Reentrant pinning of a He3 overlayer in a He3−He4 mixture film

We performed quartz crystal microbalance experiments for a $³\mathrm{He}\text{\ensuremath{-}}⁴\mathrm{He}$ mixture film on an exfoliated single-crystalline graphite using a 32-kHz quartz tuning fork. The decoupled $³\mathrm{He}$ overlayer on the superfluid $⁴\mathrm{He}$ 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 $³\mathrm{He}$ 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 $⁴\mathrm{He}$ layer on the oscillating substrate.