  Using molecular dynamics simulations, we probe a suspended graphene layer by
a diamond-like-carbon tip at various temperatures. The force acting on the tip
in the sliding direction is measured to be negative at liquid-helium
temperature. This negative force is found to be associated with a spontaneous
lateral oscillation of the suspended graphene in favor of a low interface
potential corrugation. Our hypothesis is that, at low temperature, this
oscillation induces an important hidden contribution to the friction force in
the lateral direction. This functions as a particular energy dissipation
mechanism at nanoscale.
