In 1938, Eugene Wigner theorized that a collection of particles ionized with the same
polarity would assume a crystalline configuration when its potential energy
of repulsive Coulomb interactions was large relative to its kinetic
energy. Since Wuerker's 1959 observation of crystallization of
charged aluminum particles, Wigner crystallization within a Paul trap has
been demonstrated by laser-cooled Mg ions and Hg ions.