  The superconducting materials family of doped Bi2Se3 remains intensively
studied in the field of condensed matter physics due to strong experimental
evidence for topologically non-trivial superconductivity in the bulk. However,
at the surface of these materials, even the observation of superconductivity
itself is still controversial. We use scanning tunneling microscopy (STM) down
to 0.4 K to show that on the surface of bulk superconducting SrxBi2Se3, no gap
in the density of states is observed around the Fermi energy as long as clean
metallic probe tips are used. Nevertheless, using scanning electron microscopy
and energy-dispersive X-ray analysis, we find that micron-sized flakes of
SrxBi2Se3 are easily transferred from the sample onto the STM probe tip and
that such flakes consistently show a superconducting gap in the density of
states. We argue that the superconductivity in SrxBi2Se3 crystals does not
extend to the surface when the topological surface state (TSS) is intact, but
in micro-flakes the TSS has been destroyed due to strain and allows the
superconductivity to extend to the surface. To understand this phenomenon, we
propose that the local electric field, always found in electron doped Bi2Se3 in
the presence of the TSS due to an intrinsic upward band bending, works against
superconductivity at the surface.
