Superconductivity in SrTiO₃ doped with different rare earth ions

Recently, the superconducting dome of electron-doped SrTiO₃ has attracted renewed interest. For example, it becomes superconducting even at extremely low carrier densities (< 10¹⁸ cm^-3) and it has been suggested that its superconductivity cannot be explained by BCS theory. The n-type dopants used in the literature include La, Nb, and oxygen vacancies. Here we compare the superconducting transitions of SrTiO₃ films grown by molecular beam epitaxy and doped with different trivalent rare earth ions, such as La or Sm. The La-doped samples show transitions similar to those of bulk SrTiO₃ reported in the literature. The Sm-doped films show metallic resistivity at high temperatures at carrier densities in the range of 0.5 to 1 10²⁰ cm^-3. At low temperatures, around 100 – 200 mK, onset of superconductivity is observed but characteristics differ sharply from those of La-doped films. We discuss the relevance of the findings for the mechanisms of superconductivity on doped SrTiO₃.