Load dependence of the periodicity in friction force images on the NaF(100) surface

We investigated load dependence of the frictional force between a NaF(100) surface and a Si3N4 asperity on an atomic scale using a two-dimensional frictional force microscope. As a result, we found that the periodicity in the frictional image changes with the load. Below the normal load of 1.9 nN, the usual lattice periodicity of the NaF surface is observed. On the other hand, at the normal load between 1.9 and 2.8 nN, an anomalous periodicity of half the lattice periodicity is observed for the first time. At the normal load between 3.1 and 3.3 nN, the usual lattice periodicity is again observed. This load dependence of the periodicity consistent with the prediction of the computer simulation by Tang, Joachim, Devillers and Girard. Finally, above the normal load of 3.6 nN, the periodicity disappeared.