THERMODYNAMICS OF BOSON QUANTUM FILMS

Theoretical studies of films of liquid He-4 adsorbed to strongly attra ctive plane substrates indicate that the growth of such films occurs t hrough a sequence of first-order phase transitions-''layering transiti ons''-which are a direct consequence of the short-range, hard-core-lik e interaction between individual helium atoms. The present work examin es the effects of temperature on these transitions. At given temperatu res, the spinodal points and phase coexistence boundaries are determin ed for the transitions. Increasing the temperature tends to decrease t he coverage span of the transition regions, signaling the possible exi stence of a critical point terminating the two-phase equilibrium. The layering transitions depend strongly on the helium-substrate potential ; the longer-range helium-magnesium potential yields fewer transitions and noticeably lower transition temperatures than the helium-graphite potential. The temperature dependence of the chemical potential, thir d sound, static structure function, heat capacities, and superfluid de nsities are reported. The heat capacities are compared to those measur ed by Greywall and Busch [Phys Rev. Lett. 67, 3535 (1991)]. The therma l broadening of the film's density profile is also discussed. We find that below 1.2 K, thermal broadening is quite weak for coverages away from the layering transitions. The monolayer film experiences the leas t broadening whereas double-layer and triple-layer films broaden by in creasing the local density in the outer tail of their density profiles , while depleting the local density in the inner portion of the outerm ost layer.