Andreev reflection spectroscopy on Bi 2 X 3 (X = Se, Te) topological insulators: Implications for the c -axis superconducting proximity effect

To understand the superconducting proximity effect that occurs across the c-axis interface of non-ideal 3D topological insulators, we perform point-contact Andreev reflection (AR) spectroscopy on Bi$_{2}$X$_{3}$ (X=Se,Te) single crystals with Nb tips at 4.2 K. Scanning tunneling spectroscopy is also used, to locate the Fermi level in the crystals relative to the Dirac point. Robust AR spectra are observed and analyzed with the Blonder-Tinkham-Klapwijk theory, taking into account tip-induced spin-orbit coupling, Fermi-surface mismatch, and the co-presence of bulk band states (BBS) and topological surface states (TSS) at the Fermi level. Spectral analysis using realistic band structures shows that the superconductivity that can be proximitized into Bi$_{2}$X$_{3}$ has a greater non-topological component than a topological one, since the AR phase space is larger for BBS than for TSS.