Effect of barium codoping on superconductivity in Sr$_x$Bi$_2$Se$_3$

A structural reason for superconductivity in a Cu-, Sr-, or Nb-atom-doped Bi$_2$Se$_3$ topological insulator is still unclear. To understand this reason, a codoping approach has been developed and Ba$_y$Sr$_x$Bi$_2$Se$_3$ single crystals with different $x$ and $y$ values have been grown. The composition and structural and transport properties of the grown crystals have been studied. With X-ray diffraction data, it has been shown that barium and strontium intercalate the system, although barium is present in the structure in a very small amount. The addition of barium surprisingly destroys superconductivity, slightly changing the lattice constants, the strontium doping level of the crystal matrix, and the electron density. Thus, a key role of a certain coordination arrangement of positions of strontium atoms between Bi$_2$Se$_3$ quintuples for achieving superconductivity in this material has been demonstrated.