Nanoscale Andreev Reflection Spectroscopy on Bismuth-Chalcogenide Topological Insulators*

Andreev reflection (AR) is the basic mechanism underlying the superconducting proximity effect which, at the \mbox{interface} between a \mbox{topological} insulator (TI) and a spin-singlet \mbox{superconductor}, can \mbox{induce} chiral $p$-wave pairing in the TI. Despite this novel \mbox{importance}, it is not well understood how AR is affected by the unique attributes of a three-dimensional TI, namely the Dirac dispersion and helical spin-polarization of its surface states. In this work, we use both \mbox{$s$-wave} and $d$-wave\footnote{C. S. Turel et al., \textbf{Appl. Phys. Lett.} 99, 192508 (2011)} superconducting tips to perform AR spectroscopy at \mbox{4.2 K} on flux-grown Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ single crystals, as well as \mbox{epitaxial} Bi$_{2}$Se$_{3}$ thin films grown on SrTiO$_{3}$ substrates by molecular beam \mbox{epitaxy}. These AR measurements are complemented by scanning \mbox{tunneling} \mbox{spectroscopy}, in order to characterize the superconducting tip as well as the doping level and surface condition of the TI sample. Our data are \mbox{analyzed} using BTK theory, in light of the characteristic band structure of bismuth chalcogenides, to elucidate how the band structure affects the AR process.