ABSTRACT
In his classic paper Wigner (1938) showed that upon cooling an electron gas can condense and form an ordered crystalline structure, the so-called ‘Wigner crystal’. The formation of a Wigner crystal (as well as crystallization of a quantum electron fluid) has been investigated experimentally. The crystal structures were also observed in electrostatic vacuum traps or charged macroparticles and in Paul and Penning traps with Mg and Be ions that are cooled to very low temperatures (∼10−3 K). A Coulomb crystal is also realized in colloidal suspensions. The colloidal crystals consist of almost mono-dispersive micronsized particles suspended in an electrolyte where they become charged negatively, having electron charges as high as 103-104. The particles are screened by ions of both signs in the electrolyte. The Coulomb interaction between the particles renders the formation of a crystal structure energetically more favourable. A strong coupling between the particles takes place at distances less than the screening radius, which in colloidal suspensions is very small. This leads to the result that for crystallization rather high particle number density (Np ∼ 1012) is necessary. Consequently, colloidal crystals are usually opaque, hindering an experimental study of their bulk properties. The drawback of the colloidal crystals is that they have a long equilibrium relaxation time, amounting to several weeks.
