  Topological superconductor is a very interesting and frontier topic in
condensed matter physics1. Despite the tremendous efforts in exploring the
topological superconductivity, its presence is however still under heavy
debates. The Dirac electrons are supposed to exist in a thin layer of the
surface of a topological insulator. Due to the finite spin-orbital coupling,
these electrons will have a spin-momentum locking effect. In this case, the
superfluid with the spin singlet Cooper pairing is not completely comforted by
the Dirac electrons. It thus remains unclear whether and how the Dirac
electrons fall into Cooper pairing in an intrinsic superconductor with the
topological surface states. In this work, we show the systematic study of
scanning tunneling microscope/spectroscopy on the possible topological
superconductor SrxBi2Se3. We first show that only the intercalated (or
inserted), not the substituted Sr atoms can induce superconductivity. Then we
show the full superconducting gaps without any abnormal in-gap density of
states as expected theoretically for the bulk topological superconductivity.
However, we find that the surface Dirac electrons will simultaneously condense
into the superconducting state when the energy is smaller than the bulk
superconducting gap. This vividly demonstrates how the surface Dirac electrons
are driven into Cooper pairs.
