high-frequency surface phonons associated with the presence of an overlayer of light atoms, for all values of the coupling strength. This is illustrated with the detailed solution of a simple model.     

Determination of surface roughness from light scattering measurements

We have measured the angular dependence of the p- and s-pollarized light scattered on films, which are supported by fine polished Suprasil substrates. To study the roughness of the surface we have varied the evaporation process and the thickness of the silver film. We quantitatively determine the roughness structure by using a non scalar scattering theory. The roughness structure is found in good agreement with measurements on the same films using ATR-techniques.     

Viscoelasticity of a Fermi liquid

Infinite homogeneous Fermi systems in the degenerate regime are described by the Uehling-Uhlenbeck equation. The eigenvalue problem associated with the linearized collision operator is solved analytically. Initial value problems are studied with the help of the spectral representation of the time evolution operator. The dynamic transport coefficients of the system can then be calculated in the framework of linear response theory. As an example the viscoelastic behaviour of the Fermi liquid is related to the relaxation of a quadrupole deformation in momentum space. In this connection also the coupling of the driving field to 2p-2h excitations will be discussed. The theory is applied to normal liquid³He and to nuclear matter.     

Assessment of acoustic pressure holograms from membrane velocity measurements

The aim of this paper is to demonstrate experimentally the possibility of acquiring acoustic pressure holograms using a light membrane and a scanning laser vibrometer. The velocity of a light membrane placed in an acoustic field can be measured without contact by means of a laser vibrometer. The ideal membrane must be optically reflective, acoustically transparent (having as little mass as possible), impermeable, and mounted without tension. The measured velocity is equal for continuity reasons to the normal acoustic velocity, but differs from the acoustic velocity without the membrane because the membrane is never completely transparent to acoustic waves. The effect of the mass of the membrane can be taken into account to correct this difference. Then, acoustic pressure holograms can be deduced from velocity holograms using the 2D Discrete Fourier Transform. An experimental validation is carried out; acoustic pressures derived from laser measurements are compared with microphone measurements, with a very satisfying match over a wide frequency range.     

Simple depolarizers for spectrophotometric measurements of anisotropic samples

Spectrophotometric measurements on optically anisotropic samples are considerably affected by the partial polarization of light in the instrument. It is shown that by depolarizing the incident beam, more realistic results are obtained even for strong directional anisotropic materials. Two types of depolarizers were investigated.     

The pressure dependence of superconducting energy gaps from high pressure critical field measurements

It is shown that the Toxen relationship Δ/kT_c = T_c/H_c(0) (dH_c/dT)_Tc can be used to obtain Δ/kT_c as a function of pressure from critical field curves taken at high pressures. Data for Sn, In, Zn, Al, Cd, Ga, and Th are analyzed and Δ(p)/kT_c are tabulated. For Sn and In a comparison was made of energy gaps so determined with those measured directly by tunnelling.     

Buckling in a solid Langmuir monolayer: light scattering measurements and elastic model

Above a surface pressure threshold , we detect a buckled state in the low temperature solid phase of a phospholipid monolayer spread at the air-formamide interface. Stable ripples are observed with a Brewster angle microscope, and light scattering provides measurements mNm^-1, wavelength m and amplitude of a few nm for the deformation. A model, coupling the monolayer thickness and elongation, and consistent with the monolayer texture, is also presented. Received: 18 August 1997 / Revised: 9 December 1997 / Accepted: 29 January 1998