The lowest-energy state of spherical clusters made up of single-species charged particles in a three-dimensional confining potential is investigated by molecular dynamics simulations for a system size of 5 x 10(3) to 1.2 x 10(5). The energy per particle is compared between shell-structured clusters and spherical finite-bcc lattices with relaxed surfaces. The shell structure in the interior is the lowest-energy configuration for ion numbers lower than about 10(4), while for higher ion numbers, an interior with bcc ordering surrounded by a few shells on the outside has lower energy. The formation of a small bcc lattice (nucleation) in the shell-structured cluster of 2 x 10(4) ions is observed.
Parameters characterizing the structure of a confined Yukawa system are estimated for ''dusty plasmas,'' clouds of charged macroscopic particles formed near the boundary between a plasma and the sheath, and levitated by a negatively biased electrode. When we have dust particles with different charge-to-mass ratios, they form a two-dimensional Yukawa mixture or separate two-dimensional one-component Yukawa systems, depending on the charge density in the sheath and the number density of dust particles. Pointed out is the possibility that dust particles with a larger charge-to-mass ratio have layered structures in the domain of neutral plasma, being supported by those with a smaller charge-to-mass ratio in the domain of sheath.
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