Apparent new phase of monolayer3He and4He films adsorbed on Grafoil as determined from heat capacity measurements

Heat capacity measurements in monolayer³He and⁴He films adsorbed on Grafoil at densities higher than the one corresponding to the substrate lattice registered phase show a series of sharp, narrow peaks at 1 K for densities between 0.072 and 0.077Å ^–2. The exact nature of the transition cannot be determined from this measurement alone, but several possibilities are discussed. It has been determined that the melting line of two-dimensional solid films starts atn=0.078Å ^–2 for both isotopes. Extensive heat capacity measurements at and above this density are presented for³He, and some new measurements for⁴He are shown to complement measurements reported elsewhere. The solid³He measurements are compared to predictions of recent models for melting in two dimensions.Work supported by National Science Foundation Grant # DMR72-03003A04.