  Using high-resolution angle-resolved photoemission spectroscopy and scanning
tunneling microscopy/spectroscopy, the atomic and low energy electronic
structure of the Sr-doped superconducting topological insulators (SrxBi2Se3)
was studied. Scanning tunneling microscopy shows that most of the Sr atoms are
not in the van der Waals gap. After Sr doping, the Fermi level was found to
move further upwards when compared with the parent compound Bi2Se3, which is
consistent with the low carrier density in this system. The topological surface
state was clearly observed, and the position of the Dirac point was determined
in all doped samples. The surface state is well separated from the bulk
conduction bands in the momentum space. The persistence of separated
topological surface state combined with small Fermi energy makes this
superconducting material a very promising candidate for the time reversal
invariant topological superconductor
