Sliding friction of helium films in the metastable state and its relaxation

We measured the sliding friction of $^{4}\mathrm{He}$ films adsorbed on Grafoil using the quartz-crystal microbalance technique. In the low-temperature regime, this friction remains metastable after switching off the oscillation amplitude before relaxing to the value determined by the amplitude. The relaxation of this friction is qualitatively different for decreasing and increasing amplitudes. In the former case, the relaxation depends strongly on temperature and has a quasiexponential time dependence, while it has a quasilogarithmic one in the latter case. We can explain both cases of relaxation by a model of a small low-friction domain, which is created or annihilated by overcoming a potential barrier.