Sympathetic cooling of a large ${}^{113}\mathrm{{Cd}}^{+}$ ion crystal with ${}^{40}\mathrm{{Ca}}^{+}$ in a linear Paul trap

We have sympathetically cooled and crystallized ${}^{113}\mathrm{{Cd}}^{+}$ ions with laser-cooled ${}^{40}\mathrm{{Ca}}^{+}$ ions, and directly observed the complete large bicrystal structure in a linear Paul trap. The large two-component crystal contains up to $3.5\times10^3$ ${}^{40}\mathrm{{Ca}}^{+}$ ions and $6.8\times10^3$ ${}^{113}\mathrm{{Cd}}^{+}$ ions. The temperature of the crystallized ${}^{113}\mathrm{{Cd}}^{+}$ ions was measured to be as low as 41 mK with a large mass ratio. Moreover, we have studied several properties and structures of the ultracold sample. The factors affecting the sympathetic cooling effect were studied, including the electrical parameters and the number ratio between laser-cooled ions and sympathetically-cooled ions. The results of this paper enrich the experimental research of large two-component ion crystals, and the ultracold sample of ${}^{113}\mathrm{{Cd}}^{+}$ ions makes it possible to further improve the accuracy of the cadmium-ion microwave frequency standard.