Superlubricity: a state of vanishing friction

The state of vanishing friction called “superlubricity” is discussed from theoretical and experimental viewpoints. We study the friction system consisting of two contacting surfaces, and derive the condition for the non-adiabatic motion of atoms to occur. By examining it for various systems, it is concluded that atoms move adiabatically, that is, that the superlubricity appears in realistic systems. It is shown that superlubricity appears when the system satisfies two conditions: each atom follows its equilibrium position adiabatically, and the contacting crystal surfaces are incommensurate. The adiabatic motion of atoms is likely to occur in two- and three-dimensional system rather than in one-dimensional system, it is then emphasized that the high dimensionality of contacting surfaces is crucial for the appearance of superlubricity. Friction measurements of atomically clean surfaces has been performed by using the scanning tunneling microscopy (STM) in ultra-high vacuum. The STM achieves the sliding with the elastic contact of the surfaces as in the attractive mode of atomic force microscopy (AFM) operation. It is shown that friction of magnitude 8×10−8 N, which is comparable to the calculated value, is observed when the contact is commensurate, while friction is not observed in this measurement, which can resolve the friction forces of 3×10−9 N, when the contact is incommensurate. It is thus concluded that the observed frictional anisotropies stemming from the differences in the commensurability of the contacting surfaces imply the existence of superlubricity.