  We report measurements of noncontact friction between surfaces of NbSe$_{2}$
and SrTiO$_{3}$, and a sharp Pt-Ir tip that is oscillated laterally by a quartz
tuning fork cantilever. At 4.2 K, the friction coefficients on both the
metallic and insulating materials show a giant maximum at the tip-surface
distance of several nanometers. The maximum is strongly correlated with an
increase in the spring constant of the cantilever. These features can be
understood phenomenologically by a distance-dependent relaxation mechanism with
distributed time scales.

Materials Science