If the tip-surface interaction is described by a sinusoidal potential with amplitude V0 and periodicity a then where k is the spring constant.
If η>1, the tip motion consists in abrupt jumps between the minima of the energy landscape (stick-slip regime).
In reality it was Ludwig Prandtl who suggested in 1928 this model to describe the plastic deformations in crystals as well as the dry friction.
In Russia this model was introduced by the Soviet physicists Yakov Frenkel and T. Kontorova.
The Frenkel defect became firmly fixed in the physics of solids and liquids.