  The combination of superconductivity and the helical spin-momentum locking at
the surface state of a topological insulator (TI) has been predicted to give
rise to p-wave superconductivity and Majorana bound states. The
superconductivity can be induced by the proximity effect of a an s-wave
superconductor (S) into the TI. To probe the superconducting correlations
inside the TI, dI/dV spectroscopy has been performed across such S-TI
interfaces. Both the alloyed Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ and the
stoichiometric BiSbTeSe$_2$ have been used as three dimensional TI. In the case
of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$, the presence of disorder induced
electron-electron interactions can give rise to an additional zero-bias
resistance peak. For the stoichiometric BiSbTeSe$_2$ with less disorder, tunnel
barriers were employed in order to enhance the signal from the interface. The
general observations in the spectra of a large variety of samples are
conductance dips at the induced gap voltage, combined with an increased sub-gap
conductance, consistent with p-wave predictions. The induced gap voltage is
typically smaller than the gap of the Nb superconducting electrode, especially
in the presence of an intentional tunnel barrier. Additional uncovered
spectroscopic features are oscillations that are linearly spaced in energy, as
well as a possible second order parameter component.
