P. L. Elliott

Pools of ions trapped below the surface of superfluid helium: modes of response in a steady vertical magnetic field

The paper is concerned with two-dimensional classical fluids and solids composed of charged particles that interact only through Coulomb forces. An example of this type of system, and one with which the paper is particularly concerned, is a circular pool of either positive or negative ions trapped just below the free surface of superfluid helium. A calculation is presented of the modes of response of such a system in its own plane when exposed to an oscillating electric potential, especially in the long-wavelength limit and in the presence of a steady magnetic field applied in a direction normal to the plane of the system. The calculations are relevant to the experimental detection and study of these modes, which include shear modes in the crystal phase and viscous modes in the fluid phase.

Experimental investigation of low-frequency edge magnetoplasma modes in two-dimensional sheets of ions trapped below the surface of superfluid helium

We report the experimental observation of unconventional low-frequency magnetoplasma modes in 2D sheets of ions. Some of them are multipole edge modes of the type predicted to exist by Nazin & Shikin; others have a dependence of frequency on magnetic field similar to that for conventional edge modes and may be related to satellite modes discussed by Ye & Zaremba.

Damage and annealing in two-dimensional Coulomb crystals

We have demonstrated experimentally that strong excitation of shear modes, exceeding the shear strength of the crystal, results in severe damage in a 2D crystal of positive ions trapped below the free surface of superfluid helium. Annealing takes place in times that are of order 500 s and not strongly temperature dependent.

Inter-edge magnetoplasma modes in two-dimensional sheets of positive ions trapped below the surface of superfluid helium

Abstract We report an experimental investigation of inter-edge magnetoplasma modes in circular 2D sheets of positive ions within which a step in ion density has been imposed. The modes were detected through a non-linear coupling with an axisymmetric plasma mode of the sheet, when both types of mode were driven simultaneously. Evidence is given for the existence of a possible new family of inter-edge modes.

Pools of ions trapped below the surface of superfluid helium

Pools of ions trapped below the surface of superfluid helium form an ideal arena for the study of the fascinating properties of two-dimensional classical Coulomb solids and liquids. This system can support many different families of collective motion, and furthermore exhibits interesting mode-coupling effects; both of these features are employed to probe its behaviour. A description is given of some of the work that has been of interest over the past few years, focusing in particular on our understanding of the crystalline phase.

Modes of transverse response in a two-dimensional Coulomb system above the melting temperature

Abstract Experiments are described that were designed to measure the viscosity of a 2D Coulomb fluid formed from helium ions trapped below the free surface of superfluid helium, especially at temperatures just above melting. The results and limitations of these experiments are discussed, and new effects associated with steps in the density profile are described.

Shear modes in 2D ion crystals trapped below the surface of superfluid helium

An experimental study is reported of shear modes in the crystal phase of a circular 2D pool of charged ions trapped just below the free surface of superfluid 4He. The frequencies of the modes are in good agreement with theory. Preliminary studies of the damping of the modes, the temperature dependence of the mode frequencies, and the effect on the modes of damage to the crystal are included.

Shear modes in two-dimensional ionic Coulomb crystals

We report an experimental investigation of shear modes propagating in two-dimensional Coulomb crystals of positive ions trapped below the surface of superfluid helium. The dependence of the shear-mode resonant frequencies and linewidths on external drive, temperature, magnetic field, and pool radius has been investigated.