Structural properties of two-component Coulomb crystals in linear Paul traps

We report on structural properties of two-component Coulomb crystals in a l inear Paul trap. The crystals consist of two laser cooled ion species. Mg-2 4(+) and Ca-40(+). The lighter Mg-24(+) ions form an inner cylindrical crys tal structure surprisingly similar to that of an infinitely long single com ponent crystal, while the outermost shell of the surrounding Ca-40(+) ions have a spheroidal shape, which is highly insensitive to the presence of the Mg-24(+) ions. Observed changes in the radial separation of the two ion sp ecies with the radius of the inner cylindrical crystal is explained by a si mple model.