  Proximity-induced superconductivity in a 3D topological insulator represents
a new avenue for observing zero-energy Majorana fermions inside vortex cores.
Relatively small gaps and low transition temperatures of conventional s-wave
superconductors put the hard constraints on these experiments. Significantly
larger gaps and higher transition temperatures in cuprate superconductors might
be an attractive alternative to considerably relax these constraints, but it is
not clear whether the proximity effect would be effective in heterostructures
involving cuprates and topological insulators. Here, we present angle-resolved
photoemission studies of thin Bi2Se3 films grown in-situ on optimally doped
Bi2Sr2CaCu2O8 substrates that show the absence of proximity-induced gaps on the
surfaces of Bi2Se3 films as thin as a 1.5 quintuple layer. These results
suggest that the superconducting proximity effect between a cuprate
superconductor and a topological insulator is strongly suppressed, likely due
to a very short coherence length along the c-axis, incompatible crystal and
pairing symmetries at the interface, small size of the topological surface
state Fermi surface and adverse effects of a strong spin-orbit coupling in the
topological material.
