Reentrant pinning of a 3He overlayer in a 3He-4He mixture film

We performed quartz crystal microbalance experiments for a He-3-He-4 mixture film on an exfoliated single-crystalline graphite using a 32-kHz quartz tuning fork. The decoupled He-3 overlayer on the superfluid He-4 layer shows a reentrant pinning at a certain temperature T-3d under a large oscillation amplitude. The pinning state below T-3d is metastable. After reducing the oscillation amplitude, the He-3 overlayer relaxes to a stable pinning state via a depinning state. It is found that the reentrant pinning is triggered by a structure change of the underlying localized He-4 layer on the oscillating substrate.