Investigation of spatial structure and sympathetic cooling in the ⁹Be⁺–⁴⁰Ca⁺ bi-component Coulomb crystals

We study the spatial structure and sympathetic cooling of the bi-component Coulomb crystal (CC), which consists of approximate 450 ⁹Be⁺ ions and 450 ⁴⁰Ca⁺ ions with a mass ratio of 0.225 in a segmented linear ion trap. By two-dimensional imaging of the bi-component CC, the ⁹Be⁺ ions are found to be surrounded by the ⁴⁰Ca⁺ ions in the radial direction with a separation ratio of ∼ 2.0, and the axial length of the ⁹Be⁺ ions occupied area is much larger than that of the ⁴⁰Ca⁺ ions occupied area. Combined with the previous experimental results, the structure of the ⁹Be⁺–⁴⁰Ca⁺ CC shows the larger the difference in the mass–charge ratio, the larger the separation between the two species. The comparison of the fluorescence spectra of the ⁹Be⁺ ions in the bi-component CC and the pure CC indicates that the ⁹Be⁺ ions can be sympathetically cooled and stably localized by the laser-cooled ⁴⁰Ca⁺ ions during the recording of the fluorescence spectrum.