Two-step melting of spherical Yukawa and Coulomb clusters

By extensive Monte Carlo simulations, two separate peaks are found in the specific heat of relatively small spherical clusters of particles with 100 ≤ N ≤ 400, interacting via the Yukawa and Coulomb potentials and confined in the external field. The analyses of the diffusion and positional fluctuations show that these peaks at lower and higher temperatures reflect the loss of intrashell two-dimensional ordering and of the radial ordering, respectively. The magnitudes of the fluctuations in the outermost shell are consistent with the conventional Lindemann criterion. The results can be verified by experiments in ion traps and in dusty plasmas under microgravity or equivalent isotropic environments.