Evidence for contact delocalization in atomic scale friction

We analyze an advanced two-spring model with an ultralow effective tip mass to predict nontrivial and physically rich "fine structure" in the atomic stick-slip motion in friction force microscopy (FFM) experiments. We demonstrate that this fine structure is present in recent, puzzling experiments. This shows that the tip apex can be completely or partially delocalized, thus shedding new light on what is measured in FFM and, possibly, what can happen with the asperities that establish the contact between macroscopic sliding bodies.     

Superlubricity of graphite

Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resolution down to 15 pN, we have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring atomic-scale friction as a function of the rotational angle between two contacting bodies, we show that the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of "superlubricity" [Phys. Rev. B 41, 11 837 (1990)]].     

An improved error bound for a finite element approximation of a model for phase separation of a multi-component alloy

Using the approach of Rulla (1996 SIAM J. Numer. Anal. 33, 68-87)for analysing the time discretization error and assuming moreregularity on the initial data, we improve on the error boundderived by Barrett and Blowey (1996 IMA J. Numer. Anal. 16,257-287) for a fully practical piecewise linear finite elementapproximation with a backward Euler time discretization of amodel for phase separation of a multi-component alloy.     

Reversible formation of a PdC(x) phase in Pd nanoparticles upon CO and O2 exposure

The structure and chemical composition of Pd nanoparticles exposed to pure CO and mixtures of CO and O(2) at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar and from room temperature to a few hundred degrees celsius. Our investigation shows that under CO exposure, above a certain temperature, carbon dissolves into the Pd particles forming a carbide phase. Upon exposure to CO and O(2) mixtures, the carbide phase forms and disappears reversibly, switching at the stoichiometric ratio for CO oxidation. This finding opens new scenarios for the understanding of catalytic oxidation of C-based molecules.