We report an experimental study of the crystallization of sheets of positive ions trapped below the surface of superfluid helium. The technique relies on the generation and interference of capillary waves on the helium surface by the ions when they are subject to an oscillating electric field. Effects analogous to elastic and inelastic x-ray or neutron scattering are observed, the inelastic processes involving the absorption or emission of plasmons. The formation of a triangular lattice in the crystal phase is confirmed, and effects are observed that are associated with the temperature dependence of the structure factor and the corresponding loss of long-range crystalline order in a two-dimensional system at a finite temperature.

• Received 30 July 1999

DOI:https://doi.org/10.1103/PhysRevB.61.1396