CLASSICAL 2-DIMENSIONAL ATOMS

A classical two-dimensional system consisting of charged particles whi ch are laterally confined by an artificial potential is investigated. This system is the classical analog of the well-known quantum dot prob lem. Using Monte Carlo techniques and molecular dynamics simulations w e obtained the possible ordered structures and phase transitions for s uch a system. The particles group together in rings. A Mendeleev-type table for such classical atoms was obtained. When the size of the 'cla ssical atom' is sufficiently large, the simple ring structure graduall y disappears in the center and features of a Wigner lattice appear. Th e excitation spectrum and corresponding normal modes for these classic al atoms are obtained. For atoms with a small number of charged partic les the lowest excitation corresponds to an intershell rotation. Magic numbers are associated to clusters which are most stable against inte rshell rotation. For large systems the lowest excitation consists of a vortex/anti-vortex pair. The effect of a magnetic field on the excita tion spectrum was calculated. We found that with increasing held the s pectrum collapses into two branches. The upper branch corresponds to t he cyclotron resonance energy, and the lower branch to the one of skip ping orbits in a dot. Phase transitions in these ordered structures we re investigated as a function of temperature. A two-step order-disorde r transition was found: with increasing temperature first intershell r otation becomes possible and intershell rotational order disappears. A t a second transition temperature intershell diffusion sets in. For la rge systems both transition temperatures coincide and equal the Wigner transition temperature.