Abstract
Small three-dimensional strongly coupled clusters of charged particles in a spherical confinement potential arrange themselves in nested concentric shells. If the particles are immersed into a background plasma the interaction is screened. The cluster shell configuration is known to be sensitive to the screening strength. With increased screening, an increased population of the inner shell(s) is observed. Here, we present a detailed analysis of the ground state shell configurations and configuration changes in a wide range of screening parameters for clusters with particle numbers N in the range of 11 to 60. We report three types of anomalous behaviors which are observed upon increase of screening, at fixed N or for an increase of N at fixed screening. The results are obtained by means of extensive first principle molecular dynamics simulations.
Highlights
(iii) at very large κ there exist cases of re-entrent shell fillings: one particle returns from the inner to the outer shell
Focusing on a finite range of particle numbers, 11 N 60, we presented a complete overview on all existing changes of the shell configurations for κ 5
There are, three nontrivial deviations (‘anomalies’) from this shell filling sequence which were analyzed: (i) upon κ increase two particles move to the inner shell(s) at once, (ii) when the particle number is increased by one, at a fixed κ, one particle moves from the inner to the outer shell, and (iii) at very large κ there exist cases of re-entrent shell fillings: one particle returns from the inner to the outer shell
Summary
We consider N identical Yukawa interacting classical particles with mass m and charge q in a three-dimensional isotropic harmonic confinement potential, with the confinement strength α, described by the Hamiltonian. It was observed previously for Coulomb systems that there exist different states with the same shell configuration, which differ with respect to the particle arrangement within the shells (fine structure) [19, 21, 22] These energy differences which are less than 10−8 in dimensionless units will not be resolved since this would blow up the whole analysis and we record only the energetically lowest shell configurations for a given value of κ. When for some N a configuration change at some critical κ was detected, the calculation around this point was repeated with a substantial smaller κ step to ensure an accuracy of ±0.05 in κ at points of structural transitions The choice of this interval of screening parameters is motivated by the situation of typical dusty plasma experiments where κ is around 1.
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