  Topological insulators are insulating in the bulk but possess metallic
surface states protected by time-reversal symmetry. Here, we report a detailed
electronic transport study in high quality Bi2Se3 topological insulator thin
films contacted by superconducting (In, Al and W) electrodes. The resistance of
the film shows an abrupt and significant upturn when the electrodes become
superconducting. In turn, the Bi2Se3 film strongly weakens the
superconductivity of the electrodes, significantly reducing both their
transition temperatures and critical fields. A possible interpretation of these
results is that the superconducting electrodes are accessing the surface states
and the experimental results are the consequence of the interplay between the
Cooper pairs of the electrodes and the spin polarized current of the surface
states in Bi2Se3.
