Influence of velocity in nanoscale friction processes

Force-microscopy images of boric acid crystals were obtained experimentally and simulated with the use of a two-dimensional mechanical model. An analysis of the stick and slip movement of the microscope tip shows that the energy-dissipation mechanism is strongly influenced by the non-linear dynamics of the sliding system. The contributions of stick and viscous forces on the energy dissipation (or friction forces) are studied as a function of the relative scanning velocity. At low relative velocities, the stick forces are shown to be responsible for the energy dissipation. This energy is velocity-dependent, due to the coupling between the two degrees of freedom of the sliding system. As the scanning velocity increases the stick forces are damped; the viscous force is then predominant in the energy-dissipation process. Received: 30 October 2001 / Accepted: 17 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +55-21/2295-9397, E-mail: prioli@vdg.fis.puc-rio.br     

A nonterminating 8φ7 summation for the root system Cr

Using multiple q-integrals and a determinant evaluation, we establish a nonterminating ₈φ₇ summation for the root system C_r. We also give some important specializations explicitly.     

互相关分析在近红外光谱预处理中的应用

采用互相关分析的光谱预处理方法与一元线性回归结合 ,利用近红外光谱法定量检测了苯和甲苯混合物溶液中苯的体积百分比含量 ;从理论和实验两个方面证明了在一定的条件下经过互相关变换后的光谱信号与目标物质浓度含量呈正比例关系 ,利用这个正比例关系建立一元线性回归方程可以定量检测混合物中目标物质的含量。并且深入探讨了互相关分析作为一种近红外光谱预测量方法的优缺点     

Preparation of 2-(trimethylsilyl)methyl-2-propen-1-ol derivatives by cobalt catalyzed sp2-sp3 coupling

A practical, operationally simple preparation of 2-(trimethylsilyl)methyl-2-propen-1-ol derivatives is described. The cobalt catalyzed coupling of a protected vinyl halide with trimethylsilylmethylmagnesium chloride shows excellent functional group tolerance and provides these synthetically useful allyl silanes in good overall yield. By this method, the use of highly concentrated organolithium reagents, complex reaction protocols, and expensive starting materials is avoided.     

Thermoluminescence glow curves analysis of pure and CeO2-doped Li2O-Al2O3-SiO2 glass ceramics

A comparative study of the thermoluminescence (TL) emission between beta-irradiated lithium aluminosilicates, Li₂O-Al₂O₃-SiO₂ (LAS), and beta-irradiated CeO₂-doped lithium aluminosilicates, Li₂O-Al₂O₃-SiO₂-CeO₂ (LAS:Ce), grown by sol-gel technique and preannealed at 1250 °C is presented. It is found that doping reinforces the result of preannealing the sample; the incorporation of CeO₂ at low concentrations shifts the TL curves towards higher temperatures and increases the total intensity with respect to samples without the dopant. This behavior, together with the fact that CeO₂ is a good densification agent, suggests the possibility of using CeO₂ to improve the technological properties of LAS. Deconvolution analysis of the thermoluminescence glow curves of doped materials under general order kinetics shows that the relevant temperatures at which the component signals appear are virtually the same as for pure samples. This suggests that CeO₂ does not introduce new types of traps, but only changes the population and distribution of impurities responsible for the TL in the original matrix of LAS. Finally, the kinetic parameters for the deconvoluted curves are reported. They corroborate that changes in CeO₂ concentration only vary the depth and distribution of the same kind of original traps.     

广义Segal-Bargmann空间上无界Toeplitz算子的交换

本文给出了复平面C上广义Fock空间中两个Toeplitz算子T_u和T_v的性质.假设u是一个径向函数,两算子是可交换的.在一定的增长条件之下,我们证明出u也是一个径向函数.最后还构造了一个具有本性无界符号的S_p紧,Toeplitz算子.     

A 14-dimensional Module for the Symplectic Group: Orbits on Vectors

Let 𝔽 be a field, V a 6-dimensional 𝔽-vector space and f a nondegenerate alternating bilinear form on V. We consider a 14-dimensional module for the symplectic group Sp(V, f) ? Sp(6, 𝔽) associated with (V, f), and classify the orbits on vectors. For characteristic distinct from 2, this module is irreducible and isomorphic to the Weyl module of Sp(V, f) for the fundamental weight λ₃. If the characteristic is 2, then the module is reducible as it contains an 8-dimensional submodule isomorphic to the spin module of Sp(V, f).     

Doppler characterization of laser inter-satellite links for optical LEO satellite constellations

Because of ensuring very low propagation delay between satellites, and providing global space-based broadband network services, low earth orbit (LEO) satellite constellations with laser inter-satellite links (ISLs) are considered to be the enabling technology to satisfy the increasing data traffic demand. However, significant Doppler can be observed by the onboard terminals on the ISLs, due to the high relative speed of the two communicating LEO satellites. This paper describes an analytic derivation of the Doppler wavelength shift measured by the terminal onboard a satellite on the signal transmitted through the ISLs. The Point-Ahead Mechanism of the optical ISLs is considered in the analytical expression of the Doppler wavelength shift. Then, in terms of the ISLs characteristics of the satellite constellations, the expression of Doppler wavelength shift is deduced into two aspects. First, for the full time accessing ISLs, it evolves as a function of the constellation parameters. Thus, the Doppler characterization for two kinds of interorbit full time accessing ISLs of LEO satellite constellations is analyzed. Second, for the intermittent accessing ISLs, the expression of Doppler wavelength shift is given as a function of the minimum ISL distance between two communicating satellites. And the visibility duration of the destination satellite at the source satellite is estimated for the intermittent ISLs. This work is helpful to evaluate the design of constellation networking.     

Atomistic simulations of the sliding friction of graphene flakes

Using a tight-binding atomistic simulation, we simulate the recent atomic-force microscopy experiments probing the slipperiness of graphene flakes made slide against a graphite surface. Compared to previous theoretical models, where the flake was assumed to be geometrically perfect and rigid, while the substrate is represented by a static periodic potential, our fully-atomistic model includes quantum mechanics with the chemistry of bond breaking and bond formation, and the flexibility of the flake. These realistic features, include in particular the crucial role of the flake rotation in determining the static friction, in qualitative agreement with experimental observations.     

Solar radiation pressure and deviations from Keplerian orbits

Newtonian gravity and general relativity give exactly the same expression for the period of an object in circular orbit around a static central mass. However, when the effects of the curvature of spacetime and solar radiation pressure are considered simultaneously for a solar sail propelled satellite, there is a deviation from Kepler's third law. It is shown that solar radiation pressure affects the period of this satellite in two ways: by effectively decreasing the solar mass, thereby increasing the period, and by enhancing the effects of other phenomena, potentially rendering some of them detectable. In particular, we consider deviations from Keplerian orbits due to spacetime curvature, frame dragging from the rotation of the sun, the oblateness of the sun, a possible net electric charge of the sun, and a very small positive cosmological constant.