Three-dimensional strongly coupled plasma crystal under gravity conditions

Experiments were carried out to investigate a three-dimensional (3D) plasma crystal. A method of determining the positions of each individual microparticle has been developed. A crystal volume of about $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ particles in 19 horizontal planes was analyzed. Direct imaging and the 3D pair correlation function show that ``domains'' of fcc and hcp lattices coexist in the crystal. Other structures, in particular, the theoretically predicted bcc lattice, were not observed.