The Puzzling Collapse of Electronic Sliding Friction on a Superconductor Surface

In a recent paper [Phys. Rev. Lett. 80 (1998) 1690], Krim and coworkers have observed that the friction force, acting on a thin physisorbed layer of N_2 sliding on a lead film, abruptly decreases by a factor of ~2 when the lead film is cooled below its superconductivity transition temperature. We discuss the possible mechanisms for the abruptness of the sliding friction drop, and also discuss the relevance of these results to the problem of electronic friction.Comment: 5 pages, no figure

Rubber friction on smooth surfaces

We study the sliding friction for viscoelastic solids, e.g., rubber, on hard flat substrate surfaces. We consider first the fluctuating shear stress inside a viscoelastic solid which results from the thermal motion of the atoms or molecules in the solid. At the nanoscale the thermal fluctuations are very strong and give rise to stress fluctuations in the MPa-range, which is similar to the depinning stresses which typically occur at solid-rubber interfaces, indicating the crucial importance of thermal fluctuations for rubber friction on smooth surfaces. We develop a detailed model which takes into account the influence of thermal fluctuations on the depinning of small contact patches (stress domains) at the rubber-substrate interface. The theory predicts that the velocity dependence of the macroscopic shear stress has a bell-shaped f orm, and that the low-velocity side exhibits the same temperature dependence as the bulk viscoelastic modulus, in qualitative agreement with experimental data. Finally, we discuss the influence of small-amplitude substrate roughness on rubber sliding friction.Comment: 14 pages, 16 figure

Journal Staff

Gender, male and female has always been of great interest and present in almost all fiction and films, within different perspectives. Ever since the dawn of time, man has taught ethics, morals, values and existential questions using fictional stories in oral and later in written form. The aim of analyzing the concept of male and female from fiction and film is to reach deeper insight and understanding of human beings with special reference to gender in both individual, group and community perspective. The general aim is to create security in our gender and professional roles. Whole or parts of books and films are used, chosen according to the specific learning outcome. Different aspects are then discussed in lectures, seminars or groups. Aspects include projections, culture and behavior. Students learn to reach a deeper insight and understanding of gender regarding behavior, communication, attitudes, values, culture and ethnicity. Peeling off the illusion of male and female created by our social structure, and making our values and prejudices conscious, we might discern the eternally human, the inner core of being human, regardless of sex. Fiction and film make conscious behavior, communication and gender, and merge cognition and emotions. This way of teaching may lead to better treatment of patients by health care personne

Contact mechanics for randomly rough surfaces

When two solids are squeezed together they will in general not make atomic contact everywhere within the nominal (or apparent) contact area. This fact has huge practical implications and must be considered in many technological applications. In this paper I briefly review basic theories of contact mechanics. I consider in detail a recently developed contact mechanics theory. I derive boundary conditions for the stress probability distribution function for elastic, elastoplastic and adhesive contact between solids and present numerical results illustrating some aspects of the theory. I analyze contact problems for very smooth polymer (PMMA) and Pyrex glass surfaces prepared by cooling liquids of glassy materials from above the glass transition temperature. I show that the surface roughness which results from the frozen capillary waves can have a large influence on the contact between the solids. The analysis suggest a new explanation for puzzling experimental results [L. Bureau, T. Baumberger and C. Caroli, arXiv:cond-mat/0510232] about the dependence of the frictional shear stress on the load for contact between a glassy polymer lens and flat substrates. I discuss the possibility of testing the theory using numerical methods, e.g., finite element calculations.Comment: Review paper, 29 pages, 31 picture

Interfacial separation between elastic solids with randomly rough surfaces: comparison between theory and numerical techniques

We study the distribution of interfacial separations P(u) at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for P(u) using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) a Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches.Comment: 25 pages, 12 figure

Theory of Inelastic Electron Tunneling from a Localized Spin in the Impulsive Approximation

A simple expression for the conductance steps in the inelastic electron tunneling from spin excitations in a single magnetic atom adsorbed on a non-magnetic metal surfaces is derived. The inelastic coupling between the tunneling electron and the spin is via the exchange coupling and is treated in an impulsive approximation using the Tersoff-Hamann approximation for the tunneling between the tip and the sample. Our results for conductance steps justify the analysis carried out by Hirjebedin et al. [Science 317, 1199 (2007)] of observed step-like conductances by inelastic electron tunneling from spin excitations in a single magnetic adatom using a simple spin matrix element. In addition, our result gives a simple expression for the magnitudes of conductance steps and their lateral spatial variation with respect to the tip position, which can be calculated directly from spin-polarized wave functions at the Fermi level of the sample

The Fundamental Problem of Philosophy: Its Point

The fundamental problem of philosophy is whether doing it has any point, since if it does not have any point, there is no reason to do it. It is suggested that the intrinsic point of doing philosophy is to establish a rational consensus about what the answers to its main questions are. But it seems that this cannot be accomplished because philosophical arguments are bound to be inconclusive. Still, philosophical research generates an increasing number of finer grained distinctions in terms of which we try to conceptualize reality, and this is a sort of progress. But if, as is likely, our arguments do not suffice to decide between these alternatives, our personalities might slip in to do so. Our philosophy will then express our personality. This could provide philosophy with a point for us. If some of our conclusions have practical import, philosophy could have the further point of giving us something by which we can live

Inhomogeneous potentials, Hausdorff dimension and shrinking targets

Generalising a construction of Falconer, we consider classes of $G_\delta$-subsets of $\mathbb{R}^d$ with the property that sets belonging to the class have large Hausdorff dimension and the class is closed under countable intersections. We relate these classes to some inhomogeneous potentials and energies, thereby providing some useful tools to determine if a set belongs to one of the classes. As applications of this theory, we calculate, or at least estimate, the Hausdorff dimension of randomly generated limsup-sets, and sets that appear in the setting of shrinking targets in dynamical systems. For instance, we prove that for $\alpha \geq 1$, $$\mathrm{dim}_\mathrm{H}\, \{ \, y : | T_a^n (x) - y| < n^{-\alpha} \text{ infinitely often} \, \} = \frac{1}{\alpha},$$ for almost every $x \in [1-a,1]$, where $T_a$ is a quadratic map with $a$ in a set of parameters described by Benedicks and Carleson.Comment: 36 pages. Corrected and reorganised following referee's report. Accepted for publication in Annales Henri Lebesgu