Periodic evolution of an ensemble of defects in crystals under dry friction conditions

A model of the frictional destruction of the surface layer of solids is proposed. The main elements of the model are ensembles of interacting dislocations and microcracks, whose kinetics is described by a system of coupled nonlinear differential equations. The periodic change in the wear parameters of the contacting materials is explained. Fiz. Tverd. Tela (St. Petersburg) 39, 497–498 (March 1997)     

Theoretical model for describing degradation of thin hydrogen-containing films

A new theoretical model is proposed to describe the behavior of films of composite hydrogen-containing compounds during heat treatment. The model is based on the thermal generation of hydrogen atoms and atoms of the composite compounds with their subsequent diffusion to the boundaries of the film and percolation through the surface into the atmosphere. Calculations are performed for the heat treatment of films of silicon nitride. A comparison with the experimental data in the literature demonstrates the high efficiency of the proposed model. Zh. Tekh. Fiz. 67, 105–110 (August 1997)     

Diffraction peak profiles of surface relaxed spherical nanocrystals

Abstract

A model is proposed for surface relaxation of spherical nanocrystals. Besides reproducing the primary effect of changing the average unit cell parameter, the model accounts for the inhomogeneous atomic displacement caused by surface relaxation and its effect on the diffraction line profiles. Based on three parameters with clear physical meanings - extension of the sub-coordination effect, maximum radial displacement due to sub-coordination, and effective hydrostatic pressure - the model also considers elastic anisotropy and provides parametric expressions of the diffraction line profiles directly applicable in data analysis. The model was tested on spherical nanocrystals of several fcc metals, matching atomic positions with those provided by Molecular Dynamics (MD) simulations based on embedded atom potentials. Agreement was also verified between powder diffraction patterns generated by the Debye scattering equation, using atomic positions from MD and the proposed model.     

Surface flow of liquid in an electric field

An analytic solution is proposed for a model problem which demonstrates the occurrence of a surface flow of weakly conducting liquid in an electric field which was previously observed experimentally by the author. Zh. Tekh. Fiz. 69, 127–128 (June 1999)     

Surface flows of granular mixtures

We present the generalization of the minimal model for surface flows of granular mixtures, proposed by Boutreux and de Gennes [J. Phys. I France 6, 1295 (1996)]. The minimal model was valid for grains differing only in their surface properties. The present model also takes into account differences in the size of the grains. We apply the model to study segregation in two-dimensional silos of mixtures of grains differing in size and/or surface properties. When the difference in size is small, the model predicts that a continuous segregation appears in the static phase during the filling of a silo. When the difference in size is wide, we take into account the segregation of the grains in the rolling phase, and the model predicts complete segregation and stratification in agreement with experimental observations. Received 9 September 1998 and Received in final form 4 November 1998     

Dislocation configurations around a nanoindentation in the surface of a fcc metal

We report a scanning tunnelling microscopy investigation of the emission of dislocations around nanoindentations in the form of dislocation arrangements previously called hillocks , consisting of two pairs of Shockley partial dislocations, each encompassing a stacking fault. The spatial arrangement and size distribution of hillocks around the nanoindentation traces are studied. We show that standard dislocation theory for an isotropic continuum can be used to describe the stability of the hillocks, their size and spatial distribution and the broadening of the corresponding extended dislocations near the surface. A model is proposed in which hillocks originate from the split into dislocations partials of primary perfect dislocation loops punched into the crystal by the scanning tunnelling microscope tip. This model implies the operation of a novel dislocation mechanism involving long-range transport of matter across the surface.     

A viscous continuum traffic flow model with consideration of the coupling effect for two-lane freeways

In this paper, the viscous continuum traffic flow model for a single lane is extended to the traffic flow for two-lane freeways. The proposed model is a higher-order continuum model considering the coupling and lane changing effects of the vehicles on two adjacent lanes. It results from integrating the Taylor series expansion of the viscous continuum traffic flow model proposed by Ge (2006 Physica A 371 667) into the multi-lane model presented by Daganzo (1997 Transpn. Res. B 31 83). Our proposed model may be used to describe non-anisotropic behaviour because of lane changing in multi-lane traffic. A linear stability analysis is given and the neutral stability condition is obtained. Also, issues related to lane changing, shock waves and rarefaction waves, local clustering and phase transition are investigated through a simulation experiment. The simulation results show that the proposed model is capable of explaining some particular traffic phenomena commonly observable in real world traffic flow.     

On the question of short-period oscillations of the collector voltage during transverse electron focusing

Giant short-period oscillations during transverse electron focusing at high emitter voltages have been observed in bismuth samples with a superlattice on the surface. A model is proposed which explains the onset of the oscillations, their position on the magnetic-field scale, and their intensification and shift along the magnetic-field scale with increasing current (this shift depending on the direction of current flow) and which can also account for the absence of oscillations in fields which are multiples of the field of the first oscillation. In this model the oscillations are attributed to the appearance of resonant surface (edge) states and their contribution to the electron transport. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 887–892 (25 December 1998)     

Extended -wave superconductivity. Flat nodes in the gap and the low-temperature asymptotic properties of high- superconductors

Remarkable anisotropic structures have been recently observed in the order parameter of the underdoped superconductor Bi₂Sr₂CaCu₂O . Such findings are strongly suggestive of deviations from a simple d _{x2 - y2} -wave picture of high- superconductivity, i.e. . In particular, flatter nodes in are observed along the directions in -space, than within this simple model for a d-wave gap. We argue that nonlinear corrections in the -dependence of near the nodes introduce new energy scales, which would lead to deviations in the predicted power-law asymptotic behaviour of several measurable quantities, at low or intermediate temperatures. We evaluate such deviations, either analytically or numerically, within the interlayer pair-tunneling model, and within yet another phenomenological model for a d-wave order parameter. We find that such deviations are expected to be of different sign in the two cases. Moreover, the doping dependence of the flatness of the gap near the nodes is also attributable to Fermi surface effects, in addition to possible screening effects modifying the in-plane pairing kernel, as recently proposed. Received 19 November 1999     

Sulfide passivation of III-V semiconductor surfaces: role of the sulfur ionic charge and of the reaction potential of the solution

A model is proposed for describing the effect of a solution on the electronic properties of sulfided surfaces of III-V semiconductors which treats the adsorption of sulfur in terms of a Lewis oxide-base interaction. According to this model, the density of states on a sulfided surface, which pin the Fermi level, decreases as the global hardness of the electron shell of the adsorbed sulfide ions is increased. The Thomas-Fermi-Dirac method is used to calculate the global hardness of sulfide ions with different charges as a function of the dielectric constant of the medium. It is shown that the hardness of a sulfur ion is greater when its charge is lower and the dielectric constant of the solvent is lower. Zh. Tekh. Fiz. 68, 116–119 (August 1998)     

Formation of periodic surface structures by ultrashort laser pulses

The formation of periodic surface structures by ultrashort laser pulses was observed experimentally and explained theoretically. The experiments were performed on graphite with picosecond laser pulses. The spatial period of the structures is of the order of the wavelength of the incident radiation, and the orientation of the structures is correlated with the direction of polarization of the light. The key point of the theoretical model proposed is resonance excitation of surface electromagnetic waves, which under conditions such that the temperature of the electronic subsystem is decoupled from the temperature of the crystal lattice causes a “temperature grating” to be written on the flat solid surface of the sample while the laser pulse is being applied on account of the temperature dependence of the surface impedance. The formation of a periodic surface profile from the temperature grating occurs by the volume expansion of a melted layer near the surface of the material. For typical values of the surface tension and viscosity for metals, there is not enough time for the periodic profile to be resorbed before the liquid layer solidifies. The formation of periodic surface structures is delayed in time relative to the laser pulse. Zh. éksp. Teor. Fiz. 115, 675–688 (February 1999)     

Structural and electronic properties of Na/Cu(111) at different coverages by first principles

Nanostructures are presently enjoying an increasing interest in the field of materials science. In particular, importance is given to ordered monolayers prepared by deposition of atoms on a crystalline surface. The growth of these superlattices can be controlled so as to obtain an ordered structure by means of the lateral interaction of adatoms lying on the metal surface. The objective of our study is to investigate the structural and electronic properties using DFT total-energy calculations; we employ a jellium-like model to describe the substrate but we also take into account the presence of discrete surface states that are known to affect the lateral interaction. Our treatment of the substrate is based on the model proposed by E.V. Chulkov et al. [Surf. Sci. 437, 330 (1999)]; in this model one constructs a mono-dimensional potential so as to reproduce some important electronic properties of the metal surface, such as i) the energy gap in the projected bulk band-structure and ii) the energy position of surface states. We put into practice Chulkov potential implementing into an existing plane-waves code (ABINIT, URL http://www.abinit.org) an ionic potential, so as to obtain a self-consistent Kohn-Sham effective potential which corresponds to the Chulkov one. Using this effective potential in a fully three-dimensional code we are able to study the adsorption process and the interaction between adsorbates. We illustrate some details of our implementation of the Chulkov model and we present our results about the simple system of Na adatoms on a Cu(111) surface for different coverages. In particular, we compare electronic properties and adsorption energies with those obtained within a standard jellium model substrate and with those obtained for Na adsorption on a realistic Cu(111) surface.     

Modeling of diffusion and absorption processes of tritium in beryllium membranes

The permeation of tritium produced in thermonuclear fusion through metallic membranes, which is a topical problem in radiation physics, is addressed. A physical model for the permeation of hydrogen through a beryllium membrane is proposed which takes into account the oxide layer on the surface of the membrane. The model is implemented in the form of a system of differential equations, which is solved by numerical methods. As a result of the computer modeling it is shown that as the temperature is raised, the oxide layer in a certain interval blocks the permeation of tritium in the membrane, and it is found that the tritium is distributed in a rather limited region of the membrane over its thickness. This circumstance will permit the use of special processing by etching or mechanical treatment. Zh. Tekh. Fiz. 69, 38–41 (July 1999) Deceased     

Comparison of density-functional approaches and Monte Carlo simulations for free planar films of liquid

Density functionals proposed in the literature for describing the behaviour of liquid helium at T =0 K are examined. In so doing, several properties of the ground states of free films of superfluid ⁴ He are calculated by using zero- and finite-range density functional theories and these results are compared to that computed with Monte Carlo simulations. We mainly focus the attention on the energy per particle of the slabs, the surface tension and the width of the liquid-vacuum interfaces, all as a function of the inverse of coverage. The largest differences are found in the case of the surface widths. Received 26 July 1999     

Network topology and subgap resonances observed by Fourier transform scanning tunnelling microscopy of cuprate high-temperature superconductors

Fourier transform scanning tunnelling microscopy (STM) on Bi₂Sr₂CaCu₂O_{8+ δ} (BSCCO) subgap resonances has deciphered an octet of ‘quasiparticle’ states that are consistent with the Fermi surface and energy gap observed by angle-resolved photoemission spectroscopy (ARPES), but the origin of the high-intensity k-space octets and the sharply defined r-space chequerboard is unexplained. The filamentary ferroelastic nanodomain model that predicted the r-space chequerboard also explains the k-space octets and the origin of the apparent anisotropic surface d-wave gap by using strong electron–phonon interactions outside the CuO₂ planes. The topological model identifies the factors that stabilize high-intensity k-space octets in the presence of a very high level of irregular r-space chequerboard noise.     

Enlarged surface meshes and normalization conditions for columns and rows of matrices in the COSMO method

Earlier, normalization conditions for the columns of the PCM (Polarized Continuum Model) were determined and a method of enlarged surface meshes was developed. We developed similar methods for the COSMO (COnductor-like Screening MOdel). These methods make it possible to introduce larger surface meshes without loss of accuracy and perform fast calculations of the solvation energy and the Born radii in the SGB (Surface Generalized Born) method. In addition, the corrections proposed in this work provide a significant enhancement in the accuracy of numerical calculations.     

Instability of an elastically compressed silicon surface under etching

An unusual relief in the form of linear defects resembling quasicracks or grooves was observed on a compressionally stressed surface of a bent silicon surface subjected to chemical etching. The average distance between the forming defects was determined by the magnitude of the surface mechanical strain. The reason for this is assumed to be an Asaro-Tiller-Srolovitz instability in the compressed-surface-etching-agent system. A simple technique is proposed for producing considerable mechanical strains, up to 0.5%, in a silicon surface at room temperature. Fiz. Tverd. Tela (St. Petersburg) 41, 1416–1418 (August 1999)     

Electronic properties of CuO 2 -planes: A DMFT study

We study one-particle spectra and the electronic band-structure of a CuO ₂ -plane within the three-band Hubbard model. The Dynamical Mean-Field Theory (DMFT) is used to solve the many particle problem. The calculations show that the optical gap is given by excitations from the lower Hubbard band into the so called Zhang-Rice singlet band. The optical gap turns out to be considerably smaller than the bare charge transfer energy () for a typical set of parameters, which is in agreement with experiment. We also investigate the dependence of the shape of the Fermi surface on the different hopping parameters t CuO and t OO. A value t OO / t CuO >0 leads to a Fermi surface surrounding the M point. Received 21 September 1998 and Received in final form 8 June 1999     

Transient spectroscopy of surface states in a constant subthreshold current mode for MIS transistors

The spectrum of surface states at a semiconductor-insulator interface is studied through the relaxation of the gate voltage of an MIS transistor measured with a constant subthreshold current. The proposed method makes permits study of these states in both halves of the semiconductor gap and is convenient for testing integrated circuits. The possibilities of this method are illustrated using the example of radiation induced changes in the distribution of surface states. Zh. Tekh. Fiz. 69, 60–64 (August 1999)