Andreev reflection spectroscopy on Bi₂X₃ (X = Se, Te) topological insulators: Implications for the <i>c</i>-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 Bi₂X₃ (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 Bi₂X₃ has a greater non-topological
component than a topological one, since the AR phase space is larger for BBS than for TSS. [1]

[1] C. R. Granstrom et al., https://arxiv.org/abs/1711.00144