The state of vanishing friction called superlubricity are discussed from theoretical and experimental viewpoints. We study the friction system consisting of two contacting surfaces, and derive the condition for the nonadiabatic 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 easurement by using scanning tunne
ling microscope method. In this experiment, Si(001) (n-type, 0.01 Ømega \cdot cm) is one-dimensionally scanned against a W tip in ultra-high vacuum, and the W tip-deflection caused by the friction appearing between the scanned Si(001) and the W(011) on the polycrystalline W tip is measured. This scanning tunneling microscope method achieves the sliding with the elastic contact of the surfaces as in the attractive-mode of atomic force microscope operation. It is shown that friction of magnitude 10 ^-7 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 10 ^-8 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.