(260y) An Asymmetric Dimer in a Periodic Potential: A Minimal Model for Friction of Graphene Flakes

For nanometer sized graphene flakes sliding on graphite, the friction as a function of rotation angle between flake and substrate displays peaks for the perfect commensurate case (flake and substrate have the same orientation) and decreases rapidly for incommensurate cases (superlubricity). In experiments and simulations, superlubric sliding is usually lost due to irreversible rotations to a commensurate contact [1]. However, it has been recently experimentally shown that friction can be increased reversibly by applying load to the incommensurate flake [2]. This reversible transition from superlubric to stick-slip sliding occurs while the flake remains at incommensurate contact and can be attributed to vertical motion of the carbon atoms at the edge of the flake. We propose a minimal model to describe this effects [3].
We discuss the friction and motion of a model of a dimer with asymmetric interactions with a substrate potential. Starting from the consideration that a rigid dimer with spacing equal to half of the period of the potential has exactly zero static friction like the infinite incommensurate Frenkel Kontorova model, we show how stick-slip behaviour and friction arise as a function of asymmetry. We argue that this model can yield a simple yet insightful description of the frictional behaviour of graphene flakes on graphite and of superlubricity. The results can also be of interest for diatomic molecules on surfaces.
References:
[1] A.E. Filippov, M. Dienwiebel, J.W.M. Frenken, J. Klafter, M. Urbakh, Phys. Rev. Lett. 100, 046102 (2008)
[2] M.M. van Wijk, M. Dienwiebel, J.W.M. Frenken, A. Fasolino, Phys. Rev. B 88, 235423 (2013)
[3] R. Hens, A. Fasolino, An asymmetric dimer in a periodic potential: a minimal model for friction of graphene flakes. Manuscript submitted for publication (2016).