Ion Pools Underneath the Surface of Superfluid 4He: A Playground of Classical Two-Dimensional Physics

Positive and negative ions in superfluid He-4 (He II) are interesting objects of experimental and theoretical research and serve as important probes to study various aspects of superfluidity. Additionally, with the help of externally applied electric field, they can be trapped below the free surface of He II, forming pools of nearly constant areal density, suitable to study broad range of collective phenomena of classical two-dimensional (2D) physics. A rich research program of this kind, in which the author participated, was run by Joe Vinen at the University of Birmingham, UK, for more than a decade. This article reviews the main results of this program, such as investigations of various vibrational modes of response of the ion pools and interactions of the ions with ripplons, detection of the two-dimensional Coulomb crystal and studies of its physical properties.