Phase boundaries and critical and tricritical properties of monolayer 4He adsorbed on graphite.
We report heat-capacity measurements on 4He films adsorbed on graphite for surface densities at and smaller than that required to form a commensurate (√3 × √3 )R30° monolayer in the temperature range 0.4 K K. At densities below 0.039 A˚−2, a single transition is observed at T<1.3 K. For densities larger than 0.039 A˚−2, two transitions are observed, the higher-T one developing into the order-disorder transition previously studied by several authors. Our measurements are consistent with a phase diagram showing a large region of existence of a single-phase commensurate (with vacancies) film, and a two-phase region of coexisting commensurate and vapor phases. For this phase diagram a tricritical point exists at TTCP≃1.3 K and a density nTCP≃0.039 A˚−2. We observe strong evidence for Fisher renormalization of the constant-density specific heat along the critical line. The effects of inhomogeneities on all transitions are discussed.
Physical Review B