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While viscoelastic, adhesive contact rupture of simple indenters is well studied, contact formation has received much less attention. Here, we present simulations of the formation of contact between various power law indenters and an adhesive, viscoelastic foundation. For all investigated indenters, we find that the macroscopic relaxation time τ scales approximately with 1/ρ1.8, where ρ is the range of adhesion. The prolongation of contact formation with Tabor parameter is rationalized by the increased dissipation that short-range adhesion causes on a moving crack. [ABSTRACT FROM AUTHOR]

The orientational diffusion of a nitrogen molecule N2 in a three-dimensional crystal of argon atoms is studied by using the reaction coordinate formalism and the Bennett–Chandler approach. To study the two-dimensional orientational diffusion the reaction coordinate formalism has to be generalized to multichannel reactions. The rate coefficient for the motion between the six stable orientational states is determined from correlation function expressions, which include corrections to transition-state theory due to recrossings of free energy barriers. At high temperatures the simulation results are compared with results obtained by standard molecular dynamics methods. At low temperatures the transmission coefficient is computed and the dependence of the plateau value upon the density of the surrounding argon matrix and upon the isotopes, chosen to form the N2 molecule, is investigated. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Understanding the origin of the pseudogap is an essential step toward elucidating the pairing mechanism in the cuprate superconductors. Recently there has been strong experimental evidence showing that C4 symmetry breaking occurs on the formation of the pseudogap. This form of symmetry breaking was predicted by the fluctuating bond model (FBM), an empirical model based on a strong, local coupling of electrons to the square of the planar oxygen vibrator amplitudes. In this paper we approach the FBM theory from a new direction, starting from ab initio molecular dynamics simulations. The simulations demonstrate a doping-dependent instability of the in-plane oxygens toward displacement off the Cu-O-Cu bond axis. From these results and perturbation theory we derive an improved and quantitative form of the FBM. A mean-field solution of the FBM leads to C4 symmetry breaking in the oxygen vibrational amplitudes and to a d-type pseudogap in the electronic spectrum, the features linked by recent experimental data. The phase diagram of the pseudogap derived from mean-field theory, its doping and temperature dependences, including the phase boundary T*, agree well with experimental data. We extend the theory to include the long-range Coulomb interaction on the same basis as the FBM interaction. When the long-range Coulomb interaction is included in the FBM, a charge density wave (CDW) instability in the charge channel is predicted, which explains the nanoscale, rather than spatially uniform, behavior of the C4 symmetry breaking. Taking the CDW into account, with the theoretical k dependence of the pseudogap, enables the Fermi surface arc phenomenon to be understood. [ABSTRACT FROM AUTHOR]

A microscopic theory for the ubiquitous phenomenon of static friction is presented. Interactions between two surfaces are modeled by an energy penalty that increases exponentially with the degree of surface overlap. The resulting static friction is proportional to load, in accordance with Amontons's laws. However, the friction coefficient between bare surfaces vanishes as the area of individual contacts grows, except in the rare case of commensurate surfaces. An area independent friction coefficient is obtained for any surface geometry when an adsorbed layer of mobile atoms is introduced between the surfaces. The predictions from our simple analytic model are confirmed by detailed molecular dynamics simulations.