PHASE-TRANSITIONS IN THE GROWTH OF HE-4 FILMS

Using a microscopic, variational approach we examine the growth of He- 4 absorbed to graphite and alkali substrates. We find that superfluid layers are formed and their behavior as a function of coverage is clos ely related to the one of a purely two-dimensional superfluid. The gro wth of a new layer undergoes a phase transition from a cluster formati on into the connected superfluid when the coverage is increased. Based on the important connection to the two-dimensional fluid we propose a microscopic theory of quantum vortices in He-4 films at zero temperat ure, in which single vortices are treated as quasiparticles. We calcul ate the energy needed to create the single vortex, vortex inertial mas s, microscopic interaction between vortices and binding energy of the vortex-antivortex pair as a function of density. We predict that at th e He-4 superfluid density less than about 0. 037 angstrom-2 the bindin g energy of the pair becomes negative, indicating a phase transition i nto a new state where vortex-antivortex pairs are spontaneously create d.