Thermodynamics of boson quantum films

Theoretical studies of films of liquid He-4 adsorbed to strongly attractive plane substrates indicate that the growth of such films occurs through a sequence of first-order phase transitions-''layering transitions''-which are a direct consequence of the short-range, hard-core-like interaction between individual helium atoms. The present work examines the effects of temperature on these transitions. At given temperatures, the spinodal points and phase coexistence boundaries are determined for the transitions. Increasing the temperature tends to decrease the coverage span of the transition regions, signaling the possible existence 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, third sound, static structure function, heat capacities, and superfluid densities are reported. The heat capacities are compared to those measured by Greywall and Busch [Phys Rev. Lett. 67, 3535 (1991)]. The thermal 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 least broadening whereas double-layer and triple-layer films broaden by increasing the local density in the outer tail of their density profiles, while depleting the local density in the inner portion of the outermost layer.