Andreev reflection spectroscopy on Bi${}_2$X${}_3$ (X = Se, Te) topological insulators: Implications for the c-axis superconducting proximity effect

Using Andreev reflection (AR) as an experimental gauge of the superconducting proximity effect (PE), we assess the topological purity of the superconductivity that is induced by the c-axis PE between an s-wave superconductor and the topological insulators Bi${}_2$X${}_3$ (X=Se,Te). Point-contact AR spectroscopy is performed with Nb tips on Bi${}_2$X${}_3$ single crystals at 4.2 K. Scanning tunneling spectroscopy is also used, to locate the Fermi level $E_F$ relative to the Dirac point in the crystals. The AR data is analyzed with Blonder-Tinkham-Klapwijk theory, taking into account tip-induced spin-orbit coupling, Fermi-surface mismatch, and the co-presence of bulk band and topological surface states at $E_F$. Our results indicate that the superconductivity that can be proximity-induced into Bi${}_2$X${}_3$ is predominantly non-topological.