The Ion Crystal

Abstract

Most of the contributions to this volume have been concerned with electrons trapped above the surface of liquid helium. However, it is also possible to trap charged particles below the surface of liquid helium, and this chapter will be concerned primarily with the study of two-dimensional crystals formed from arrays of such trapped particles at temperatures below about 200 mK. The plan of the chapter is as follows. In § 2 we describe the nature of two types of charged particle, or “ion”, that can be produced very easily in liquid helium. In § 3 we explain how such particles can be trapped below the free surface of the liquid, and we describe the characteristics of the ion pools created by making use of this trapping mechanism. In § 4 we describe briefly the observation of plasma modes in the trapped pools and how the observed linewidths of these modes lead to values of the ionic mobility as a function of temperature and trapping depth. Mode coupling effects, which have been used extensively in the study of the crystalline phase of the pools, will be introduced in § 5. In § 6 we first introduce the predicted crystallization of the pools, and then go on to describe how this crystallization is likely to affect the modes of collective oscillation of the pools. §§7 and 8 deal respectively with the observation of Shikin modes and shear modes, both of which are characteristic of the crystal phase; they describe also the information that has been obtained from these observations. § 9 deals briefly with the melting transition and the nature of the high temperature phase. § 10 summarizes the chapter and outlines prospects for the future.