Phase diagram and Wigner crystal of dipolar complexes in liquid helium

Abstract

The phase diagram of the two-dimensional dipolar system formed by tightly bound pairs of surface electrons above a superfluid helium film and positive ions lying inside the film over a solid substrate is determined on the basis of Lindeman's criterion and using the Kosterlitz-Thouless dislocation-mediated melting mechanism. The ground-state energy, the phonon-dispersion curves, and the longitudinal and transversal sound velocities of the dipolar crystal are evaluated within the harmonic approximation as a function of the distance between the layers and for different substrates.