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We observe mechanical effects of an exfoliated graphene monolayer depositedon a quartz crystal substrate designed to operate as an extremely low-lossbulk-acoustic-wave cavity at liquid-helium temperature.This is achieved bysensing overtones of the three thickness eigen-modes of the so-called SC-cut,since all three modes, two shear mode and one extensional mode, can beelectrically probed with such a crystal cut. From quality-factor measurements,the mechanical losses of the adhesive graphene monolayer are assessed to beabout 8 x 10-4 at 4 K in the best case. They are therefore significantlygreater than those already reported for suspended membranes but also foradherent layers on SiO2/Si substrates operating in torsional modes. In fact,results reveal that surface scattering occurs due to a roughness degradation ofa factor 7. In addition, the mechanical losses presented here are also placedin the context of a device submitted to thermomechanical stresses, but whichare not the only ones existing. Some of them could be intrinsic ones related tothe deposition process of the graphene layer. Based on a force-frequency theoryapplied to the three thickness modes which react differently to stresses, it isdemonstrated that this stress effect actually entangled with that of massloading reconciles the experimental results.

Probing the acoustic losses of graphene with a low-loss quartz bulk-acoustic-wave resonator at cryogenic temperatures