Interfacial Friction of ⁴He Films Adsorbed on Grafoil Preplated with Kr

We report results for interfacial friction of nonsuperfluid ⁴He films adsorbed on Grafoil (exfoliated graphite) preplated with one atomic layer of Kr (⁴He/Kr/Gr). In the present experiment, the quartz‐crystal microbalance (QCM) technique was adopted, and the friction was measured down to 0.5 K for several oscillating amplitudes, ranging from 0.1 to 1 nm at 5 MHz. Above about the first layer completion, a decrease in the friction was clearly observed for a large amplitude below a certain temperature T_S. This decrease occurs abruptly at around the second layer completion. Although ⁴He/Kr/Gr has a lower T_S than ⁴He adsorbed on bare Grafoil when ⁴He is two atoms thick, the areal density dependence of T_S is similar for the two systems.