Turning a topological insulator into superconductor in Sr$_{0.06}$Bi$_{2}$Se$_3$

Topological superconductors are new states of quantum matter which are featured with a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. Up to now, possible topological superconducting state has been observed in the interface of topological insulator/superconductor heterostructures and highly-pressurized topological insulators. By intercalation of transition metal Cu in the well-known topological insulator Bi$_2$Se$_3$, possible bulk topological superconductivity has been realized in Cu$_x$Bi$_2$Se$_3$. However, Cu$_x$Bi$_2$Se$_3$ exhibits only very low superconducting volume fraction, making the compound very complicated. Thus the searching of a full topological superconductor has been a big challenge. Here we report that by intercalation of alkaline-earth metal Sr in Bi$_2$Se$_3$, superconductivity has been achieved in Sr$_{0.06}$Bi$_{2}$Se$_3$. The large shielding fraction ($\sim$88% at 0.5 K) indicates the occurrence of bulk superconductivity. This provides a new material base for investigating topological superconductivity.