Subject:

Theory of Condensed Matter 
Organization:

Theory of Condensed Matter 
Abstract:

A theoretical study of the motion of adsorbates (e. g. atoms, molecules or clusters) on solid surfaces is presented, with a focus on surface diffusion and atomicscale friction. These two phenomena are inextricably linked, because when an atomic or molecular adsorbate diffuses, or is pulled, it unavoidably experiences friction, opposing its motion. Since the adsorbate typically extends over the length scale of nanometers, the simple, empirical laws of friction known for the macroscopic world do not hold in this context. The aim of this work is to investigate the microscopic dynamical mechanisms of surfaces diffusion and to find a 'counterpart' of the macroscopic laws of friction at the nanoscale. After a brief introduction about the experimental techniques and the computational methods to study surface diffusion and nanoscale friction, we consider the onedimensional diffusion of a diatomic molecule on a periodic surface. At variance with the case of a single atom, complex dynamical features related to the interparticle interaction are observed, such as highly nonlinear behaviour, anomalous diffusion, resonances and chaos. In the last part of the work, the velocity and load dependence of atomicscale friction are presented. For a pair of macroscopic objects, friction is independent of their relative sliding velocity and is proportional to the applied load. At the nanoscale, we find an appreciable nonlinear dependence of friction on sliding velocity in the framework of the Tomlinson model, whose specific form depends on the presence or not of thermal effects. The load dependence is studied for a model of friction on graphite, using molecular dynamics simulations. It turns out to be nonlinear, with an exponent larger than one. We also report dynamical effects observed in the motion of extended flakes of graphite, where the role of surface registry is important to achieve very low friction.
