STRUCTURE AND MELTING OF DIPOLE CLUSTERS

Two-dimensional clusters of particles, repelling due to dipole-dipole interactions and confined by an external parabolic potential, are cons idered. The model describes different physical systems, particularly e lectrons in semiconductor structures, or electrons above a drop of He near a metal electrode, a drop of colloid liquid etc. Two kinds of ord ering are in competition in the clusters: a triangular lattice and a s hell structure. The ground-state configurations corresponding to the l ocal and global minima of the potential energy for clusters with N = 1 - 40 ''particles'' are calculated. The structure, the potential energ y and the radial and angular r.m.s. displacements as functions of temp erature are also calculated. Analysing these quantities the melting of clusters is studied. One-or two-stage melting occurs depending on the number of particles in the cluster. in the case of clusters consistin g of two shells melting has two stages: at lower temperature reorienta tion of neighbouring shells (''orientational melting'') arises; at muc h higher temperatures the radial shell order disappears. In clusters c onsisting of more than two shells total melting occurs as a first-orde r one-stage transition (analogously to a dipole crystal). This is conn ected with the barrier of rotation being less than the barrier of inte rchange of particles between shells for small microclusters while the barriers are of equal order for clusters with a greater number of part icles. (C) 1997 Elsevier Science B.V.