Critical diameters and temperature domains for MBE growth of III–V nanowires on lattice mismatched substrates

We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au‐assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation‐free III–V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of impurities on surface self-diffusion and surface structure

Measurements using field emission techniques of the activation energy for surface selfdiffusion of several of the refractory transition metals when carbon or silicon is present on the surface show large increases which are dependent on the degree of surface coverage. Maximum values obtained were: 8.5 eV for carbon on tungsten, 7.0 eV for silicon on tungsten, 4.9 eV for carbon on tantalum, 4.5 eV for carbon on molybdenum and 2.8 eV for silicon on molybdenum. In addition, two anomalous effects have been observed in which surface changes occur at critical temperatures, (a) Sharp discontinuities occur in the plots of activation energy versus temperature for carbon on tungsten at about 2300 °K and for silicon on tungsten at about 2000 °K. In both cases the activation energy drops from the respective high value to that for the clean substrate material of 3.0 eV. Concomitant with this transition the emission patterns change in appearance from those typical of a contaminated surface to those typical of a clean surface, (b) For carbon on tungsten and silicon on tungsten, (433) planes are observed which decrease in size with temperature and suddenly disappear at a very sharp critical temperature. It is suggested that the presence of these impurities causes a restructuring of the surface layers even when present in much less than stoichiometric amounts and that surface phase changes occur independent of bulk changes.     

Upper bounds on the private capacity for bosonic Gaussian channels

Recently, there have been considerable progresses on the bounds of various quantum channel capacities for bosonic Gaussian channels. Especially, several upper bounds for the classical capacity and the quantum capacity on the bosonic Gaussian channels, via a technique known as quantum entropy power inequality, have been shed light on understanding the mysterious quantum-channel-capacity problems. However, upper bounds for the private capacity on quantum channels are still missing for the study on certain universal upper bounds. Here, we derive upper bounds on the private capacity for bosonic Gaussian channels involving a general Gaussian-noise case through the conditional quantum entropy power inequality.     

移测显微镜在测量金属杨氏模量中的应用

本分析了移测显微镜的测量精密度,根据拉伸法测量金属杨氏模量的原理,设计了一套应用移测显微镜测量金属杨氏模量的方法。     

Sticking coefficients of CO,O2 and Xe on the (110) and (100) planes of tungsten

Sticking coefficients for oxygen, CO and Xe on the (110) and (100) planes of tungsten are presented as functions of absolute coverage, surface and gas temperature. Sticking coefficients for CO are in general higher and show simpler behavior than those for oxygen. Sticking coefficients for all gases studied are higher on (100) than on (110) under comparable conditions. It is concluded that simple Kisliuk models are applicable over much of the temperature range for CO on (110) and (100), but not for oxygen, where adsorption and reflection coefficients on first impact vary with coverage, and surface and gas temperatures in complicated fashion. For Xe, at low surface temperatures sticking coefficients are close to unity and increase with coverage, indicating better energy accomodation on Xe-covered than on clean tungsten.     

Path integration in non-relativistic quantum mechanics

A new method for computing path integrals explicitly is developed and applied to problems in non-relativistic quantum mechanics, such as: wave functions, propagators on configuration spaces and on phase space, caustic problems, bound states. Path integrals for paths on curved spaces and for paths on multiply-connected spaces are computed.     

Trends in atomic adsorption on titanium carbide and nitride

Extensive density-functional calculations on atomic chemisorption of H, B, C, N, O, F, Al, Si, P, S, and Cl on the polar TiC(1 1 1) and TiN(1 1 1) yield similar adsorption trends for the two surfaces: (i) pyramid-like adsorption-energy trends along the adatom periods; (ii) strongest adsorption for O, C, N, S, and F; (iii) large adsorption variety; (iv) record-high adsorption energy for O (8.4-8.8 eV). However, a stronger adsorption on TiN is found for elements on the left of the periodic table and on TiC for elements on the right. The results support that a concerted-coupling model, proposed for chemisorption on TiC, applies also to TiN.     

Single atom self-diffusion on nickel surfaces

Results of a field ion microscope study of single atom self-diffusion on Ni(311), (331), (110), (111) and (100) planes are presented, including detailed information on the self-diffusion parameters on (311), (331), and (110) surfaces, and activation energies for diffusion on the (111), and (100) surfaces. Evidence is presented for the existence of two types of adsorption site and surface site geometry for single nickel atoms on the (111) surface. The presence of adsorbed hydrogen on the (110), (311), and (331) surfaces is shown to lower the onset temperature for self-diffusion on these planes.     

A theoretical interpretation of photoelectron energy spectra for oxygen chemisorbed on nickel

Ultraviolet photoemission spectra (UPS) measured by Eastman and Cashion for oxygen moderately to heavily chemisorbed on nickel are uniquely different from UPS data for bulk nickel and relatively low coverages of oxygen adsorbed on nickel. A theoretical model is proposed suggesting that the unique structural features of the UPS data for oxygen moderately to heavily chemisorbed on nickel are evidence for the incipient formation of surface nickel oxide. The theoretical model is based on the self-consistent-field Xα scattered-wave SCF-Xα-SW) cluster method and “transition-state” theory recently developed by Slater and Johnson. The same theoretical model is used to explain the similarities (and differences) between the UPS data for oxygen chemisorbed on nickel and X-ray photo-electron spectra (XPS) for bulk nickel oxide.     

Effect of the Dielectric Anisotropy on the Propagation Properties of Microstrip Tapers with Height Variations

This paper deals with the design and application of nonuniform microstrip transmission lines on anisotropic substrates. A rigorous analysis is based on the use of Hertz vector potentials, moment method and transmission line theory to determine the dispersion characteristics of single and coupled tapered microstrip lines for accurate performance prediction. Results are presented for the main parameters providing the necessary information to design several devices on tapered microstrip, with variation on the strip width and dielectric height, for (M)MIC and antennas applications. A good agreement was observed with the results available in the literature for tapered lines on isotropic substrates.     

The Bethe ansatz for the six-vertex model with rotated boundary conditions

A method is suggested for derivation of the Bethe ansatz equations for the six-vertex model on a square lattice rotated at an arbitrary angle with respect to the coordinate axes. The method is based on the random walk representation for configurations of the model. The equations for the ice model on the rotated lattice are derived and some numerical results are obtained.     

Noisy scale-free networks

The impact of observational noise on the analysis of scale-free networks is studied. Various noise sources are modeled as random link removal, random link exchange and random link addition. Emphasis is on the resulting modifications for the node-degree distribution and for a functional ranking based on betweenness centrality. The implications for estimated gene-expressed networks for childhood acute lymphoblastic leukemia are discussed.     

低温下用3ω法测量热导率的研究

3ω法是测量体材料和薄膜材料热导率的主要方法之一。低温真空条件下测量热物性往往受接触热阻以及加热器自热效应的影响比较大。文中针对低温下体材料热导率的测量,通过总结国内外近年来用3ω法测量热导率的主要研究成果,对如何选择合适的加热频率以减小这两方面所带来的影响做了一些研究,并指出了低温下用3ω法测量体材料热导率的进一步研究工作。     

Active systems with Nambu dynamics: with applications to rod wielding for haptic length perception and self-propagating systems on two-spheres

We generalize Nambu dynamics to describe self-excited, active systems that evolve on attractors and account for the up-take of energy and activation processes, on the one hand, and damping and inhibitory processes, on the other. An application to rod wielding for haptic length perception and a model for self-propagating systems on two-spheres are discussed.     

Exact Solutions of Covariant Wave Equations with a Multipole Source Term on Curved Spacetimes

A formalism is presented for calculating exactsolutions of covariant inhomogeneous scalar and tensorwave equations whose source terms are arbitrary ordermultipoles on a curved background spacetime. The developed formalism is based on the theory ofthe higher-order fundamental solutions for wave equationwhich are the distributions that satisfy theinhomogeneous wave equation with the corresponding order covariant derivatives of the Dirac deltafunction on the right-hand side. Like the classicalGreen's function for a scalar wave equation, thehigher-order fundamental solutions contain a direct termwhich has support on the light cone as well as a tailterm which has support inside the light cone. Knowinghow to compute the fundamental solutions of arbitraryorder, one can find exact multipole solutions of wave equations on curved spacetimes. Wepresent complete recurrent algorithms for calculatingthe arbitrary-order fundamental solutions and the exactmultipole solutions in a form convenient for practical computations. As an example we apply thealgorithm to a massless scalar wave field on aparticular Robertson-Walker spacetime.     

Rheology of dense granular mixtures: boundary pressures

Models for dense sheared granular materials indicate that their rheological properties depend on particle size, but the representative size for mixtures is not obvious. Here, we computationally study pressure on a boundary due to sheared granular mixtures to determine its dependence on particle size distribution. We find that the pressure does not depend monotonically on average particle size. Instead it has an additional dependence on a measure of the effective free volume per particle we adapt from an expression for packing of monosized particles near the jammed state.     

H_2在Ni,Pd与Cu表面的解离吸附

用EAM方法(embeded-atommethod)研究H_2在Ni,Pd与Cu的(100),(110)与(111)面上的解离吸附.首先通过拟合单个H原子在Ni,Pd与Cu不同表面上的吸附能和吸附键长,得到H与这些金属表面相互作用的EAM势,然后计算H_2在这些表面上以不同方式进行解离吸附时的活化势垒E_a,吸附热q_(ad)与吸附键长R.并给出H_2在(110)面上解离吸附的势能曲线.计算结果表明H_2的解离吸附与衬底种类、衬底表面取向及解离方式有关.H_2在Ni表面上解离时活化势垒很低,而在Cu表面解 关键词：     

Universal LCAO parameters for III–V compound semiconductors

A set of semi-empirical expressions for the two-center integrals which enter the Slater-Koster parameters are constructed for Group III–V compound semiconductors These expressions show their explicit dependence on the atomic characteristics, on the dielectric constant, on the lomcity of solids, and also on the interatomic separations, and they are valid for atomic separations as far as second-neighbor distances For zero ionicity they reduce to those obtained previously by us for Group IV semiconductors The Slater-Koster parameters are then constructed from these expressions for the two-center integrals The band structures of various gallium and indium compounds are obtained and are compared with other theoretical calculations and with experimental photoemission results     

Comparison of algorithms for the symbolic computation of the NP spin coefficients and curvature components

Algorithms for the symbolic computation of the NP spin coefficients and curvature components for a given null coframe based on the structural equations of Cartan and the complex vectorial formalism of Debever are described. The efficiency of the algorithms is compared theoretically and also empirically in a number of test cases using implementations in the computer algebra system Maple. The test results confirm the theoretical superiority of the algorithm based on Debever's formalism over the one based directly on Cartan's first structural equations for the computation of the spin coefficients both with respect to execution time and storage requirements. The algorithm for the computation of the curvature components based on Debever's formalism is generally superior to the one based on Cartan's second structural equations but the advantage is not as marked as for the spin coefficients.