Structure and Madelung energy of spherical Coulomb crystals
Abstract
With the help of moleculardynamics computer simulations, we study the equilibrium configurations of systems of N=25000 strongly correlated charged particles under the influence of a radial harmonic external confining force and their mutual Coulomb forces. The temperature is well below the crystallization point; i.e., the ratio of Coulomb to kinetic energy is as large as Γ=10^{9}. The particles arrange in concentric spherical shells with approximately constant intershell distances. On the surfaces plane hexagonal structures are well pronounced. The calculated radii, occupation numbers, and energies per particle are compared with results of classical geometrical and shell models with homogeneously charged shells corrected for hexagonal surface occupation. The closedshell particle numbers also agree well with those of multilayer icosahedra. From the computer simulations we extract a Madelung (excess) energy of 0.8926, which is close to the theoretical value of the shell model corrected for plane hexagonal surfaces, 0.8923, but larger than the one of the infinite geometrical lattice, 0.8944, and of the bcc value of 0.8959. Surfaceenergy effects are positive and of the order of N^{1/3}.
 Publication:

Physical Review A
 Pub Date:
 October 1991
 DOI:
 10.1103/PhysRevA.44.4506
 Bibcode:
 1991PhRvA..44.4506H
 Keywords:

 32.80.Pj;
 52.25.b;
 52.65.+z;
 52.75.Di;
 Optical cooling of atoms;
 trapping;
 Plasma properties;
 Ion and plasma propulsion