Local Magnetic Imaging of Proximity Effect-Induced Superconductivity at the Bi$_{2}$Se$_{3}$-Nb Interface

The interface between a topological insulator (TI) and an s-wave superconductor (SC) is predicted to host Majorana bound states analogous to vortices in a spinless p$_{\mathrm{x}}+$ip$_{\mathrm{y}}$ superconductor. For 3D TIs coupled to s-wave superconductors, the winding of the superconducting vortices can counteract the TI pi-Berry's phase, resulting in zero-energy Majorana fermion excitations at the interface. Transport measurements of Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ superconducting junctions have shown Josephson junction effects[1,3,4] and established the existence of a supercurrent that is tunable with gate voltage [2], but the relative contributions of the bulk and bound states to the supercurrent is not well-understood. We report on measurements of the local superfluid density at the interface between Bi$_{2}$Se$_{3}$ nanoplatelets and Nb using a scanning SQUID microscope and quartz tuning fork sensor for simultaneous AFM characterization. We demonstrate that the local penetration depth measurements have increased accuracy and provide an experimentally tractable method for studying proximity effect-induced superconductivity at the SC-TI interface, which is a precursor for observation of the elusive Majorana fermion in Bi$_{2}$Se$_{3}$ and other 3D TIs.\\[4pt] [1] arXiv:1209.5830 (2012). [2] Nat. Comm. \textbf{2} (2011). [3] Nat. Mat. \textbf{11}, 421 (2012). [4] Phys. Rev. Lett. \textbf{109}, 056803 (2012). [5] Phys. Rev. B \textbf{84}, 165120 (2011).