LT-STM study of Nb islands on Bi2Se3 with W and Nb Tips

Proximity effect between an s-wave superconductor (SC) and a topological insulator (TI) are expected to induce px $+$ ipy superconductor like state at the SC-TI interface. The vortex cores of that state are predicted to host Majorana fermions. In this work we study the TI Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ using W and Nb tips at 4.2 K with and without Nb islands. The W tip shows no SC gap on top and around the islands. The Nb tip shows variable SC gaps and a verity of zero bias conductance peaks. The possible sources for these observations and the part of the TI, Nb islands and/or Nb tip will be discussed.