Electronic and phononic friction of solids at low temperatures

It has recently been shown both experimentally and theoretically that there is no static friction in a contact of atomically flat crystalline solids provided the periods of their lattices are incommensurate and their interaction does not exceed some critical value. The only mechanisms of friction in this case are phonon generation and excitation of conducting electrons. It is shown that, at low temperatures, the phonon contribution to the coefficient of friction can be very small by virtue of the quantum mechanical nature of the elementary excitations in a solid. Incommensurate dielectric crystals could therefore slip at low temperatures practically without friction. In metals, on the contrary, the excitation of electrons leads to a finite dynamic friction force at any temperature. Presently, both phonon and electron contributions to the friction force are estimated (the latter both in the normal and the superconducting state of metal).