Ionic Coulomb crystals in superfluid helium

An account is given of experimental studies of quasi-two-dimensional classical Coulomb crystals formed from pools of positive or negative ions trapped below the free surface of superfluids ⁴He. The studies are concerned with various modes of oscillation of the crystals, which are coupled in an important way to ripplons on the helium surface. They include the detection of the Shikin mode, which confirms the existence of a crystal, the discovery of interesting nonlinear effects and intermode coupling, the search for shear modes, and the possible discovery of interesting modes in what may be a hexatic phase above the melting temperature of the crystals.