Impurity Effects on the Slippage of Nonsuperfluid Helium Films

We measured the mechanical response of 4 He films adsorbed on an oscillating substrate with a small amount of N2 molecules using an ultrasonic technique. Compared with the substrate with no N2 molecules, it was found that nonsuperfluid 4 He films undergo slipping at lower temperature. This means that the frictional force acting on these films is increased by N2 molecules adsorbed on the substrate.