Discontinuous growth of solid 4He on graphene

We have studied the growth of solid 4He on multilayer graphene immersed in the superfluid well below the bulk freezing pressure using the torsional oscillator method at temperatures from 1.65 to 0.1 K. Below 1.2 K, the growth of one solid layer is observed as a series of discontinuous steps, each accompanied by energy dissipation. We speculate that the discontinuities result from a series of two-dimensional structural phase transitions of the uppermost solid layer, between several commensurate and incommensurate states.