Secular Motion Frequencies of 9Be+ Ions and 40Ca+ Ions in Bi-component Coulomb Crystals**Supported by the National Natural Science Foundation of China under Grant Nos 91636216, 11504410 and 11474317, and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant No XDB21020200.

Abstract

We obtain bi-component Coulomb crystals using laser-cooled 40Ca+ ions to sympathetically cool 9Be+ ions in a linear Paul trap. The shell structures of the bi-component Coulomb crystals are investigated. The secular motion frequencies of the two different ions are determined and compared with those in the single-component Coulomb crystals. In the radial direction, the resonant motion frequencies of the two ionic species shift toward each other due to the strong motion coupling in the ion trap. In the axial direction, the motion frequency of the laser-cooled 40Ca+ is impervious to the sympathetically cooled 9Be+ ions because the spatially separation of the two different ionic species leads to the weak motion coupling in the axial direction.